31100df60e9d3dc7cb5058f2c838c74eabb82ec792ee30deba0d2b9143712498

ensimple/2470.html.txt

@@ -0,0 +1,25 @@
+An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system.
+
+In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by the Kepler space observatory.[3] Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]
+
+Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition.[8]
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One".[9]
+
+The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 1992.
+
+In 1992, radio astronomers announced the discovery of planets around another pulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
+
+On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[12] This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.
+
+In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]
+
+Extrasolar planets can have many different forms.
+
+
+
+The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.[14]  The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.[15][16]
+
+Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.

ensimple/2471.html.txt

@@ -0,0 +1,55 @@
+A star is a very large ball of bright glowing hot matter in space. That matter is called plasma. Stars are held together by gravity. They give out heat and light because they are very hot.
+
+Stars are hot because nuclear reactions happen inside them. Those reactions are called nuclear fusion.  Nuclear fusion makes light and heat and makes bigger and bigger chemical elements. Stars have a lot of hydrogen. Nuclear fusion changes hydrogen into helium. When a star gets old, it starts to change the helium into other bigger chemical elements, like carbon and oxygen. Fusion makes a lot of energy. The energy makes the star very hot. The energy produced by stars moves (radiates) away from them. Much of the energy leaves as light. The rest leaves as other kinds of electromagnetic radiation.
+
+The star nearest to Earth is the Sun. The energy from the Sun supports almost all life on Earth by providing light for plants. Plants turn the light into energy in a process called photosynthesis.[1] The energy from the Sun also causes weather and humidity on Earth.
+
+We can see other stars in the night sky when the Sun goes down. Like the Sun, they are made mostly of hydrogen and a little bit of helium plus other elements. Astronomers often compare those other stars to the Sun. For example, their mass is given in solar masses. A small star may be 0.2 solar masses, a big one 4.0 solar masses.
+
+The Earth and other planets move around (orbit) the Sun. The Sun and all things that orbit the Sun are called the Solar System. Many other stars have planets orbiting them: those planets are called exoplanets. If you were on an exoplanet, our Sun would look like a star in the sky, but you could not see the Earth because it would be too far away.
+
+Proxima Centauri is the star that is closest to our Sun. It is 39.9 trillion kilometres away. This is 4.2 light years away. This means that light from Proxima Centauri takes 4.2 years to reach Earth.
+
+Astronomers think there is a very large number of stars in the Universe. The observable Universe contains more than 2 trillion (1012) galaxies[2] and, overall, as many as an estimated 1×1024 stars[3][4] (more stars than all the grains of sand on planet Earth).[5] That is, 1,000,000,000,000,000,000,000,000 stars, which is many times more than the few hundred billion stars in the Milky Way (our galaxy).
+
+Most stars are very old. They are usually thought to be between 1 billion and 10 billion years old. The oldest stars are 13.7 billion years old. That is as old as the Universe. Some young stars are only a few million years old. Young stars are mostly brighter than old ones.
+
+Stars are different sizes. The smallest stars are neutron stars, which are actually dead stars. They are no bigger than a city. A neutron star has a large amount of mass in a very small space.
+
+Hypergiant stars are the largest stars in the Universe. They have a diameter over 1,500 times bigger than the Sun. If the Sun was a hypergiant star, it would reach out to as far as Jupiter.
+
+The star Betelgeuse is a red supergiant star. Although these stars are very large, they also have low density.
+
+Some stars look brighter than other stars. This difference is measured in terms of apparent magnitude. There are two reasons why stars have different apparent magnitude. If a star is very close to us it will appear much brighter. This is just like a candle. A candle that is close to us appears brighter. The other reason a star can appear brighter is that it is hotter than another cooler star.
+
+Stars give off light but also give off a solar wind and neutrinos. These are very small particles of matter.
+
+Stars are made of mass and mass makes gravity. Gravity makes planets orbit stars. This is why the Earth orbits the Sun. The gravity of two stars can make them go around each other. Stars that orbit each other are called binary stars. Scientists think there are many binary stars. There are even groups of three or more stars that orbit each other. Proxima Centauri is a small star that orbits other stars.
+
+Stars are not spread evenly across all of space. They are grouped into galaxies. A galaxy contains hundreds of billions of stars.
+
+Stars have been important to people all over the world for all of history. Stars have been part of religious practices. Long ago, people believed that stars could never die.
+
+Astronomers organized stars into groups called constellations. They used the constellations to help them see the motion of the planets and to guess the position of the Sun.[6] The motion of the Sun and the stars was used to make calendars. The calendars were used by farmers to decide when to plant crops and when to harvest them.[8]
+
+Stars are made in nebulae. These are areas that have more gas than normal space. The gas in a nebula is pulled together by gravity. The Orion nebula is an example of a place where gas is coming together to form stars.
+
+Stars spend most of their lives combining (fusing) hydrogen with hydrogen to make energy. When hydrogen is fused it makes helium and it makes a lot of energy. To fuse hydrogen into helium it must be very hot and the pressure must be very high. Fusion happens at the center of stars, called "the core".
+
+The smallest stars (red dwarfs) fuse their hydrogen slowly and live for 100 billion years. Red dwarfs live longer than any other type of star. At the end of their lives, they become dimmer and dimmer. Red dwarfs do not explode.
+
+When very heavy stars die, they explode. This explosion is called a supernova. When a supernova happens in a nebula, the explosion pushes the gas in the nebula together. This makes the gas in the nebula very thick (dense). Gravity and exploding stars both help to bring the gas together to make new stars in nebulas.
+
+Most stars use up the hydrogen at their core. When they do, their core becomes smaller and becomes hotter. It becomes so hot it pushes away the outer part of the star. The outer part expands and it makes a red giant star. Astro-physicists think that in about 5 billion years, the Sun will be a red giant. Our Sun will be so large it will eat the Earth. After our Sun stops using hydrogen to make energy, it will use helium in its very hot core. It will be hotter than when it was fusing hydrogen. Heavy stars will also make elements heavier than helium. As a star makes heavier and heavier elements, it makes less and less energy. Iron is a heavy element made in heavy stars.
+
+Our star is an average star. Average stars will push away their outer gases. The gas it pushes away makes a cloud called a planetary nebula. The core part of the star will remain. It will be a ball as big as the Earth and called a white dwarf. It will fade into a black dwarf over a very long time.
+
+Later in large stars,  heavier elements are made by fusion. Finally the star makes a supernova explosion. Most things happen in the universe so slowly we do not notice. But supernova explosions happen in only 100 seconds. When a supernova explodes its flash is as bright as a 100 billion stars. The dying star is so bright it can be seen during the day. Supernova means "new star" because people used to think it was the beginning of a new star. Today we know that a supernova is the death of an old star. The gas of the star is pushed away by the explosion. It forms a giant cloud of gas called a planetary nebula. The crab nebula is a good example. All that remains is a neutron star. If the star was very heavy, the star will make a black hole. Gravity in a black hole is extremely strong. It is so strong that even light cannot escape from a black hole.
+
+The heaviest elements are made in the explosion of a supernova. After billions of years of floating in space, the gas and dust come together to make new stars and new planets.  Much of the gas and dust in space comes from supernovae. Our Sun, the Earth, and all living things are made from star dust.
+
+Astronomers have known for centuries that stars have different colors. When looking at an electromagnetic spectrum, ultraviolet waves are the shortest, and infrared are the longest.[9]   The visible spectrum has wavelengths between these two extremes.
+
+Modern instruments can measure very precisely the color of a star.  This allows astronomers to determine that star's temperature, because a hotter star's black-body radiation has shorter wavelengths.  The hottest stars are blue and violet, then white, then yellow, and the coolest are red.[10] Knowing the color and absolute magnitude, astronomers can place the star on the Hertzsprung-Russell diagram, and estimate its habitable zone and other facts about it.
+
+For example, our Sun is white, and the Earth is the perfect distance away for life. If our Sun was a hotter, blue star, however, Earth would have to be much farther away or else it would be too hot to have water and sustain life.

ensimple/2472.html.txt

@@ -0,0 +1,25 @@
+An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system.
+
+In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by the Kepler space observatory.[3] Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]
+
+Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition.[8]
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One".[9]
+
+The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 1992.
+
+In 1992, radio astronomers announced the discovery of planets around another pulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
+
+On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[12] This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.
+
+In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]
+
+Extrasolar planets can have many different forms.
+
+
+
+The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.[14]  The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.[15][16]
+
+Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.

ensimple/2473.html.txt

@@ -0,0 +1,55 @@
+A star is a very large ball of bright glowing hot matter in space. That matter is called plasma. Stars are held together by gravity. They give out heat and light because they are very hot.
+
+Stars are hot because nuclear reactions happen inside them. Those reactions are called nuclear fusion.  Nuclear fusion makes light and heat and makes bigger and bigger chemical elements. Stars have a lot of hydrogen. Nuclear fusion changes hydrogen into helium. When a star gets old, it starts to change the helium into other bigger chemical elements, like carbon and oxygen. Fusion makes a lot of energy. The energy makes the star very hot. The energy produced by stars moves (radiates) away from them. Much of the energy leaves as light. The rest leaves as other kinds of electromagnetic radiation.
+
+The star nearest to Earth is the Sun. The energy from the Sun supports almost all life on Earth by providing light for plants. Plants turn the light into energy in a process called photosynthesis.[1] The energy from the Sun also causes weather and humidity on Earth.
+
+We can see other stars in the night sky when the Sun goes down. Like the Sun, they are made mostly of hydrogen and a little bit of helium plus other elements. Astronomers often compare those other stars to the Sun. For example, their mass is given in solar masses. A small star may be 0.2 solar masses, a big one 4.0 solar masses.
+
+The Earth and other planets move around (orbit) the Sun. The Sun and all things that orbit the Sun are called the Solar System. Many other stars have planets orbiting them: those planets are called exoplanets. If you were on an exoplanet, our Sun would look like a star in the sky, but you could not see the Earth because it would be too far away.
+
+Proxima Centauri is the star that is closest to our Sun. It is 39.9 trillion kilometres away. This is 4.2 light years away. This means that light from Proxima Centauri takes 4.2 years to reach Earth.
+
+Astronomers think there is a very large number of stars in the Universe. The observable Universe contains more than 2 trillion (1012) galaxies[2] and, overall, as many as an estimated 1×1024 stars[3][4] (more stars than all the grains of sand on planet Earth).[5] That is, 1,000,000,000,000,000,000,000,000 stars, which is many times more than the few hundred billion stars in the Milky Way (our galaxy).
+
+Most stars are very old. They are usually thought to be between 1 billion and 10 billion years old. The oldest stars are 13.7 billion years old. That is as old as the Universe. Some young stars are only a few million years old. Young stars are mostly brighter than old ones.
+
+Stars are different sizes. The smallest stars are neutron stars, which are actually dead stars. They are no bigger than a city. A neutron star has a large amount of mass in a very small space.
+
+Hypergiant stars are the largest stars in the Universe. They have a diameter over 1,500 times bigger than the Sun. If the Sun was a hypergiant star, it would reach out to as far as Jupiter.
+
+The star Betelgeuse is a red supergiant star. Although these stars are very large, they also have low density.
+
+Some stars look brighter than other stars. This difference is measured in terms of apparent magnitude. There are two reasons why stars have different apparent magnitude. If a star is very close to us it will appear much brighter. This is just like a candle. A candle that is close to us appears brighter. The other reason a star can appear brighter is that it is hotter than another cooler star.
+
+Stars give off light but also give off a solar wind and neutrinos. These are very small particles of matter.
+
+Stars are made of mass and mass makes gravity. Gravity makes planets orbit stars. This is why the Earth orbits the Sun. The gravity of two stars can make them go around each other. Stars that orbit each other are called binary stars. Scientists think there are many binary stars. There are even groups of three or more stars that orbit each other. Proxima Centauri is a small star that orbits other stars.
+
+Stars are not spread evenly across all of space. They are grouped into galaxies. A galaxy contains hundreds of billions of stars.
+
+Stars have been important to people all over the world for all of history. Stars have been part of religious practices. Long ago, people believed that stars could never die.
+
+Astronomers organized stars into groups called constellations. They used the constellations to help them see the motion of the planets and to guess the position of the Sun.[6] The motion of the Sun and the stars was used to make calendars. The calendars were used by farmers to decide when to plant crops and when to harvest them.[8]
+
+Stars are made in nebulae. These are areas that have more gas than normal space. The gas in a nebula is pulled together by gravity. The Orion nebula is an example of a place where gas is coming together to form stars.
+
+Stars spend most of their lives combining (fusing) hydrogen with hydrogen to make energy. When hydrogen is fused it makes helium and it makes a lot of energy. To fuse hydrogen into helium it must be very hot and the pressure must be very high. Fusion happens at the center of stars, called "the core".
+
+The smallest stars (red dwarfs) fuse their hydrogen slowly and live for 100 billion years. Red dwarfs live longer than any other type of star. At the end of their lives, they become dimmer and dimmer. Red dwarfs do not explode.
+
+When very heavy stars die, they explode. This explosion is called a supernova. When a supernova happens in a nebula, the explosion pushes the gas in the nebula together. This makes the gas in the nebula very thick (dense). Gravity and exploding stars both help to bring the gas together to make new stars in nebulas.
+
+Most stars use up the hydrogen at their core. When they do, their core becomes smaller and becomes hotter. It becomes so hot it pushes away the outer part of the star. The outer part expands and it makes a red giant star. Astro-physicists think that in about 5 billion years, the Sun will be a red giant. Our Sun will be so large it will eat the Earth. After our Sun stops using hydrogen to make energy, it will use helium in its very hot core. It will be hotter than when it was fusing hydrogen. Heavy stars will also make elements heavier than helium. As a star makes heavier and heavier elements, it makes less and less energy. Iron is a heavy element made in heavy stars.
+
+Our star is an average star. Average stars will push away their outer gases. The gas it pushes away makes a cloud called a planetary nebula. The core part of the star will remain. It will be a ball as big as the Earth and called a white dwarf. It will fade into a black dwarf over a very long time.
+
+Later in large stars,  heavier elements are made by fusion. Finally the star makes a supernova explosion. Most things happen in the universe so slowly we do not notice. But supernova explosions happen in only 100 seconds. When a supernova explodes its flash is as bright as a 100 billion stars. The dying star is so bright it can be seen during the day. Supernova means "new star" because people used to think it was the beginning of a new star. Today we know that a supernova is the death of an old star. The gas of the star is pushed away by the explosion. It forms a giant cloud of gas called a planetary nebula. The crab nebula is a good example. All that remains is a neutron star. If the star was very heavy, the star will make a black hole. Gravity in a black hole is extremely strong. It is so strong that even light cannot escape from a black hole.
+
+The heaviest elements are made in the explosion of a supernova. After billions of years of floating in space, the gas and dust come together to make new stars and new planets.  Much of the gas and dust in space comes from supernovae. Our Sun, the Earth, and all living things are made from star dust.
+
+Astronomers have known for centuries that stars have different colors. When looking at an electromagnetic spectrum, ultraviolet waves are the shortest, and infrared are the longest.[9]   The visible spectrum has wavelengths between these two extremes.
+
+Modern instruments can measure very precisely the color of a star.  This allows astronomers to determine that star's temperature, because a hotter star's black-body radiation has shorter wavelengths.  The hottest stars are blue and violet, then white, then yellow, and the coolest are red.[10] Knowing the color and absolute magnitude, astronomers can place the star on the Hertzsprung-Russell diagram, and estimate its habitable zone and other facts about it.
+
+For example, our Sun is white, and the Earth is the perfect distance away for life. If our Sun was a hotter, blue star, however, Earth would have to be much farther away or else it would be too hot to have water and sustain life.

ensimple/2474.html.txt

@@ -0,0 +1,25 @@
+An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system.
+
+In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by the Kepler space observatory.[3] Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]
+
+Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition.[8]
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One".[9]
+
+The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 1992.
+
+In 1992, radio astronomers announced the discovery of planets around another pulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
+
+On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[12] This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.
+
+In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]
+
+Extrasolar planets can have many different forms.
+
+
+
+The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.[14]  The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.[15][16]
+
+Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.

ensimple/2475.html.txt

@@ -0,0 +1,25 @@
+An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system.
+
+In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by the Kepler space observatory.[3] Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]
+
+Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition.[8]
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One".[9]
+
+The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 1992.
+
+In 1992, radio astronomers announced the discovery of planets around another pulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
+
+On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[12] This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.
+
+In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]
+
+Extrasolar planets can have many different forms.
+
+
+
+The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.[14]  The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.[15][16]
+
+Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.

ensimple/2476.html.txt

@@ -0,0 +1,55 @@
+A star is a very large ball of bright glowing hot matter in space. That matter is called plasma. Stars are held together by gravity. They give out heat and light because they are very hot.
+
+Stars are hot because nuclear reactions happen inside them. Those reactions are called nuclear fusion.  Nuclear fusion makes light and heat and makes bigger and bigger chemical elements. Stars have a lot of hydrogen. Nuclear fusion changes hydrogen into helium. When a star gets old, it starts to change the helium into other bigger chemical elements, like carbon and oxygen. Fusion makes a lot of energy. The energy makes the star very hot. The energy produced by stars moves (radiates) away from them. Much of the energy leaves as light. The rest leaves as other kinds of electromagnetic radiation.
+
+The star nearest to Earth is the Sun. The energy from the Sun supports almost all life on Earth by providing light for plants. Plants turn the light into energy in a process called photosynthesis.[1] The energy from the Sun also causes weather and humidity on Earth.
+
+We can see other stars in the night sky when the Sun goes down. Like the Sun, they are made mostly of hydrogen and a little bit of helium plus other elements. Astronomers often compare those other stars to the Sun. For example, their mass is given in solar masses. A small star may be 0.2 solar masses, a big one 4.0 solar masses.
+
+The Earth and other planets move around (orbit) the Sun. The Sun and all things that orbit the Sun are called the Solar System. Many other stars have planets orbiting them: those planets are called exoplanets. If you were on an exoplanet, our Sun would look like a star in the sky, but you could not see the Earth because it would be too far away.
+
+Proxima Centauri is the star that is closest to our Sun. It is 39.9 trillion kilometres away. This is 4.2 light years away. This means that light from Proxima Centauri takes 4.2 years to reach Earth.
+
+Astronomers think there is a very large number of stars in the Universe. The observable Universe contains more than 2 trillion (1012) galaxies[2] and, overall, as many as an estimated 1×1024 stars[3][4] (more stars than all the grains of sand on planet Earth).[5] That is, 1,000,000,000,000,000,000,000,000 stars, which is many times more than the few hundred billion stars in the Milky Way (our galaxy).
+
+Most stars are very old. They are usually thought to be between 1 billion and 10 billion years old. The oldest stars are 13.7 billion years old. That is as old as the Universe. Some young stars are only a few million years old. Young stars are mostly brighter than old ones.
+
+Stars are different sizes. The smallest stars are neutron stars, which are actually dead stars. They are no bigger than a city. A neutron star has a large amount of mass in a very small space.
+
+Hypergiant stars are the largest stars in the Universe. They have a diameter over 1,500 times bigger than the Sun. If the Sun was a hypergiant star, it would reach out to as far as Jupiter.
+
+The star Betelgeuse is a red supergiant star. Although these stars are very large, they also have low density.
+
+Some stars look brighter than other stars. This difference is measured in terms of apparent magnitude. There are two reasons why stars have different apparent magnitude. If a star is very close to us it will appear much brighter. This is just like a candle. A candle that is close to us appears brighter. The other reason a star can appear brighter is that it is hotter than another cooler star.
+
+Stars give off light but also give off a solar wind and neutrinos. These are very small particles of matter.
+
+Stars are made of mass and mass makes gravity. Gravity makes planets orbit stars. This is why the Earth orbits the Sun. The gravity of two stars can make them go around each other. Stars that orbit each other are called binary stars. Scientists think there are many binary stars. There are even groups of three or more stars that orbit each other. Proxima Centauri is a small star that orbits other stars.
+
+Stars are not spread evenly across all of space. They are grouped into galaxies. A galaxy contains hundreds of billions of stars.
+
+Stars have been important to people all over the world for all of history. Stars have been part of religious practices. Long ago, people believed that stars could never die.
+
+Astronomers organized stars into groups called constellations. They used the constellations to help them see the motion of the planets and to guess the position of the Sun.[6] The motion of the Sun and the stars was used to make calendars. The calendars were used by farmers to decide when to plant crops and when to harvest them.[8]
+
+Stars are made in nebulae. These are areas that have more gas than normal space. The gas in a nebula is pulled together by gravity. The Orion nebula is an example of a place where gas is coming together to form stars.
+
+Stars spend most of their lives combining (fusing) hydrogen with hydrogen to make energy. When hydrogen is fused it makes helium and it makes a lot of energy. To fuse hydrogen into helium it must be very hot and the pressure must be very high. Fusion happens at the center of stars, called "the core".
+
+The smallest stars (red dwarfs) fuse their hydrogen slowly and live for 100 billion years. Red dwarfs live longer than any other type of star. At the end of their lives, they become dimmer and dimmer. Red dwarfs do not explode.
+
+When very heavy stars die, they explode. This explosion is called a supernova. When a supernova happens in a nebula, the explosion pushes the gas in the nebula together. This makes the gas in the nebula very thick (dense). Gravity and exploding stars both help to bring the gas together to make new stars in nebulas.
+
+Most stars use up the hydrogen at their core. When they do, their core becomes smaller and becomes hotter. It becomes so hot it pushes away the outer part of the star. The outer part expands and it makes a red giant star. Astro-physicists think that in about 5 billion years, the Sun will be a red giant. Our Sun will be so large it will eat the Earth. After our Sun stops using hydrogen to make energy, it will use helium in its very hot core. It will be hotter than when it was fusing hydrogen. Heavy stars will also make elements heavier than helium. As a star makes heavier and heavier elements, it makes less and less energy. Iron is a heavy element made in heavy stars.
+
+Our star is an average star. Average stars will push away their outer gases. The gas it pushes away makes a cloud called a planetary nebula. The core part of the star will remain. It will be a ball as big as the Earth and called a white dwarf. It will fade into a black dwarf over a very long time.
+
+Later in large stars,  heavier elements are made by fusion. Finally the star makes a supernova explosion. Most things happen in the universe so slowly we do not notice. But supernova explosions happen in only 100 seconds. When a supernova explodes its flash is as bright as a 100 billion stars. The dying star is so bright it can be seen during the day. Supernova means "new star" because people used to think it was the beginning of a new star. Today we know that a supernova is the death of an old star. The gas of the star is pushed away by the explosion. It forms a giant cloud of gas called a planetary nebula. The crab nebula is a good example. All that remains is a neutron star. If the star was very heavy, the star will make a black hole. Gravity in a black hole is extremely strong. It is so strong that even light cannot escape from a black hole.
+
+The heaviest elements are made in the explosion of a supernova. After billions of years of floating in space, the gas and dust come together to make new stars and new planets.  Much of the gas and dust in space comes from supernovae. Our Sun, the Earth, and all living things are made from star dust.
+
+Astronomers have known for centuries that stars have different colors. When looking at an electromagnetic spectrum, ultraviolet waves are the shortest, and infrared are the longest.[9]   The visible spectrum has wavelengths between these two extremes.
+
+Modern instruments can measure very precisely the color of a star.  This allows astronomers to determine that star's temperature, because a hotter star's black-body radiation has shorter wavelengths.  The hottest stars are blue and violet, then white, then yellow, and the coolest are red.[10] Knowing the color and absolute magnitude, astronomers can place the star on the Hertzsprung-Russell diagram, and estimate its habitable zone and other facts about it.
+
+For example, our Sun is white, and the Earth is the perfect distance away for life. If our Sun was a hotter, blue star, however, Earth would have to be much farther away or else it would be too hot to have water and sustain life.

ensimple/2477.html.txt

@@ -0,0 +1,25 @@
+An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system.
+
+In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by the Kepler space observatory.[3] Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]
+
+Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition.[8]
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One".[9]
+
+The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 1992.
+
+In 1992, radio astronomers announced the discovery of planets around another pulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
+
+On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[12] This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.
+
+In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]
+
+Extrasolar planets can have many different forms.
+
+
+
+The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.[14]  The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.[15][16]
+
+Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.

ensimple/2478.html.txt

@@ -0,0 +1,25 @@
+An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system.
+
+In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by the Kepler space observatory.[3] Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]
+
+Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition.[8]
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One".[9]
+
+The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 1992.
+
+In 1992, radio astronomers announced the discovery of planets around another pulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
+
+On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[12] This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.
+
+In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]
+
+Extrasolar planets can have many different forms.
+
+
+
+The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.[14]  The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.[15][16]
+
+Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.

ensimple/2479.html.txt

@@ -0,0 +1,55 @@
+A star is a very large ball of bright glowing hot matter in space. That matter is called plasma. Stars are held together by gravity. They give out heat and light because they are very hot.
+
+Stars are hot because nuclear reactions happen inside them. Those reactions are called nuclear fusion.  Nuclear fusion makes light and heat and makes bigger and bigger chemical elements. Stars have a lot of hydrogen. Nuclear fusion changes hydrogen into helium. When a star gets old, it starts to change the helium into other bigger chemical elements, like carbon and oxygen. Fusion makes a lot of energy. The energy makes the star very hot. The energy produced by stars moves (radiates) away from them. Much of the energy leaves as light. The rest leaves as other kinds of electromagnetic radiation.
+
+The star nearest to Earth is the Sun. The energy from the Sun supports almost all life on Earth by providing light for plants. Plants turn the light into energy in a process called photosynthesis.[1] The energy from the Sun also causes weather and humidity on Earth.
+
+We can see other stars in the night sky when the Sun goes down. Like the Sun, they are made mostly of hydrogen and a little bit of helium plus other elements. Astronomers often compare those other stars to the Sun. For example, their mass is given in solar masses. A small star may be 0.2 solar masses, a big one 4.0 solar masses.
+
+The Earth and other planets move around (orbit) the Sun. The Sun and all things that orbit the Sun are called the Solar System. Many other stars have planets orbiting them: those planets are called exoplanets. If you were on an exoplanet, our Sun would look like a star in the sky, but you could not see the Earth because it would be too far away.
+
+Proxima Centauri is the star that is closest to our Sun. It is 39.9 trillion kilometres away. This is 4.2 light years away. This means that light from Proxima Centauri takes 4.2 years to reach Earth.
+
+Astronomers think there is a very large number of stars in the Universe. The observable Universe contains more than 2 trillion (1012) galaxies[2] and, overall, as many as an estimated 1×1024 stars[3][4] (more stars than all the grains of sand on planet Earth).[5] That is, 1,000,000,000,000,000,000,000,000 stars, which is many times more than the few hundred billion stars in the Milky Way (our galaxy).
+
+Most stars are very old. They are usually thought to be between 1 billion and 10 billion years old. The oldest stars are 13.7 billion years old. That is as old as the Universe. Some young stars are only a few million years old. Young stars are mostly brighter than old ones.
+
+Stars are different sizes. The smallest stars are neutron stars, which are actually dead stars. They are no bigger than a city. A neutron star has a large amount of mass in a very small space.
+
+Hypergiant stars are the largest stars in the Universe. They have a diameter over 1,500 times bigger than the Sun. If the Sun was a hypergiant star, it would reach out to as far as Jupiter.
+
+The star Betelgeuse is a red supergiant star. Although these stars are very large, they also have low density.
+
+Some stars look brighter than other stars. This difference is measured in terms of apparent magnitude. There are two reasons why stars have different apparent magnitude. If a star is very close to us it will appear much brighter. This is just like a candle. A candle that is close to us appears brighter. The other reason a star can appear brighter is that it is hotter than another cooler star.
+
+Stars give off light but also give off a solar wind and neutrinos. These are very small particles of matter.
+
+Stars are made of mass and mass makes gravity. Gravity makes planets orbit stars. This is why the Earth orbits the Sun. The gravity of two stars can make them go around each other. Stars that orbit each other are called binary stars. Scientists think there are many binary stars. There are even groups of three or more stars that orbit each other. Proxima Centauri is a small star that orbits other stars.
+
+Stars are not spread evenly across all of space. They are grouped into galaxies. A galaxy contains hundreds of billions of stars.
+
+Stars have been important to people all over the world for all of history. Stars have been part of religious practices. Long ago, people believed that stars could never die.
+
+Astronomers organized stars into groups called constellations. They used the constellations to help them see the motion of the planets and to guess the position of the Sun.[6] The motion of the Sun and the stars was used to make calendars. The calendars were used by farmers to decide when to plant crops and when to harvest them.[8]
+
+Stars are made in nebulae. These are areas that have more gas than normal space. The gas in a nebula is pulled together by gravity. The Orion nebula is an example of a place where gas is coming together to form stars.
+
+Stars spend most of their lives combining (fusing) hydrogen with hydrogen to make energy. When hydrogen is fused it makes helium and it makes a lot of energy. To fuse hydrogen into helium it must be very hot and the pressure must be very high. Fusion happens at the center of stars, called "the core".
+
+The smallest stars (red dwarfs) fuse their hydrogen slowly and live for 100 billion years. Red dwarfs live longer than any other type of star. At the end of their lives, they become dimmer and dimmer. Red dwarfs do not explode.
+
+When very heavy stars die, they explode. This explosion is called a supernova. When a supernova happens in a nebula, the explosion pushes the gas in the nebula together. This makes the gas in the nebula very thick (dense). Gravity and exploding stars both help to bring the gas together to make new stars in nebulas.
+
+Most stars use up the hydrogen at their core. When they do, their core becomes smaller and becomes hotter. It becomes so hot it pushes away the outer part of the star. The outer part expands and it makes a red giant star. Astro-physicists think that in about 5 billion years, the Sun will be a red giant. Our Sun will be so large it will eat the Earth. After our Sun stops using hydrogen to make energy, it will use helium in its very hot core. It will be hotter than when it was fusing hydrogen. Heavy stars will also make elements heavier than helium. As a star makes heavier and heavier elements, it makes less and less energy. Iron is a heavy element made in heavy stars.
+
+Our star is an average star. Average stars will push away their outer gases. The gas it pushes away makes a cloud called a planetary nebula. The core part of the star will remain. It will be a ball as big as the Earth and called a white dwarf. It will fade into a black dwarf over a very long time.
+
+Later in large stars,  heavier elements are made by fusion. Finally the star makes a supernova explosion. Most things happen in the universe so slowly we do not notice. But supernova explosions happen in only 100 seconds. When a supernova explodes its flash is as bright as a 100 billion stars. The dying star is so bright it can be seen during the day. Supernova means "new star" because people used to think it was the beginning of a new star. Today we know that a supernova is the death of an old star. The gas of the star is pushed away by the explosion. It forms a giant cloud of gas called a planetary nebula. The crab nebula is a good example. All that remains is a neutron star. If the star was very heavy, the star will make a black hole. Gravity in a black hole is extremely strong. It is so strong that even light cannot escape from a black hole.
+
+The heaviest elements are made in the explosion of a supernova. After billions of years of floating in space, the gas and dust come together to make new stars and new planets.  Much of the gas and dust in space comes from supernovae. Our Sun, the Earth, and all living things are made from star dust.
+
+Astronomers have known for centuries that stars have different colors. When looking at an electromagnetic spectrum, ultraviolet waves are the shortest, and infrared are the longest.[9]   The visible spectrum has wavelengths between these two extremes.
+
+Modern instruments can measure very precisely the color of a star.  This allows astronomers to determine that star's temperature, because a hotter star's black-body radiation has shorter wavelengths.  The hottest stars are blue and violet, then white, then yellow, and the coolest are red.[10] Knowing the color and absolute magnitude, astronomers can place the star on the Hertzsprung-Russell diagram, and estimate its habitable zone and other facts about it.
+
+For example, our Sun is white, and the Earth is the perfect distance away for life. If our Sun was a hotter, blue star, however, Earth would have to be much farther away or else it would be too hot to have water and sustain life.

ensimple/248.html.txt

@@ -0,0 +1,11 @@
+
+
+The electric eel, Electrophorus electricus, is a species of fish. It can weigh up to six pounds. It can give electric shocks of up to six hundred fifty watts of electricity. The animal uses these shocks both for hunting and to defend itself. It lives in the stagnant muddy river bottoms of the Orinoco and the Amazon, and uses low-voltage electric fields to find its prey.[1]
+
+The electric eel is an apex predator in the parts of South America where it lives. This means it usually has no enemies except other animals of its own species. It can kill animals larger than itself. Its electric organs evolved from muscles, and make up four fifths of its body.[1]
+
+Despite its name, the electric eel is not an eel. It is a knifefish. It is an obligatory air breather (must breath air).[1] This makes sense, because the muddy water is very low in oxygen, and it needs oxygen to power its electric organs.
+
+Because they can shock, very few of these fish are kept as pets. But if the eel repeatedly shocks, its electric organs become completely discharged. Then a person can touch it without being shocked.
+
+Media related to Electric eels at Wikimedia Commons

ensimple/2480.html.txt

@@ -0,0 +1,25 @@
+An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system.
+
+In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by the Kepler space observatory.[3] Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]
+
+Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition.[8]
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One".[9]
+
+The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 1992.
+
+In 1992, radio astronomers announced the discovery of planets around another pulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
+
+On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[12] This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.
+
+In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]
+
+Extrasolar planets can have many different forms.
+
+
+
+The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.[14]  The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.[15][16]
+
+Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.

ensimple/2481.html.txt

@@ -0,0 +1,25 @@
+An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system.
+
+In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by the Kepler space observatory.[3] Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]
+
+Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition.[8]
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One".[9]
+
+The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 1992.
+
+In 1992, radio astronomers announced the discovery of planets around another pulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
+
+On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[12] This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.
+
+In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]
+
+Extrasolar planets can have many different forms.
+
+
+
+The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.[14]  The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.[15][16]
+
+Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.

ensimple/2482.html.txt

@@ -0,0 +1,25 @@
+An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system.
+
+In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by the Kepler space observatory.[3] Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]
+
+Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition.[8]
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One".[9]
+
+The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 1992.
+
+In 1992, radio astronomers announced the discovery of planets around another pulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
+
+On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[12] This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.
+
+In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]
+
+Extrasolar planets can have many different forms.
+
+
+
+The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.[14]  The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.[15][16]
+
+Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.

ensimple/2483.html.txt

@@ -0,0 +1,55 @@
+A star is a very large ball of bright glowing hot matter in space. That matter is called plasma. Stars are held together by gravity. They give out heat and light because they are very hot.
+
+Stars are hot because nuclear reactions happen inside them. Those reactions are called nuclear fusion.  Nuclear fusion makes light and heat and makes bigger and bigger chemical elements. Stars have a lot of hydrogen. Nuclear fusion changes hydrogen into helium. When a star gets old, it starts to change the helium into other bigger chemical elements, like carbon and oxygen. Fusion makes a lot of energy. The energy makes the star very hot. The energy produced by stars moves (radiates) away from them. Much of the energy leaves as light. The rest leaves as other kinds of electromagnetic radiation.
+
+The star nearest to Earth is the Sun. The energy from the Sun supports almost all life on Earth by providing light for plants. Plants turn the light into energy in a process called photosynthesis.[1] The energy from the Sun also causes weather and humidity on Earth.
+
+We can see other stars in the night sky when the Sun goes down. Like the Sun, they are made mostly of hydrogen and a little bit of helium plus other elements. Astronomers often compare those other stars to the Sun. For example, their mass is given in solar masses. A small star may be 0.2 solar masses, a big one 4.0 solar masses.
+
+The Earth and other planets move around (orbit) the Sun. The Sun and all things that orbit the Sun are called the Solar System. Many other stars have planets orbiting them: those planets are called exoplanets. If you were on an exoplanet, our Sun would look like a star in the sky, but you could not see the Earth because it would be too far away.
+
+Proxima Centauri is the star that is closest to our Sun. It is 39.9 trillion kilometres away. This is 4.2 light years away. This means that light from Proxima Centauri takes 4.2 years to reach Earth.
+
+Astronomers think there is a very large number of stars in the Universe. The observable Universe contains more than 2 trillion (1012) galaxies[2] and, overall, as many as an estimated 1×1024 stars[3][4] (more stars than all the grains of sand on planet Earth).[5] That is, 1,000,000,000,000,000,000,000,000 stars, which is many times more than the few hundred billion stars in the Milky Way (our galaxy).
+
+Most stars are very old. They are usually thought to be between 1 billion and 10 billion years old. The oldest stars are 13.7 billion years old. That is as old as the Universe. Some young stars are only a few million years old. Young stars are mostly brighter than old ones.
+
+Stars are different sizes. The smallest stars are neutron stars, which are actually dead stars. They are no bigger than a city. A neutron star has a large amount of mass in a very small space.
+
+Hypergiant stars are the largest stars in the Universe. They have a diameter over 1,500 times bigger than the Sun. If the Sun was a hypergiant star, it would reach out to as far as Jupiter.
+
+The star Betelgeuse is a red supergiant star. Although these stars are very large, they also have low density.
+
+Some stars look brighter than other stars. This difference is measured in terms of apparent magnitude. There are two reasons why stars have different apparent magnitude. If a star is very close to us it will appear much brighter. This is just like a candle. A candle that is close to us appears brighter. The other reason a star can appear brighter is that it is hotter than another cooler star.
+
+Stars give off light but also give off a solar wind and neutrinos. These are very small particles of matter.
+
+Stars are made of mass and mass makes gravity. Gravity makes planets orbit stars. This is why the Earth orbits the Sun. The gravity of two stars can make them go around each other. Stars that orbit each other are called binary stars. Scientists think there are many binary stars. There are even groups of three or more stars that orbit each other. Proxima Centauri is a small star that orbits other stars.
+
+Stars are not spread evenly across all of space. They are grouped into galaxies. A galaxy contains hundreds of billions of stars.
+
+Stars have been important to people all over the world for all of history. Stars have been part of religious practices. Long ago, people believed that stars could never die.
+
+Astronomers organized stars into groups called constellations. They used the constellations to help them see the motion of the planets and to guess the position of the Sun.[6] The motion of the Sun and the stars was used to make calendars. The calendars were used by farmers to decide when to plant crops and when to harvest them.[8]
+
+Stars are made in nebulae. These are areas that have more gas than normal space. The gas in a nebula is pulled together by gravity. The Orion nebula is an example of a place where gas is coming together to form stars.
+
+Stars spend most of their lives combining (fusing) hydrogen with hydrogen to make energy. When hydrogen is fused it makes helium and it makes a lot of energy. To fuse hydrogen into helium it must be very hot and the pressure must be very high. Fusion happens at the center of stars, called "the core".
+
+The smallest stars (red dwarfs) fuse their hydrogen slowly and live for 100 billion years. Red dwarfs live longer than any other type of star. At the end of their lives, they become dimmer and dimmer. Red dwarfs do not explode.
+
+When very heavy stars die, they explode. This explosion is called a supernova. When a supernova happens in a nebula, the explosion pushes the gas in the nebula together. This makes the gas in the nebula very thick (dense). Gravity and exploding stars both help to bring the gas together to make new stars in nebulas.
+
+Most stars use up the hydrogen at their core. When they do, their core becomes smaller and becomes hotter. It becomes so hot it pushes away the outer part of the star. The outer part expands and it makes a red giant star. Astro-physicists think that in about 5 billion years, the Sun will be a red giant. Our Sun will be so large it will eat the Earth. After our Sun stops using hydrogen to make energy, it will use helium in its very hot core. It will be hotter than when it was fusing hydrogen. Heavy stars will also make elements heavier than helium. As a star makes heavier and heavier elements, it makes less and less energy. Iron is a heavy element made in heavy stars.
+
+Our star is an average star. Average stars will push away their outer gases. The gas it pushes away makes a cloud called a planetary nebula. The core part of the star will remain. It will be a ball as big as the Earth and called a white dwarf. It will fade into a black dwarf over a very long time.
+
+Later in large stars,  heavier elements are made by fusion. Finally the star makes a supernova explosion. Most things happen in the universe so slowly we do not notice. But supernova explosions happen in only 100 seconds. When a supernova explodes its flash is as bright as a 100 billion stars. The dying star is so bright it can be seen during the day. Supernova means "new star" because people used to think it was the beginning of a new star. Today we know that a supernova is the death of an old star. The gas of the star is pushed away by the explosion. It forms a giant cloud of gas called a planetary nebula. The crab nebula is a good example. All that remains is a neutron star. If the star was very heavy, the star will make a black hole. Gravity in a black hole is extremely strong. It is so strong that even light cannot escape from a black hole.
+
+The heaviest elements are made in the explosion of a supernova. After billions of years of floating in space, the gas and dust come together to make new stars and new planets.  Much of the gas and dust in space comes from supernovae. Our Sun, the Earth, and all living things are made from star dust.
+
+Astronomers have known for centuries that stars have different colors. When looking at an electromagnetic spectrum, ultraviolet waves are the shortest, and infrared are the longest.[9]   The visible spectrum has wavelengths between these two extremes.
+
+Modern instruments can measure very precisely the color of a star.  This allows astronomers to determine that star's temperature, because a hotter star's black-body radiation has shorter wavelengths.  The hottest stars are blue and violet, then white, then yellow, and the coolest are red.[10] Knowing the color and absolute magnitude, astronomers can place the star on the Hertzsprung-Russell diagram, and estimate its habitable zone and other facts about it.
+
+For example, our Sun is white, and the Earth is the perfect distance away for life. If our Sun was a hotter, blue star, however, Earth would have to be much farther away or else it would be too hot to have water and sustain life.

ensimple/2484.html.txt

@@ -0,0 +1,25 @@
+An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system.
+
+In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by the Kepler space observatory.[3] Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]
+
+Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition.[8]
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One".[9]
+
+The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 1992.
+
+In 1992, radio astronomers announced the discovery of planets around another pulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
+
+On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[12] This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.
+
+In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]
+
+Extrasolar planets can have many different forms.
+
+
+
+The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.[14]  The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.[15][16]
+
+Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.

ensimple/2485.html.txt

@@ -0,0 +1,55 @@
+A star is a very large ball of bright glowing hot matter in space. That matter is called plasma. Stars are held together by gravity. They give out heat and light because they are very hot.
+
+Stars are hot because nuclear reactions happen inside them. Those reactions are called nuclear fusion.  Nuclear fusion makes light and heat and makes bigger and bigger chemical elements. Stars have a lot of hydrogen. Nuclear fusion changes hydrogen into helium. When a star gets old, it starts to change the helium into other bigger chemical elements, like carbon and oxygen. Fusion makes a lot of energy. The energy makes the star very hot. The energy produced by stars moves (radiates) away from them. Much of the energy leaves as light. The rest leaves as other kinds of electromagnetic radiation.
+
+The star nearest to Earth is the Sun. The energy from the Sun supports almost all life on Earth by providing light for plants. Plants turn the light into energy in a process called photosynthesis.[1] The energy from the Sun also causes weather and humidity on Earth.
+
+We can see other stars in the night sky when the Sun goes down. Like the Sun, they are made mostly of hydrogen and a little bit of helium plus other elements. Astronomers often compare those other stars to the Sun. For example, their mass is given in solar masses. A small star may be 0.2 solar masses, a big one 4.0 solar masses.
+
+The Earth and other planets move around (orbit) the Sun. The Sun and all things that orbit the Sun are called the Solar System. Many other stars have planets orbiting them: those planets are called exoplanets. If you were on an exoplanet, our Sun would look like a star in the sky, but you could not see the Earth because it would be too far away.
+
+Proxima Centauri is the star that is closest to our Sun. It is 39.9 trillion kilometres away. This is 4.2 light years away. This means that light from Proxima Centauri takes 4.2 years to reach Earth.
+
+Astronomers think there is a very large number of stars in the Universe. The observable Universe contains more than 2 trillion (1012) galaxies[2] and, overall, as many as an estimated 1×1024 stars[3][4] (more stars than all the grains of sand on planet Earth).[5] That is, 1,000,000,000,000,000,000,000,000 stars, which is many times more than the few hundred billion stars in the Milky Way (our galaxy).
+
+Most stars are very old. They are usually thought to be between 1 billion and 10 billion years old. The oldest stars are 13.7 billion years old. That is as old as the Universe. Some young stars are only a few million years old. Young stars are mostly brighter than old ones.
+
+Stars are different sizes. The smallest stars are neutron stars, which are actually dead stars. They are no bigger than a city. A neutron star has a large amount of mass in a very small space.
+
+Hypergiant stars are the largest stars in the Universe. They have a diameter over 1,500 times bigger than the Sun. If the Sun was a hypergiant star, it would reach out to as far as Jupiter.
+
+The star Betelgeuse is a red supergiant star. Although these stars are very large, they also have low density.
+
+Some stars look brighter than other stars. This difference is measured in terms of apparent magnitude. There are two reasons why stars have different apparent magnitude. If a star is very close to us it will appear much brighter. This is just like a candle. A candle that is close to us appears brighter. The other reason a star can appear brighter is that it is hotter than another cooler star.
+
+Stars give off light but also give off a solar wind and neutrinos. These are very small particles of matter.
+
+Stars are made of mass and mass makes gravity. Gravity makes planets orbit stars. This is why the Earth orbits the Sun. The gravity of two stars can make them go around each other. Stars that orbit each other are called binary stars. Scientists think there are many binary stars. There are even groups of three or more stars that orbit each other. Proxima Centauri is a small star that orbits other stars.
+
+Stars are not spread evenly across all of space. They are grouped into galaxies. A galaxy contains hundreds of billions of stars.
+
+Stars have been important to people all over the world for all of history. Stars have been part of religious practices. Long ago, people believed that stars could never die.
+
+Astronomers organized stars into groups called constellations. They used the constellations to help them see the motion of the planets and to guess the position of the Sun.[6] The motion of the Sun and the stars was used to make calendars. The calendars were used by farmers to decide when to plant crops and when to harvest them.[8]
+
+Stars are made in nebulae. These are areas that have more gas than normal space. The gas in a nebula is pulled together by gravity. The Orion nebula is an example of a place where gas is coming together to form stars.
+
+Stars spend most of their lives combining (fusing) hydrogen with hydrogen to make energy. When hydrogen is fused it makes helium and it makes a lot of energy. To fuse hydrogen into helium it must be very hot and the pressure must be very high. Fusion happens at the center of stars, called "the core".
+
+The smallest stars (red dwarfs) fuse their hydrogen slowly and live for 100 billion years. Red dwarfs live longer than any other type of star. At the end of their lives, they become dimmer and dimmer. Red dwarfs do not explode.
+
+When very heavy stars die, they explode. This explosion is called a supernova. When a supernova happens in a nebula, the explosion pushes the gas in the nebula together. This makes the gas in the nebula very thick (dense). Gravity and exploding stars both help to bring the gas together to make new stars in nebulas.
+
+Most stars use up the hydrogen at their core. When they do, their core becomes smaller and becomes hotter. It becomes so hot it pushes away the outer part of the star. The outer part expands and it makes a red giant star. Astro-physicists think that in about 5 billion years, the Sun will be a red giant. Our Sun will be so large it will eat the Earth. After our Sun stops using hydrogen to make energy, it will use helium in its very hot core. It will be hotter than when it was fusing hydrogen. Heavy stars will also make elements heavier than helium. As a star makes heavier and heavier elements, it makes less and less energy. Iron is a heavy element made in heavy stars.
+
+Our star is an average star. Average stars will push away their outer gases. The gas it pushes away makes a cloud called a planetary nebula. The core part of the star will remain. It will be a ball as big as the Earth and called a white dwarf. It will fade into a black dwarf over a very long time.
+
+Later in large stars,  heavier elements are made by fusion. Finally the star makes a supernova explosion. Most things happen in the universe so slowly we do not notice. But supernova explosions happen in only 100 seconds. When a supernova explodes its flash is as bright as a 100 billion stars. The dying star is so bright it can be seen during the day. Supernova means "new star" because people used to think it was the beginning of a new star. Today we know that a supernova is the death of an old star. The gas of the star is pushed away by the explosion. It forms a giant cloud of gas called a planetary nebula. The crab nebula is a good example. All that remains is a neutron star. If the star was very heavy, the star will make a black hole. Gravity in a black hole is extremely strong. It is so strong that even light cannot escape from a black hole.
+
+The heaviest elements are made in the explosion of a supernova. After billions of years of floating in space, the gas and dust come together to make new stars and new planets.  Much of the gas and dust in space comes from supernovae. Our Sun, the Earth, and all living things are made from star dust.
+
+Astronomers have known for centuries that stars have different colors. When looking at an electromagnetic spectrum, ultraviolet waves are the shortest, and infrared are the longest.[9]   The visible spectrum has wavelengths between these two extremes.
+
+Modern instruments can measure very precisely the color of a star.  This allows astronomers to determine that star's temperature, because a hotter star's black-body radiation has shorter wavelengths.  The hottest stars are blue and violet, then white, then yellow, and the coolest are red.[10] Knowing the color and absolute magnitude, astronomers can place the star on the Hertzsprung-Russell diagram, and estimate its habitable zone and other facts about it.
+
+For example, our Sun is white, and the Earth is the perfect distance away for life. If our Sun was a hotter, blue star, however, Earth would have to be much farther away or else it would be too hot to have water and sustain life.

ensimple/2486.html.txt

@@ -0,0 +1,25 @@
+An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system.
+
+In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by the Kepler space observatory.[3] Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]
+
+Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition.[8]
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One".[9]
+
+The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 1992.
+
+In 1992, radio astronomers announced the discovery of planets around another pulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
+
+On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[12] This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.
+
+In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]
+
+Extrasolar planets can have many different forms.
+
+
+
+The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.[14]  The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.[15][16]
+
+Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.

ensimple/2487.html.txt

@@ -0,0 +1,55 @@
+A star is a very large ball of bright glowing hot matter in space. That matter is called plasma. Stars are held together by gravity. They give out heat and light because they are very hot.
+
+Stars are hot because nuclear reactions happen inside them. Those reactions are called nuclear fusion.  Nuclear fusion makes light and heat and makes bigger and bigger chemical elements. Stars have a lot of hydrogen. Nuclear fusion changes hydrogen into helium. When a star gets old, it starts to change the helium into other bigger chemical elements, like carbon and oxygen. Fusion makes a lot of energy. The energy makes the star very hot. The energy produced by stars moves (radiates) away from them. Much of the energy leaves as light. The rest leaves as other kinds of electromagnetic radiation.
+
+The star nearest to Earth is the Sun. The energy from the Sun supports almost all life on Earth by providing light for plants. Plants turn the light into energy in a process called photosynthesis.[1] The energy from the Sun also causes weather and humidity on Earth.
+
+We can see other stars in the night sky when the Sun goes down. Like the Sun, they are made mostly of hydrogen and a little bit of helium plus other elements. Astronomers often compare those other stars to the Sun. For example, their mass is given in solar masses. A small star may be 0.2 solar masses, a big one 4.0 solar masses.
+
+The Earth and other planets move around (orbit) the Sun. The Sun and all things that orbit the Sun are called the Solar System. Many other stars have planets orbiting them: those planets are called exoplanets. If you were on an exoplanet, our Sun would look like a star in the sky, but you could not see the Earth because it would be too far away.
+
+Proxima Centauri is the star that is closest to our Sun. It is 39.9 trillion kilometres away. This is 4.2 light years away. This means that light from Proxima Centauri takes 4.2 years to reach Earth.
+
+Astronomers think there is a very large number of stars in the Universe. The observable Universe contains more than 2 trillion (1012) galaxies[2] and, overall, as many as an estimated 1×1024 stars[3][4] (more stars than all the grains of sand on planet Earth).[5] That is, 1,000,000,000,000,000,000,000,000 stars, which is many times more than the few hundred billion stars in the Milky Way (our galaxy).
+
+Most stars are very old. They are usually thought to be between 1 billion and 10 billion years old. The oldest stars are 13.7 billion years old. That is as old as the Universe. Some young stars are only a few million years old. Young stars are mostly brighter than old ones.
+
+Stars are different sizes. The smallest stars are neutron stars, which are actually dead stars. They are no bigger than a city. A neutron star has a large amount of mass in a very small space.
+
+Hypergiant stars are the largest stars in the Universe. They have a diameter over 1,500 times bigger than the Sun. If the Sun was a hypergiant star, it would reach out to as far as Jupiter.
+
+The star Betelgeuse is a red supergiant star. Although these stars are very large, they also have low density.
+
+Some stars look brighter than other stars. This difference is measured in terms of apparent magnitude. There are two reasons why stars have different apparent magnitude. If a star is very close to us it will appear much brighter. This is just like a candle. A candle that is close to us appears brighter. The other reason a star can appear brighter is that it is hotter than another cooler star.
+
+Stars give off light but also give off a solar wind and neutrinos. These are very small particles of matter.
+
+Stars are made of mass and mass makes gravity. Gravity makes planets orbit stars. This is why the Earth orbits the Sun. The gravity of two stars can make them go around each other. Stars that orbit each other are called binary stars. Scientists think there are many binary stars. There are even groups of three or more stars that orbit each other. Proxima Centauri is a small star that orbits other stars.
+
+Stars are not spread evenly across all of space. They are grouped into galaxies. A galaxy contains hundreds of billions of stars.
+
+Stars have been important to people all over the world for all of history. Stars have been part of religious practices. Long ago, people believed that stars could never die.
+
+Astronomers organized stars into groups called constellations. They used the constellations to help them see the motion of the planets and to guess the position of the Sun.[6] The motion of the Sun and the stars was used to make calendars. The calendars were used by farmers to decide when to plant crops and when to harvest them.[8]
+
+Stars are made in nebulae. These are areas that have more gas than normal space. The gas in a nebula is pulled together by gravity. The Orion nebula is an example of a place where gas is coming together to form stars.
+
+Stars spend most of their lives combining (fusing) hydrogen with hydrogen to make energy. When hydrogen is fused it makes helium and it makes a lot of energy. To fuse hydrogen into helium it must be very hot and the pressure must be very high. Fusion happens at the center of stars, called "the core".
+
+The smallest stars (red dwarfs) fuse their hydrogen slowly and live for 100 billion years. Red dwarfs live longer than any other type of star. At the end of their lives, they become dimmer and dimmer. Red dwarfs do not explode.
+
+When very heavy stars die, they explode. This explosion is called a supernova. When a supernova happens in a nebula, the explosion pushes the gas in the nebula together. This makes the gas in the nebula very thick (dense). Gravity and exploding stars both help to bring the gas together to make new stars in nebulas.
+
+Most stars use up the hydrogen at their core. When they do, their core becomes smaller and becomes hotter. It becomes so hot it pushes away the outer part of the star. The outer part expands and it makes a red giant star. Astro-physicists think that in about 5 billion years, the Sun will be a red giant. Our Sun will be so large it will eat the Earth. After our Sun stops using hydrogen to make energy, it will use helium in its very hot core. It will be hotter than when it was fusing hydrogen. Heavy stars will also make elements heavier than helium. As a star makes heavier and heavier elements, it makes less and less energy. Iron is a heavy element made in heavy stars.
+
+Our star is an average star. Average stars will push away their outer gases. The gas it pushes away makes a cloud called a planetary nebula. The core part of the star will remain. It will be a ball as big as the Earth and called a white dwarf. It will fade into a black dwarf over a very long time.
+
+Later in large stars,  heavier elements are made by fusion. Finally the star makes a supernova explosion. Most things happen in the universe so slowly we do not notice. But supernova explosions happen in only 100 seconds. When a supernova explodes its flash is as bright as a 100 billion stars. The dying star is so bright it can be seen during the day. Supernova means "new star" because people used to think it was the beginning of a new star. Today we know that a supernova is the death of an old star. The gas of the star is pushed away by the explosion. It forms a giant cloud of gas called a planetary nebula. The crab nebula is a good example. All that remains is a neutron star. If the star was very heavy, the star will make a black hole. Gravity in a black hole is extremely strong. It is so strong that even light cannot escape from a black hole.
+
+The heaviest elements are made in the explosion of a supernova. After billions of years of floating in space, the gas and dust come together to make new stars and new planets.  Much of the gas and dust in space comes from supernovae. Our Sun, the Earth, and all living things are made from star dust.
+
+Astronomers have known for centuries that stars have different colors. When looking at an electromagnetic spectrum, ultraviolet waves are the shortest, and infrared are the longest.[9]   The visible spectrum has wavelengths between these two extremes.
+
+Modern instruments can measure very precisely the color of a star.  This allows astronomers to determine that star's temperature, because a hotter star's black-body radiation has shorter wavelengths.  The hottest stars are blue and violet, then white, then yellow, and the coolest are red.[10] Knowing the color and absolute magnitude, astronomers can place the star on the Hertzsprung-Russell diagram, and estimate its habitable zone and other facts about it.
+
+For example, our Sun is white, and the Earth is the perfect distance away for life. If our Sun was a hotter, blue star, however, Earth would have to be much farther away or else it would be too hot to have water and sustain life.

ensimple/2488.html.txt

@@ -0,0 +1,25 @@
+An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system.
+
+In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by the Kepler space observatory.[3] Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]
+
+Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition.[8]
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One".[9]
+
+The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 1992.
+
+In 1992, radio astronomers announced the discovery of planets around another pulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
+
+On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[12] This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.
+
+In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]
+
+Extrasolar planets can have many different forms.
+
+
+
+The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.[14]  The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.[15][16]
+
+Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.

ensimple/2489.html.txt

@@ -0,0 +1,25 @@
+An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system.
+
+In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by the Kepler space observatory.[3] Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]
+
+Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition.[8]
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One".[9]
+
+The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 1992.
+
+In 1992, radio astronomers announced the discovery of planets around another pulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
+
+On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[12] This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.
+
+In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]
+
+Extrasolar planets can have many different forms.
+
+
+
+The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.[14]  The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.[15][16]
+
+Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.

ensimple/249.html.txt

@@ -0,0 +1,20 @@
+One year is about 365 days long (except in a leap year). It is the time it takes the Earth to go completely around (orbit) the sun once. A year is actually 365.2422 days long, but a calendar has 365 days, except in a leap year.
+
+The year starts on January 1 and ends on December 31 in the Gregorian calendar, but a fiscal year or a school year can start on a different day of the year.
+
+There are several ways used to measure the length of a year.
+
+Solar and lunar years are used by different calendars for daily life.  The other measurements are used by astronomers.
+
+January | 
+February | 
+March | 
+April | 
+May | 
+June | 
+July | 
+August | 
+September | 
+October |
+November | 
+December

ensimple/2490.html.txt

@@ -0,0 +1,25 @@
+An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system.
+
+In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by the Kepler space observatory.[3] Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]
+
+Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition.[8]
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One".[9]
+
+The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 1992.
+
+In 1992, radio astronomers announced the discovery of planets around another pulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
+
+On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[12] This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.
+
+In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]
+
+Extrasolar planets can have many different forms.
+
+
+
+The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.[14]  The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.[15][16]
+
+Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.

ensimple/2491.html.txt

@@ -0,0 +1,25 @@
+An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system.
+
+In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by the Kepler space observatory.[3] Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]
+
+Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition.[8]
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One".[9]
+
+The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 1992.
+
+In 1992, radio astronomers announced the discovery of planets around another pulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
+
+On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[12] This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.
+
+In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]
+
+Extrasolar planets can have many different forms.
+
+
+
+The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.[14]  The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.[15][16]
+
+Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.

ensimple/2492.html.txt

@@ -0,0 +1,25 @@
+An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system.
+
+In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by the Kepler space observatory.[3] Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]
+
+Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition.[8]
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One".[9]
+
+The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 1992.
+
+In 1992, radio astronomers announced the discovery of planets around another pulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
+
+On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[12] This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.
+
+In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]
+
+Extrasolar planets can have many different forms.
+
+
+
+The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.[14]  The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.[15][16]
+
+Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.

ensimple/2493.html.txt

@@ -0,0 +1,25 @@
+An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system.
+
+In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by the Kepler space observatory.[3] Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]
+
+Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition.[8]
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One".[9]
+
+The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 1992.
+
+In 1992, radio astronomers announced the discovery of planets around another pulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
+
+On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[12] This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.
+
+In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]
+
+Extrasolar planets can have many different forms.
+
+
+
+The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.[14]  The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.[15][16]
+
+Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.

ensimple/2494.html.txt

@@ -0,0 +1,55 @@
+A star is a very large ball of bright glowing hot matter in space. That matter is called plasma. Stars are held together by gravity. They give out heat and light because they are very hot.
+
+Stars are hot because nuclear reactions happen inside them. Those reactions are called nuclear fusion.  Nuclear fusion makes light and heat and makes bigger and bigger chemical elements. Stars have a lot of hydrogen. Nuclear fusion changes hydrogen into helium. When a star gets old, it starts to change the helium into other bigger chemical elements, like carbon and oxygen. Fusion makes a lot of energy. The energy makes the star very hot. The energy produced by stars moves (radiates) away from them. Much of the energy leaves as light. The rest leaves as other kinds of electromagnetic radiation.
+
+The star nearest to Earth is the Sun. The energy from the Sun supports almost all life on Earth by providing light for plants. Plants turn the light into energy in a process called photosynthesis.[1] The energy from the Sun also causes weather and humidity on Earth.
+
+We can see other stars in the night sky when the Sun goes down. Like the Sun, they are made mostly of hydrogen and a little bit of helium plus other elements. Astronomers often compare those other stars to the Sun. For example, their mass is given in solar masses. A small star may be 0.2 solar masses, a big one 4.0 solar masses.
+
+The Earth and other planets move around (orbit) the Sun. The Sun and all things that orbit the Sun are called the Solar System. Many other stars have planets orbiting them: those planets are called exoplanets. If you were on an exoplanet, our Sun would look like a star in the sky, but you could not see the Earth because it would be too far away.
+
+Proxima Centauri is the star that is closest to our Sun. It is 39.9 trillion kilometres away. This is 4.2 light years away. This means that light from Proxima Centauri takes 4.2 years to reach Earth.
+
+Astronomers think there is a very large number of stars in the Universe. The observable Universe contains more than 2 trillion (1012) galaxies[2] and, overall, as many as an estimated 1×1024 stars[3][4] (more stars than all the grains of sand on planet Earth).[5] That is, 1,000,000,000,000,000,000,000,000 stars, which is many times more than the few hundred billion stars in the Milky Way (our galaxy).
+
+Most stars are very old. They are usually thought to be between 1 billion and 10 billion years old. The oldest stars are 13.7 billion years old. That is as old as the Universe. Some young stars are only a few million years old. Young stars are mostly brighter than old ones.
+
+Stars are different sizes. The smallest stars are neutron stars, which are actually dead stars. They are no bigger than a city. A neutron star has a large amount of mass in a very small space.
+
+Hypergiant stars are the largest stars in the Universe. They have a diameter over 1,500 times bigger than the Sun. If the Sun was a hypergiant star, it would reach out to as far as Jupiter.
+
+The star Betelgeuse is a red supergiant star. Although these stars are very large, they also have low density.
+
+Some stars look brighter than other stars. This difference is measured in terms of apparent magnitude. There are two reasons why stars have different apparent magnitude. If a star is very close to us it will appear much brighter. This is just like a candle. A candle that is close to us appears brighter. The other reason a star can appear brighter is that it is hotter than another cooler star.
+
+Stars give off light but also give off a solar wind and neutrinos. These are very small particles of matter.
+
+Stars are made of mass and mass makes gravity. Gravity makes planets orbit stars. This is why the Earth orbits the Sun. The gravity of two stars can make them go around each other. Stars that orbit each other are called binary stars. Scientists think there are many binary stars. There are even groups of three or more stars that orbit each other. Proxima Centauri is a small star that orbits other stars.
+
+Stars are not spread evenly across all of space. They are grouped into galaxies. A galaxy contains hundreds of billions of stars.
+
+Stars have been important to people all over the world for all of history. Stars have been part of religious practices. Long ago, people believed that stars could never die.
+
+Astronomers organized stars into groups called constellations. They used the constellations to help them see the motion of the planets and to guess the position of the Sun.[6] The motion of the Sun and the stars was used to make calendars. The calendars were used by farmers to decide when to plant crops and when to harvest them.[8]
+
+Stars are made in nebulae. These are areas that have more gas than normal space. The gas in a nebula is pulled together by gravity. The Orion nebula is an example of a place where gas is coming together to form stars.
+
+Stars spend most of their lives combining (fusing) hydrogen with hydrogen to make energy. When hydrogen is fused it makes helium and it makes a lot of energy. To fuse hydrogen into helium it must be very hot and the pressure must be very high. Fusion happens at the center of stars, called "the core".
+
+The smallest stars (red dwarfs) fuse their hydrogen slowly and live for 100 billion years. Red dwarfs live longer than any other type of star. At the end of their lives, they become dimmer and dimmer. Red dwarfs do not explode.
+
+When very heavy stars die, they explode. This explosion is called a supernova. When a supernova happens in a nebula, the explosion pushes the gas in the nebula together. This makes the gas in the nebula very thick (dense). Gravity and exploding stars both help to bring the gas together to make new stars in nebulas.
+
+Most stars use up the hydrogen at their core. When they do, their core becomes smaller and becomes hotter. It becomes so hot it pushes away the outer part of the star. The outer part expands and it makes a red giant star. Astro-physicists think that in about 5 billion years, the Sun will be a red giant. Our Sun will be so large it will eat the Earth. After our Sun stops using hydrogen to make energy, it will use helium in its very hot core. It will be hotter than when it was fusing hydrogen. Heavy stars will also make elements heavier than helium. As a star makes heavier and heavier elements, it makes less and less energy. Iron is a heavy element made in heavy stars.
+
+Our star is an average star. Average stars will push away their outer gases. The gas it pushes away makes a cloud called a planetary nebula. The core part of the star will remain. It will be a ball as big as the Earth and called a white dwarf. It will fade into a black dwarf over a very long time.
+
+Later in large stars,  heavier elements are made by fusion. Finally the star makes a supernova explosion. Most things happen in the universe so slowly we do not notice. But supernova explosions happen in only 100 seconds. When a supernova explodes its flash is as bright as a 100 billion stars. The dying star is so bright it can be seen during the day. Supernova means "new star" because people used to think it was the beginning of a new star. Today we know that a supernova is the death of an old star. The gas of the star is pushed away by the explosion. It forms a giant cloud of gas called a planetary nebula. The crab nebula is a good example. All that remains is a neutron star. If the star was very heavy, the star will make a black hole. Gravity in a black hole is extremely strong. It is so strong that even light cannot escape from a black hole.
+
+The heaviest elements are made in the explosion of a supernova. After billions of years of floating in space, the gas and dust come together to make new stars and new planets.  Much of the gas and dust in space comes from supernovae. Our Sun, the Earth, and all living things are made from star dust.
+
+Astronomers have known for centuries that stars have different colors. When looking at an electromagnetic spectrum, ultraviolet waves are the shortest, and infrared are the longest.[9]   The visible spectrum has wavelengths between these two extremes.
+
+Modern instruments can measure very precisely the color of a star.  This allows astronomers to determine that star's temperature, because a hotter star's black-body radiation has shorter wavelengths.  The hottest stars are blue and violet, then white, then yellow, and the coolest are red.[10] Knowing the color and absolute magnitude, astronomers can place the star on the Hertzsprung-Russell diagram, and estimate its habitable zone and other facts about it.
+
+For example, our Sun is white, and the Earth is the perfect distance away for life. If our Sun was a hotter, blue star, however, Earth would have to be much farther away or else it would be too hot to have water and sustain life.

ensimple/2495.html.txt

@@ -0,0 +1,25 @@
+An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system.
+
+In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by the Kepler space observatory.[3] Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]
+
+Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition.[8]
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One".[9]
+
+The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 1992.
+
+In 1992, radio astronomers announced the discovery of planets around another pulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
+
+On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[12] This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.
+
+In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]
+
+Extrasolar planets can have many different forms.
+
+
+
+The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.[14]  The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.[15][16]
+
+Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.

ensimple/2496.html.txt

@@ -0,0 +1,55 @@
+A star is a very large ball of bright glowing hot matter in space. That matter is called plasma. Stars are held together by gravity. They give out heat and light because they are very hot.
+
+Stars are hot because nuclear reactions happen inside them. Those reactions are called nuclear fusion.  Nuclear fusion makes light and heat and makes bigger and bigger chemical elements. Stars have a lot of hydrogen. Nuclear fusion changes hydrogen into helium. When a star gets old, it starts to change the helium into other bigger chemical elements, like carbon and oxygen. Fusion makes a lot of energy. The energy makes the star very hot. The energy produced by stars moves (radiates) away from them. Much of the energy leaves as light. The rest leaves as other kinds of electromagnetic radiation.
+
+The star nearest to Earth is the Sun. The energy from the Sun supports almost all life on Earth by providing light for plants. Plants turn the light into energy in a process called photosynthesis.[1] The energy from the Sun also causes weather and humidity on Earth.
+
+We can see other stars in the night sky when the Sun goes down. Like the Sun, they are made mostly of hydrogen and a little bit of helium plus other elements. Astronomers often compare those other stars to the Sun. For example, their mass is given in solar masses. A small star may be 0.2 solar masses, a big one 4.0 solar masses.
+
+The Earth and other planets move around (orbit) the Sun. The Sun and all things that orbit the Sun are called the Solar System. Many other stars have planets orbiting them: those planets are called exoplanets. If you were on an exoplanet, our Sun would look like a star in the sky, but you could not see the Earth because it would be too far away.
+
+Proxima Centauri is the star that is closest to our Sun. It is 39.9 trillion kilometres away. This is 4.2 light years away. This means that light from Proxima Centauri takes 4.2 years to reach Earth.
+
+Astronomers think there is a very large number of stars in the Universe. The observable Universe contains more than 2 trillion (1012) galaxies[2] and, overall, as many as an estimated 1×1024 stars[3][4] (more stars than all the grains of sand on planet Earth).[5] That is, 1,000,000,000,000,000,000,000,000 stars, which is many times more than the few hundred billion stars in the Milky Way (our galaxy).
+
+Most stars are very old. They are usually thought to be between 1 billion and 10 billion years old. The oldest stars are 13.7 billion years old. That is as old as the Universe. Some young stars are only a few million years old. Young stars are mostly brighter than old ones.
+
+Stars are different sizes. The smallest stars are neutron stars, which are actually dead stars. They are no bigger than a city. A neutron star has a large amount of mass in a very small space.
+
+Hypergiant stars are the largest stars in the Universe. They have a diameter over 1,500 times bigger than the Sun. If the Sun was a hypergiant star, it would reach out to as far as Jupiter.
+
+The star Betelgeuse is a red supergiant star. Although these stars are very large, they also have low density.
+
+Some stars look brighter than other stars. This difference is measured in terms of apparent magnitude. There are two reasons why stars have different apparent magnitude. If a star is very close to us it will appear much brighter. This is just like a candle. A candle that is close to us appears brighter. The other reason a star can appear brighter is that it is hotter than another cooler star.
+
+Stars give off light but also give off a solar wind and neutrinos. These are very small particles of matter.
+
+Stars are made of mass and mass makes gravity. Gravity makes planets orbit stars. This is why the Earth orbits the Sun. The gravity of two stars can make them go around each other. Stars that orbit each other are called binary stars. Scientists think there are many binary stars. There are even groups of three or more stars that orbit each other. Proxima Centauri is a small star that orbits other stars.
+
+Stars are not spread evenly across all of space. They are grouped into galaxies. A galaxy contains hundreds of billions of stars.
+
+Stars have been important to people all over the world for all of history. Stars have been part of religious practices. Long ago, people believed that stars could never die.
+
+Astronomers organized stars into groups called constellations. They used the constellations to help them see the motion of the planets and to guess the position of the Sun.[6] The motion of the Sun and the stars was used to make calendars. The calendars were used by farmers to decide when to plant crops and when to harvest them.[8]
+
+Stars are made in nebulae. These are areas that have more gas than normal space. The gas in a nebula is pulled together by gravity. The Orion nebula is an example of a place where gas is coming together to form stars.
+
+Stars spend most of their lives combining (fusing) hydrogen with hydrogen to make energy. When hydrogen is fused it makes helium and it makes a lot of energy. To fuse hydrogen into helium it must be very hot and the pressure must be very high. Fusion happens at the center of stars, called "the core".
+
+The smallest stars (red dwarfs) fuse their hydrogen slowly and live for 100 billion years. Red dwarfs live longer than any other type of star. At the end of their lives, they become dimmer and dimmer. Red dwarfs do not explode.
+
+When very heavy stars die, they explode. This explosion is called a supernova. When a supernova happens in a nebula, the explosion pushes the gas in the nebula together. This makes the gas in the nebula very thick (dense). Gravity and exploding stars both help to bring the gas together to make new stars in nebulas.
+
+Most stars use up the hydrogen at their core. When they do, their core becomes smaller and becomes hotter. It becomes so hot it pushes away the outer part of the star. The outer part expands and it makes a red giant star. Astro-physicists think that in about 5 billion years, the Sun will be a red giant. Our Sun will be so large it will eat the Earth. After our Sun stops using hydrogen to make energy, it will use helium in its very hot core. It will be hotter than when it was fusing hydrogen. Heavy stars will also make elements heavier than helium. As a star makes heavier and heavier elements, it makes less and less energy. Iron is a heavy element made in heavy stars.
+
+Our star is an average star. Average stars will push away their outer gases. The gas it pushes away makes a cloud called a planetary nebula. The core part of the star will remain. It will be a ball as big as the Earth and called a white dwarf. It will fade into a black dwarf over a very long time.
+
+Later in large stars,  heavier elements are made by fusion. Finally the star makes a supernova explosion. Most things happen in the universe so slowly we do not notice. But supernova explosions happen in only 100 seconds. When a supernova explodes its flash is as bright as a 100 billion stars. The dying star is so bright it can be seen during the day. Supernova means "new star" because people used to think it was the beginning of a new star. Today we know that a supernova is the death of an old star. The gas of the star is pushed away by the explosion. It forms a giant cloud of gas called a planetary nebula. The crab nebula is a good example. All that remains is a neutron star. If the star was very heavy, the star will make a black hole. Gravity in a black hole is extremely strong. It is so strong that even light cannot escape from a black hole.
+
+The heaviest elements are made in the explosion of a supernova. After billions of years of floating in space, the gas and dust come together to make new stars and new planets.  Much of the gas and dust in space comes from supernovae. Our Sun, the Earth, and all living things are made from star dust.
+
+Astronomers have known for centuries that stars have different colors. When looking at an electromagnetic spectrum, ultraviolet waves are the shortest, and infrared are the longest.[9]   The visible spectrum has wavelengths between these two extremes.
+
+Modern instruments can measure very precisely the color of a star.  This allows astronomers to determine that star's temperature, because a hotter star's black-body radiation has shorter wavelengths.  The hottest stars are blue and violet, then white, then yellow, and the coolest are red.[10] Knowing the color and absolute magnitude, astronomers can place the star on the Hertzsprung-Russell diagram, and estimate its habitable zone and other facts about it.
+
+For example, our Sun is white, and the Earth is the perfect distance away for life. If our Sun was a hotter, blue star, however, Earth would have to be much farther away or else it would be too hot to have water and sustain life.

ensimple/2497.html.txt

@@ -0,0 +1,25 @@
+An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system.
+
+In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by the Kepler space observatory.[3] Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]
+
+Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition.[8]
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One".[9]
+
+The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 1992.
+
+In 1992, radio astronomers announced the discovery of planets around another pulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
+
+On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[12] This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.
+
+In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]
+
+Extrasolar planets can have many different forms.
+
+
+
+The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.[14]  The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.[15][16]
+
+Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.

ensimple/2498.html.txt

@@ -0,0 +1,25 @@
+An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system.
+
+In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by the Kepler space observatory.[3] Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]
+
+Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition.[8]
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One".[9]
+
+The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 1992.
+
+In 1992, radio astronomers announced the discovery of planets around another pulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
+
+On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[12] This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.
+
+In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]
+
+Extrasolar planets can have many different forms.
+
+
+
+The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.[14]  The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.[15][16]
+
+Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.

ensimple/2499.html.txt

@@ -0,0 +1,25 @@
+An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system.
+
+In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by the Kepler space observatory.[3] Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]
+
+Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition.[8]
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One".[9]
+
+The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 1992.
+
+In 1992, radio astronomers announced the discovery of planets around another pulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
+
+On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[12] This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.
+
+In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]
+
+Extrasolar planets can have many different forms.
+
+
+
+The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.[14]  The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.[15][16]
+
+Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.

ensimple/25.html.txt

@@ -0,0 +1,86 @@
+When writing articles here:
+
+Evolution is a scientific theory used by biologists. It explains how animals and plants changed over a long time, and how they have come to be the way they are.
+
+Earth is very old, about 4.5 billion years. By studying the layers of rock that make up Earth's crust, scientists can find out about its past. This kind of research is called historical geology.
+
+We know that living things have changed over time, because we can see their remains in the rocks. These remains are called 'fossils'. So we know that the animals and plants of today are different from those of long ago. And the further we go back, the more different the fossils are.
+
+How has this come about? Evolution has taken place. That evolution has taken place is a fact, because it is overwhelmingly supported by many lines of evidence. At the same time, evolutionary questions are still being actively researched by biologists.
+
+The theory of evolution is the basis of modern biology. Nothing in biology makes sense without it.
+
+From a collection of Wikipedia's articles:
+
+See the pages of the Wikimedia Foundation Governance wiki, too.
+
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+
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+
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+
+List of all Wikipedias – 
+Languages working together – 
+Start a Wikipedia for a new language

ensimple/250.html.txt

@@ -0,0 +1,8 @@
+A pet is a domesticated animal that lives with people, but is not forced to work and is not eaten, in most instances. In most cases, a pet is kept to entertain people or for companionship. Some pets such as dogs and cats are placed in an animal shelter if there is no one willing to take care of it. If no one adopts it or the pet is too old/sick, the pet may be euthanized.
+
+Dogs, cats, fish, rodents, lagomorphs, ferrets, birds, certain reptiles and amphibians, and a wide variety of arthropods such as tarantulas and hermit crabs are the most common pets in North America.  Horses, elephants, oxen, and donkeys are usually made to work, so they are not usually called pets.  Some dogs also do work for people, and it was once common for some birds (like falcons and carrier pigeons) to work for humans.
+
+Rodents are also very popular pets. The most common are guinea pigs, rabbits, hamsters (especially Syrian and dwarf hamsters), mice and rats.
+
+
+

ensimple/2500.html.txt

@@ -0,0 +1,55 @@
+A star is a very large ball of bright glowing hot matter in space. That matter is called plasma. Stars are held together by gravity. They give out heat and light because they are very hot.
+
+Stars are hot because nuclear reactions happen inside them. Those reactions are called nuclear fusion.  Nuclear fusion makes light and heat and makes bigger and bigger chemical elements. Stars have a lot of hydrogen. Nuclear fusion changes hydrogen into helium. When a star gets old, it starts to change the helium into other bigger chemical elements, like carbon and oxygen. Fusion makes a lot of energy. The energy makes the star very hot. The energy produced by stars moves (radiates) away from them. Much of the energy leaves as light. The rest leaves as other kinds of electromagnetic radiation.
+
+The star nearest to Earth is the Sun. The energy from the Sun supports almost all life on Earth by providing light for plants. Plants turn the light into energy in a process called photosynthesis.[1] The energy from the Sun also causes weather and humidity on Earth.
+
+We can see other stars in the night sky when the Sun goes down. Like the Sun, they are made mostly of hydrogen and a little bit of helium plus other elements. Astronomers often compare those other stars to the Sun. For example, their mass is given in solar masses. A small star may be 0.2 solar masses, a big one 4.0 solar masses.
+
+The Earth and other planets move around (orbit) the Sun. The Sun and all things that orbit the Sun are called the Solar System. Many other stars have planets orbiting them: those planets are called exoplanets. If you were on an exoplanet, our Sun would look like a star in the sky, but you could not see the Earth because it would be too far away.
+
+Proxima Centauri is the star that is closest to our Sun. It is 39.9 trillion kilometres away. This is 4.2 light years away. This means that light from Proxima Centauri takes 4.2 years to reach Earth.
+
+Astronomers think there is a very large number of stars in the Universe. The observable Universe contains more than 2 trillion (1012) galaxies[2] and, overall, as many as an estimated 1×1024 stars[3][4] (more stars than all the grains of sand on planet Earth).[5] That is, 1,000,000,000,000,000,000,000,000 stars, which is many times more than the few hundred billion stars in the Milky Way (our galaxy).
+
+Most stars are very old. They are usually thought to be between 1 billion and 10 billion years old. The oldest stars are 13.7 billion years old. That is as old as the Universe. Some young stars are only a few million years old. Young stars are mostly brighter than old ones.
+
+Stars are different sizes. The smallest stars are neutron stars, which are actually dead stars. They are no bigger than a city. A neutron star has a large amount of mass in a very small space.
+
+Hypergiant stars are the largest stars in the Universe. They have a diameter over 1,500 times bigger than the Sun. If the Sun was a hypergiant star, it would reach out to as far as Jupiter.
+
+The star Betelgeuse is a red supergiant star. Although these stars are very large, they also have low density.
+
+Some stars look brighter than other stars. This difference is measured in terms of apparent magnitude. There are two reasons why stars have different apparent magnitude. If a star is very close to us it will appear much brighter. This is just like a candle. A candle that is close to us appears brighter. The other reason a star can appear brighter is that it is hotter than another cooler star.
+
+Stars give off light but also give off a solar wind and neutrinos. These are very small particles of matter.
+
+Stars are made of mass and mass makes gravity. Gravity makes planets orbit stars. This is why the Earth orbits the Sun. The gravity of two stars can make them go around each other. Stars that orbit each other are called binary stars. Scientists think there are many binary stars. There are even groups of three or more stars that orbit each other. Proxima Centauri is a small star that orbits other stars.
+
+Stars are not spread evenly across all of space. They are grouped into galaxies. A galaxy contains hundreds of billions of stars.
+
+Stars have been important to people all over the world for all of history. Stars have been part of religious practices. Long ago, people believed that stars could never die.
+
+Astronomers organized stars into groups called constellations. They used the constellations to help them see the motion of the planets and to guess the position of the Sun.[6] The motion of the Sun and the stars was used to make calendars. The calendars were used by farmers to decide when to plant crops and when to harvest them.[8]
+
+Stars are made in nebulae. These are areas that have more gas than normal space. The gas in a nebula is pulled together by gravity. The Orion nebula is an example of a place where gas is coming together to form stars.
+
+Stars spend most of their lives combining (fusing) hydrogen with hydrogen to make energy. When hydrogen is fused it makes helium and it makes a lot of energy. To fuse hydrogen into helium it must be very hot and the pressure must be very high. Fusion happens at the center of stars, called "the core".
+
+The smallest stars (red dwarfs) fuse their hydrogen slowly and live for 100 billion years. Red dwarfs live longer than any other type of star. At the end of their lives, they become dimmer and dimmer. Red dwarfs do not explode.
+
+When very heavy stars die, they explode. This explosion is called a supernova. When a supernova happens in a nebula, the explosion pushes the gas in the nebula together. This makes the gas in the nebula very thick (dense). Gravity and exploding stars both help to bring the gas together to make new stars in nebulas.
+
+Most stars use up the hydrogen at their core. When they do, their core becomes smaller and becomes hotter. It becomes so hot it pushes away the outer part of the star. The outer part expands and it makes a red giant star. Astro-physicists think that in about 5 billion years, the Sun will be a red giant. Our Sun will be so large it will eat the Earth. After our Sun stops using hydrogen to make energy, it will use helium in its very hot core. It will be hotter than when it was fusing hydrogen. Heavy stars will also make elements heavier than helium. As a star makes heavier and heavier elements, it makes less and less energy. Iron is a heavy element made in heavy stars.
+
+Our star is an average star. Average stars will push away their outer gases. The gas it pushes away makes a cloud called a planetary nebula. The core part of the star will remain. It will be a ball as big as the Earth and called a white dwarf. It will fade into a black dwarf over a very long time.
+
+Later in large stars,  heavier elements are made by fusion. Finally the star makes a supernova explosion. Most things happen in the universe so slowly we do not notice. But supernova explosions happen in only 100 seconds. When a supernova explodes its flash is as bright as a 100 billion stars. The dying star is so bright it can be seen during the day. Supernova means "new star" because people used to think it was the beginning of a new star. Today we know that a supernova is the death of an old star. The gas of the star is pushed away by the explosion. It forms a giant cloud of gas called a planetary nebula. The crab nebula is a good example. All that remains is a neutron star. If the star was very heavy, the star will make a black hole. Gravity in a black hole is extremely strong. It is so strong that even light cannot escape from a black hole.
+
+The heaviest elements are made in the explosion of a supernova. After billions of years of floating in space, the gas and dust come together to make new stars and new planets.  Much of the gas and dust in space comes from supernovae. Our Sun, the Earth, and all living things are made from star dust.
+
+Astronomers have known for centuries that stars have different colors. When looking at an electromagnetic spectrum, ultraviolet waves are the shortest, and infrared are the longest.[9]   The visible spectrum has wavelengths between these two extremes.
+
+Modern instruments can measure very precisely the color of a star.  This allows astronomers to determine that star's temperature, because a hotter star's black-body radiation has shorter wavelengths.  The hottest stars are blue and violet, then white, then yellow, and the coolest are red.[10] Knowing the color and absolute magnitude, astronomers can place the star on the Hertzsprung-Russell diagram, and estimate its habitable zone and other facts about it.
+
+For example, our Sun is white, and the Earth is the perfect distance away for life. If our Sun was a hotter, blue star, however, Earth would have to be much farther away or else it would be too hot to have water and sustain life.

ensimple/2501.html.txt

@@ -0,0 +1,25 @@
+An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system.
+
+In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by the Kepler space observatory.[3] Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]
+
+Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition.[8]
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One".[9]
+
+The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 1992.
+
+In 1992, radio astronomers announced the discovery of planets around another pulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
+
+On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[12] This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.
+
+In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]
+
+Extrasolar planets can have many different forms.
+
+
+
+The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.[14]  The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.[15][16]
+
+Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.

ensimple/2502.html.txt

@@ -0,0 +1,55 @@
+A star is a very large ball of bright glowing hot matter in space. That matter is called plasma. Stars are held together by gravity. They give out heat and light because they are very hot.
+
+Stars are hot because nuclear reactions happen inside them. Those reactions are called nuclear fusion.  Nuclear fusion makes light and heat and makes bigger and bigger chemical elements. Stars have a lot of hydrogen. Nuclear fusion changes hydrogen into helium. When a star gets old, it starts to change the helium into other bigger chemical elements, like carbon and oxygen. Fusion makes a lot of energy. The energy makes the star very hot. The energy produced by stars moves (radiates) away from them. Much of the energy leaves as light. The rest leaves as other kinds of electromagnetic radiation.
+
+The star nearest to Earth is the Sun. The energy from the Sun supports almost all life on Earth by providing light for plants. Plants turn the light into energy in a process called photosynthesis.[1] The energy from the Sun also causes weather and humidity on Earth.
+
+We can see other stars in the night sky when the Sun goes down. Like the Sun, they are made mostly of hydrogen and a little bit of helium plus other elements. Astronomers often compare those other stars to the Sun. For example, their mass is given in solar masses. A small star may be 0.2 solar masses, a big one 4.0 solar masses.
+
+The Earth and other planets move around (orbit) the Sun. The Sun and all things that orbit the Sun are called the Solar System. Many other stars have planets orbiting them: those planets are called exoplanets. If you were on an exoplanet, our Sun would look like a star in the sky, but you could not see the Earth because it would be too far away.
+
+Proxima Centauri is the star that is closest to our Sun. It is 39.9 trillion kilometres away. This is 4.2 light years away. This means that light from Proxima Centauri takes 4.2 years to reach Earth.
+
+Astronomers think there is a very large number of stars in the Universe. The observable Universe contains more than 2 trillion (1012) galaxies[2] and, overall, as many as an estimated 1×1024 stars[3][4] (more stars than all the grains of sand on planet Earth).[5] That is, 1,000,000,000,000,000,000,000,000 stars, which is many times more than the few hundred billion stars in the Milky Way (our galaxy).
+
+Most stars are very old. They are usually thought to be between 1 billion and 10 billion years old. The oldest stars are 13.7 billion years old. That is as old as the Universe. Some young stars are only a few million years old. Young stars are mostly brighter than old ones.
+
+Stars are different sizes. The smallest stars are neutron stars, which are actually dead stars. They are no bigger than a city. A neutron star has a large amount of mass in a very small space.
+
+Hypergiant stars are the largest stars in the Universe. They have a diameter over 1,500 times bigger than the Sun. If the Sun was a hypergiant star, it would reach out to as far as Jupiter.
+
+The star Betelgeuse is a red supergiant star. Although these stars are very large, they also have low density.
+
+Some stars look brighter than other stars. This difference is measured in terms of apparent magnitude. There are two reasons why stars have different apparent magnitude. If a star is very close to us it will appear much brighter. This is just like a candle. A candle that is close to us appears brighter. The other reason a star can appear brighter is that it is hotter than another cooler star.
+
+Stars give off light but also give off a solar wind and neutrinos. These are very small particles of matter.
+
+Stars are made of mass and mass makes gravity. Gravity makes planets orbit stars. This is why the Earth orbits the Sun. The gravity of two stars can make them go around each other. Stars that orbit each other are called binary stars. Scientists think there are many binary stars. There are even groups of three or more stars that orbit each other. Proxima Centauri is a small star that orbits other stars.
+
+Stars are not spread evenly across all of space. They are grouped into galaxies. A galaxy contains hundreds of billions of stars.
+
+Stars have been important to people all over the world for all of history. Stars have been part of religious practices. Long ago, people believed that stars could never die.
+
+Astronomers organized stars into groups called constellations. They used the constellations to help them see the motion of the planets and to guess the position of the Sun.[6] The motion of the Sun and the stars was used to make calendars. The calendars were used by farmers to decide when to plant crops and when to harvest them.[8]
+
+Stars are made in nebulae. These are areas that have more gas than normal space. The gas in a nebula is pulled together by gravity. The Orion nebula is an example of a place where gas is coming together to form stars.
+
+Stars spend most of their lives combining (fusing) hydrogen with hydrogen to make energy. When hydrogen is fused it makes helium and it makes a lot of energy. To fuse hydrogen into helium it must be very hot and the pressure must be very high. Fusion happens at the center of stars, called "the core".
+
+The smallest stars (red dwarfs) fuse their hydrogen slowly and live for 100 billion years. Red dwarfs live longer than any other type of star. At the end of their lives, they become dimmer and dimmer. Red dwarfs do not explode.
+
+When very heavy stars die, they explode. This explosion is called a supernova. When a supernova happens in a nebula, the explosion pushes the gas in the nebula together. This makes the gas in the nebula very thick (dense). Gravity and exploding stars both help to bring the gas together to make new stars in nebulas.
+
+Most stars use up the hydrogen at their core. When they do, their core becomes smaller and becomes hotter. It becomes so hot it pushes away the outer part of the star. The outer part expands and it makes a red giant star. Astro-physicists think that in about 5 billion years, the Sun will be a red giant. Our Sun will be so large it will eat the Earth. After our Sun stops using hydrogen to make energy, it will use helium in its very hot core. It will be hotter than when it was fusing hydrogen. Heavy stars will also make elements heavier than helium. As a star makes heavier and heavier elements, it makes less and less energy. Iron is a heavy element made in heavy stars.
+
+Our star is an average star. Average stars will push away their outer gases. The gas it pushes away makes a cloud called a planetary nebula. The core part of the star will remain. It will be a ball as big as the Earth and called a white dwarf. It will fade into a black dwarf over a very long time.
+
+Later in large stars,  heavier elements are made by fusion. Finally the star makes a supernova explosion. Most things happen in the universe so slowly we do not notice. But supernova explosions happen in only 100 seconds. When a supernova explodes its flash is as bright as a 100 billion stars. The dying star is so bright it can be seen during the day. Supernova means "new star" because people used to think it was the beginning of a new star. Today we know that a supernova is the death of an old star. The gas of the star is pushed away by the explosion. It forms a giant cloud of gas called a planetary nebula. The crab nebula is a good example. All that remains is a neutron star. If the star was very heavy, the star will make a black hole. Gravity in a black hole is extremely strong. It is so strong that even light cannot escape from a black hole.
+
+The heaviest elements are made in the explosion of a supernova. After billions of years of floating in space, the gas and dust come together to make new stars and new planets.  Much of the gas and dust in space comes from supernovae. Our Sun, the Earth, and all living things are made from star dust.
+
+Astronomers have known for centuries that stars have different colors. When looking at an electromagnetic spectrum, ultraviolet waves are the shortest, and infrared are the longest.[9]   The visible spectrum has wavelengths between these two extremes.
+
+Modern instruments can measure very precisely the color of a star.  This allows astronomers to determine that star's temperature, because a hotter star's black-body radiation has shorter wavelengths.  The hottest stars are blue and violet, then white, then yellow, and the coolest are red.[10] Knowing the color and absolute magnitude, astronomers can place the star on the Hertzsprung-Russell diagram, and estimate its habitable zone and other facts about it.
+
+For example, our Sun is white, and the Earth is the perfect distance away for life. If our Sun was a hotter, blue star, however, Earth would have to be much farther away or else it would be too hot to have water and sustain life.

ensimple/2503.html.txt

@@ -0,0 +1,25 @@
+An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system.
+
+In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by the Kepler space observatory.[3] Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]
+
+Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition.[8]
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One".[9]
+
+The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 1992.
+
+In 1992, radio astronomers announced the discovery of planets around another pulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
+
+On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[12] This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.
+
+In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]
+
+Extrasolar planets can have many different forms.
+
+
+
+The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.[14]  The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.[15][16]
+
+Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.

ensimple/2504.html.txt

@@ -0,0 +1,25 @@
+An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system.
+
+In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by the Kepler space observatory.[3] Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]
+
+Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition.[8]
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One".[9]
+
+The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 1992.
+
+In 1992, radio astronomers announced the discovery of planets around another pulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
+
+On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[12] This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.
+
+In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]
+
+Extrasolar planets can have many different forms.
+
+
+
+The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.[14]  The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.[15][16]
+
+Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.

ensimple/2505.html.txt

@@ -0,0 +1,55 @@
+A star is a very large ball of bright glowing hot matter in space. That matter is called plasma. Stars are held together by gravity. They give out heat and light because they are very hot.
+
+Stars are hot because nuclear reactions happen inside them. Those reactions are called nuclear fusion.  Nuclear fusion makes light and heat and makes bigger and bigger chemical elements. Stars have a lot of hydrogen. Nuclear fusion changes hydrogen into helium. When a star gets old, it starts to change the helium into other bigger chemical elements, like carbon and oxygen. Fusion makes a lot of energy. The energy makes the star very hot. The energy produced by stars moves (radiates) away from them. Much of the energy leaves as light. The rest leaves as other kinds of electromagnetic radiation.
+
+The star nearest to Earth is the Sun. The energy from the Sun supports almost all life on Earth by providing light for plants. Plants turn the light into energy in a process called photosynthesis.[1] The energy from the Sun also causes weather and humidity on Earth.
+
+We can see other stars in the night sky when the Sun goes down. Like the Sun, they are made mostly of hydrogen and a little bit of helium plus other elements. Astronomers often compare those other stars to the Sun. For example, their mass is given in solar masses. A small star may be 0.2 solar masses, a big one 4.0 solar masses.
+
+The Earth and other planets move around (orbit) the Sun. The Sun and all things that orbit the Sun are called the Solar System. Many other stars have planets orbiting them: those planets are called exoplanets. If you were on an exoplanet, our Sun would look like a star in the sky, but you could not see the Earth because it would be too far away.
+
+Proxima Centauri is the star that is closest to our Sun. It is 39.9 trillion kilometres away. This is 4.2 light years away. This means that light from Proxima Centauri takes 4.2 years to reach Earth.
+
+Astronomers think there is a very large number of stars in the Universe. The observable Universe contains more than 2 trillion (1012) galaxies[2] and, overall, as many as an estimated 1×1024 stars[3][4] (more stars than all the grains of sand on planet Earth).[5] That is, 1,000,000,000,000,000,000,000,000 stars, which is many times more than the few hundred billion stars in the Milky Way (our galaxy).
+
+Most stars are very old. They are usually thought to be between 1 billion and 10 billion years old. The oldest stars are 13.7 billion years old. That is as old as the Universe. Some young stars are only a few million years old. Young stars are mostly brighter than old ones.
+
+Stars are different sizes. The smallest stars are neutron stars, which are actually dead stars. They are no bigger than a city. A neutron star has a large amount of mass in a very small space.
+
+Hypergiant stars are the largest stars in the Universe. They have a diameter over 1,500 times bigger than the Sun. If the Sun was a hypergiant star, it would reach out to as far as Jupiter.
+
+The star Betelgeuse is a red supergiant star. Although these stars are very large, they also have low density.
+
+Some stars look brighter than other stars. This difference is measured in terms of apparent magnitude. There are two reasons why stars have different apparent magnitude. If a star is very close to us it will appear much brighter. This is just like a candle. A candle that is close to us appears brighter. The other reason a star can appear brighter is that it is hotter than another cooler star.
+
+Stars give off light but also give off a solar wind and neutrinos. These are very small particles of matter.
+
+Stars are made of mass and mass makes gravity. Gravity makes planets orbit stars. This is why the Earth orbits the Sun. The gravity of two stars can make them go around each other. Stars that orbit each other are called binary stars. Scientists think there are many binary stars. There are even groups of three or more stars that orbit each other. Proxima Centauri is a small star that orbits other stars.
+
+Stars are not spread evenly across all of space. They are grouped into galaxies. A galaxy contains hundreds of billions of stars.
+
+Stars have been important to people all over the world for all of history. Stars have been part of religious practices. Long ago, people believed that stars could never die.
+
+Astronomers organized stars into groups called constellations. They used the constellations to help them see the motion of the planets and to guess the position of the Sun.[6] The motion of the Sun and the stars was used to make calendars. The calendars were used by farmers to decide when to plant crops and when to harvest them.[8]
+
+Stars are made in nebulae. These are areas that have more gas than normal space. The gas in a nebula is pulled together by gravity. The Orion nebula is an example of a place where gas is coming together to form stars.
+
+Stars spend most of their lives combining (fusing) hydrogen with hydrogen to make energy. When hydrogen is fused it makes helium and it makes a lot of energy. To fuse hydrogen into helium it must be very hot and the pressure must be very high. Fusion happens at the center of stars, called "the core".
+
+The smallest stars (red dwarfs) fuse their hydrogen slowly and live for 100 billion years. Red dwarfs live longer than any other type of star. At the end of their lives, they become dimmer and dimmer. Red dwarfs do not explode.
+
+When very heavy stars die, they explode. This explosion is called a supernova. When a supernova happens in a nebula, the explosion pushes the gas in the nebula together. This makes the gas in the nebula very thick (dense). Gravity and exploding stars both help to bring the gas together to make new stars in nebulas.
+
+Most stars use up the hydrogen at their core. When they do, their core becomes smaller and becomes hotter. It becomes so hot it pushes away the outer part of the star. The outer part expands and it makes a red giant star. Astro-physicists think that in about 5 billion years, the Sun will be a red giant. Our Sun will be so large it will eat the Earth. After our Sun stops using hydrogen to make energy, it will use helium in its very hot core. It will be hotter than when it was fusing hydrogen. Heavy stars will also make elements heavier than helium. As a star makes heavier and heavier elements, it makes less and less energy. Iron is a heavy element made in heavy stars.
+
+Our star is an average star. Average stars will push away their outer gases. The gas it pushes away makes a cloud called a planetary nebula. The core part of the star will remain. It will be a ball as big as the Earth and called a white dwarf. It will fade into a black dwarf over a very long time.
+
+Later in large stars,  heavier elements are made by fusion. Finally the star makes a supernova explosion. Most things happen in the universe so slowly we do not notice. But supernova explosions happen in only 100 seconds. When a supernova explodes its flash is as bright as a 100 billion stars. The dying star is so bright it can be seen during the day. Supernova means "new star" because people used to think it was the beginning of a new star. Today we know that a supernova is the death of an old star. The gas of the star is pushed away by the explosion. It forms a giant cloud of gas called a planetary nebula. The crab nebula is a good example. All that remains is a neutron star. If the star was very heavy, the star will make a black hole. Gravity in a black hole is extremely strong. It is so strong that even light cannot escape from a black hole.
+
+The heaviest elements are made in the explosion of a supernova. After billions of years of floating in space, the gas and dust come together to make new stars and new planets.  Much of the gas and dust in space comes from supernovae. Our Sun, the Earth, and all living things are made from star dust.
+
+Astronomers have known for centuries that stars have different colors. When looking at an electromagnetic spectrum, ultraviolet waves are the shortest, and infrared are the longest.[9]   The visible spectrum has wavelengths between these two extremes.
+
+Modern instruments can measure very precisely the color of a star.  This allows astronomers to determine that star's temperature, because a hotter star's black-body radiation has shorter wavelengths.  The hottest stars are blue and violet, then white, then yellow, and the coolest are red.[10] Knowing the color and absolute magnitude, astronomers can place the star on the Hertzsprung-Russell diagram, and estimate its habitable zone and other facts about it.
+
+For example, our Sun is white, and the Earth is the perfect distance away for life. If our Sun was a hotter, blue star, however, Earth would have to be much farther away or else it would be too hot to have water and sustain life.

ensimple/2506.html.txt

@@ -0,0 +1,25 @@
+An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system.
+
+In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by the Kepler space observatory.[3] Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]
+
+Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]
+
+In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition.[8]
+
+In the eighteenth century, the same possibility was mentioned by Isaac Newton in his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One".[9]
+
+The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 1992.
+
+In 1992, radio astronomers announced the discovery of planets around another pulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
+
+On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[12] This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.
+
+In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]
+
+Extrasolar planets can have many different forms.
+
+
+
+The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.[14]  The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.[15][16]
+
+Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.

ensimple/2507.html.txt

@@ -0,0 +1,55 @@
+A star is a very large ball of bright glowing hot matter in space. That matter is called plasma. Stars are held together by gravity. They give out heat and light because they are very hot.
+
+Stars are hot because nuclear reactions happen inside them. Those reactions are called nuclear fusion.  Nuclear fusion makes light and heat and makes bigger and bigger chemical elements. Stars have a lot of hydrogen. Nuclear fusion changes hydrogen into helium. When a star gets old, it starts to change the helium into other bigger chemical elements, like carbon and oxygen. Fusion makes a lot of energy. The energy makes the star very hot. The energy produced by stars moves (radiates) away from them. Much of the energy leaves as light. The rest leaves as other kinds of electromagnetic radiation.
+
+The star nearest to Earth is the Sun. The energy from the Sun supports almost all life on Earth by providing light for plants. Plants turn the light into energy in a process called photosynthesis.[1] The energy from the Sun also causes weather and humidity on Earth.
+
+We can see other stars in the night sky when the Sun goes down. Like the Sun, they are made mostly of hydrogen and a little bit of helium plus other elements. Astronomers often compare those other stars to the Sun. For example, their mass is given in solar masses. A small star may be 0.2 solar masses, a big one 4.0 solar masses.
+
+The Earth and other planets move around (orbit) the Sun. The Sun and all things that orbit the Sun are called the Solar System. Many other stars have planets orbiting them: those planets are called exoplanets. If you were on an exoplanet, our Sun would look like a star in the sky, but you could not see the Earth because it would be too far away.
+
+Proxima Centauri is the star that is closest to our Sun. It is 39.9 trillion kilometres away. This is 4.2 light years away. This means that light from Proxima Centauri takes 4.2 years to reach Earth.
+
+Astronomers think there is a very large number of stars in the Universe. The observable Universe contains more than 2 trillion (1012) galaxies[2] and, overall, as many as an estimated 1×1024 stars[3][4] (more stars than all the grains of sand on planet Earth).[5] That is, 1,000,000,000,000,000,000,000,000 stars, which is many times more than the few hundred billion stars in the Milky Way (our galaxy).
+
+Most stars are very old. They are usually thought to be between 1 billion and 10 billion years old. The oldest stars are 13.7 billion years old. That is as old as the Universe. Some young stars are only a few million years old. Young stars are mostly brighter than old ones.
+
+Stars are different sizes. The smallest stars are neutron stars, which are actually dead stars. They are no bigger than a city. A neutron star has a large amount of mass in a very small space.
+
+Hypergiant stars are the largest stars in the Universe. They have a diameter over 1,500 times bigger than the Sun. If the Sun was a hypergiant star, it would reach out to as far as Jupiter.
+
+The star Betelgeuse is a red supergiant star. Although these stars are very large, they also have low density.
+
+Some stars look brighter than other stars. This difference is measured in terms of apparent magnitude. There are two reasons why stars have different apparent magnitude. If a star is very close to us it will appear much brighter. This is just like a candle. A candle that is close to us appears brighter. The other reason a star can appear brighter is that it is hotter than another cooler star.
+
+Stars give off light but also give off a solar wind and neutrinos. These are very small particles of matter.
+
+Stars are made of mass and mass makes gravity. Gravity makes planets orbit stars. This is why the Earth orbits the Sun. The gravity of two stars can make them go around each other. Stars that orbit each other are called binary stars. Scientists think there are many binary stars. There are even groups of three or more stars that orbit each other. Proxima Centauri is a small star that orbits other stars.
+
+Stars are not spread evenly across all of space. They are grouped into galaxies. A galaxy contains hundreds of billions of stars.
+
+Stars have been important to people all over the world for all of history. Stars have been part of religious practices. Long ago, people believed that stars could never die.
+
+Astronomers organized stars into groups called constellations. They used the constellations to help them see the motion of the planets and to guess the position of the Sun.[6] The motion of the Sun and the stars was used to make calendars. The calendars were used by farmers to decide when to plant crops and when to harvest them.[8]
+
+Stars are made in nebulae. These are areas that have more gas than normal space. The gas in a nebula is pulled together by gravity. The Orion nebula is an example of a place where gas is coming together to form stars.
+
+Stars spend most of their lives combining (fusing) hydrogen with hydrogen to make energy. When hydrogen is fused it makes helium and it makes a lot of energy. To fuse hydrogen into helium it must be very hot and the pressure must be very high. Fusion happens at the center of stars, called "the core".
+
+The smallest stars (red dwarfs) fuse their hydrogen slowly and live for 100 billion years. Red dwarfs live longer than any other type of star. At the end of their lives, they become dimmer and dimmer. Red dwarfs do not explode.
+
+When very heavy stars die, they explode. This explosion is called a supernova. When a supernova happens in a nebula, the explosion pushes the gas in the nebula together. This makes the gas in the nebula very thick (dense). Gravity and exploding stars both help to bring the gas together to make new stars in nebulas.
+
+Most stars use up the hydrogen at their core. When they do, their core becomes smaller and becomes hotter. It becomes so hot it pushes away the outer part of the star. The outer part expands and it makes a red giant star. Astro-physicists think that in about 5 billion years, the Sun will be a red giant. Our Sun will be so large it will eat the Earth. After our Sun stops using hydrogen to make energy, it will use helium in its very hot core. It will be hotter than when it was fusing hydrogen. Heavy stars will also make elements heavier than helium. As a star makes heavier and heavier elements, it makes less and less energy. Iron is a heavy element made in heavy stars.
+
+Our star is an average star. Average stars will push away their outer gases. The gas it pushes away makes a cloud called a planetary nebula. The core part of the star will remain. It will be a ball as big as the Earth and called a white dwarf. It will fade into a black dwarf over a very long time.
+
+Later in large stars,  heavier elements are made by fusion. Finally the star makes a supernova explosion. Most things happen in the universe so slowly we do not notice. But supernova explosions happen in only 100 seconds. When a supernova explodes its flash is as bright as a 100 billion stars. The dying star is so bright it can be seen during the day. Supernova means "new star" because people used to think it was the beginning of a new star. Today we know that a supernova is the death of an old star. The gas of the star is pushed away by the explosion. It forms a giant cloud of gas called a planetary nebula. The crab nebula is a good example. All that remains is a neutron star. If the star was very heavy, the star will make a black hole. Gravity in a black hole is extremely strong. It is so strong that even light cannot escape from a black hole.
+
+The heaviest elements are made in the explosion of a supernova. After billions of years of floating in space, the gas and dust come together to make new stars and new planets.  Much of the gas and dust in space comes from supernovae. Our Sun, the Earth, and all living things are made from star dust.
+
+Astronomers have known for centuries that stars have different colors. When looking at an electromagnetic spectrum, ultraviolet waves are the shortest, and infrared are the longest.[9]   The visible spectrum has wavelengths between these two extremes.
+
+Modern instruments can measure very precisely the color of a star.  This allows astronomers to determine that star's temperature, because a hotter star's black-body radiation has shorter wavelengths.  The hottest stars are blue and violet, then white, then yellow, and the coolest are red.[10] Knowing the color and absolute magnitude, astronomers can place the star on the Hertzsprung-Russell diagram, and estimate its habitable zone and other facts about it.
+
+For example, our Sun is white, and the Earth is the perfect distance away for life. If our Sun was a hotter, blue star, however, Earth would have to be much farther away or else it would be too hot to have water and sustain life.

ensimple/2508.html.txt

@@ -0,0 +1,5 @@
+A week is 7 days in connecting order. There are usually 52 weeks in a year.
+
+In the English language, the days of the week are named after gods in Norse mythology, except for Saturday, which is named after a Roman god.
+
+Depending on the law of a country, the week either starts on Monday and ends on Sunday, or starts on Sunday and ends on Saturday. In most countries, Saturday and Sunday are the weekend. Friday, Saturday, and Sunday is a religious day for Muslims, Jews, and Christians, respectively.

ensimple/2509.html.txt

@@ -0,0 +1,19 @@
+Hebrew is a Semitic language. It was first spoken in Israel. Many Jewish people also speak Hebrew, as Hebrew is part of Judaism. The Academy of the Hebrew Language is the main institution of Hebrew.
+
+It was spoken by Israelites a long time ago, during the time of the Bible. After Judah was conquered by Babylonia, the Jews were taken captive to Babylon and started speaking Aramaic. Hebrew was no longer used much in daily, but it was still known by Jews who studied religious books.
+
+In the 20th century, many Jews decided to make Hebrew into a spoken language again. It became the language of the new country of Israel in 1948. People in Israel came from many places and decided to learn Hebrew, the language of their common ancestors, so that they could all speak one language. However, Modern Hebrew is quite different from Biblical Hebrew, with a simpler grammar and many loanwords from other languages, especially English.
+
+As of today,[when?] Hebrew has been the only dead language that had been made into a living language again.[3]
+
+The Bible was originally written in Biblical Hebrew, Biblical Aramaic and Koine Greek.
+
+Hebrew is a Semitic language and so that it is a lot like Arabic. Hebrew words are made by combining a root with a pattern. In Israeli Hebrew, some words are translated from European languages like English, French, German, and Russian. Many words from the Old Testament were given new meanings in Israeli Hebrew.[4] People learning Hebrew need to study the grammar first so that they can read correctly without vowels.
+
+In Israeli Hebrew, there is no verb "to be" in the present tense but only in the future and the past tenses. In Biblical Hebrew, there are no tenses but only two aspects: imperfect and perfect. The imperfect is something like the future and the present tenses. The perfect is something like the past tense. Mishnaic Hebrew was spoken as well as Judeo-Aramaic in the time of Jesus and in the time of the Bar-Kokhba revolt (2nd century AD) until the Byzantine Empire of Justinian (6th century AD).
+
+The Hebrew alphabet has been adapted to write Yiddish, another Jewish language. However, Yiddish sounds different from Hebrew since it is a Germanic language.
+
+The Hebrew alphabet has 22 letters. Five of them change when they are at the end of a word. Hebrew is read from right to left.
+
+The Hebrew alphabet is an abjad and so only the consonants are written, and readers must supply the vowels. Since that can be difficult, the vowels can be marked as dots called “nikkud” or “tinuah” (plural ”nikudot” and “tinuot” respectively.) In Modern Hebrew, some letters can denote vowels, which are called matres lectionis (mothers of the reading) since they greatly help reading. Vav (or Waw) can make the 'oo' sound (/u/ in IPA) like in food. Yodh (or Yud) can make the 'ee' sound (/i/ in IPA) like in feed.

ensimple/251.html.txt

@@ -0,0 +1,8 @@
+A pet is a domesticated animal that lives with people, but is not forced to work and is not eaten, in most instances. In most cases, a pet is kept to entertain people or for companionship. Some pets such as dogs and cats are placed in an animal shelter if there is no one willing to take care of it. If no one adopts it or the pet is too old/sick, the pet may be euthanized.
+
+Dogs, cats, fish, rodents, lagomorphs, ferrets, birds, certain reptiles and amphibians, and a wide variety of arthropods such as tarantulas and hermit crabs are the most common pets in North America.  Horses, elephants, oxen, and donkeys are usually made to work, so they are not usually called pets.  Some dogs also do work for people, and it was once common for some birds (like falcons and carrier pigeons) to work for humans.
+
+Rodents are also very popular pets. The most common are guinea pigs, rabbits, hamsters (especially Syrian and dwarf hamsters), mice and rats.
+
+
+

ensimple/2510.html.txt

@@ -0,0 +1,19 @@
+Hebrew is a Semitic language. It was first spoken in Israel. Many Jewish people also speak Hebrew, as Hebrew is part of Judaism. The Academy of the Hebrew Language is the main institution of Hebrew.
+
+It was spoken by Israelites a long time ago, during the time of the Bible. After Judah was conquered by Babylonia, the Jews were taken captive to Babylon and started speaking Aramaic. Hebrew was no longer used much in daily, but it was still known by Jews who studied religious books.
+
+In the 20th century, many Jews decided to make Hebrew into a spoken language again. It became the language of the new country of Israel in 1948. People in Israel came from many places and decided to learn Hebrew, the language of their common ancestors, so that they could all speak one language. However, Modern Hebrew is quite different from Biblical Hebrew, with a simpler grammar and many loanwords from other languages, especially English.
+
+As of today,[when?] Hebrew has been the only dead language that had been made into a living language again.[3]
+
+The Bible was originally written in Biblical Hebrew, Biblical Aramaic and Koine Greek.
+
+Hebrew is a Semitic language and so that it is a lot like Arabic. Hebrew words are made by combining a root with a pattern. In Israeli Hebrew, some words are translated from European languages like English, French, German, and Russian. Many words from the Old Testament were given new meanings in Israeli Hebrew.[4] People learning Hebrew need to study the grammar first so that they can read correctly without vowels.
+
+In Israeli Hebrew, there is no verb "to be" in the present tense but only in the future and the past tenses. In Biblical Hebrew, there are no tenses but only two aspects: imperfect and perfect. The imperfect is something like the future and the present tenses. The perfect is something like the past tense. Mishnaic Hebrew was spoken as well as Judeo-Aramaic in the time of Jesus and in the time of the Bar-Kokhba revolt (2nd century AD) until the Byzantine Empire of Justinian (6th century AD).
+
+The Hebrew alphabet has been adapted to write Yiddish, another Jewish language. However, Yiddish sounds different from Hebrew since it is a Germanic language.
+
+The Hebrew alphabet has 22 letters. Five of them change when they are at the end of a word. Hebrew is read from right to left.
+
+The Hebrew alphabet is an abjad and so only the consonants are written, and readers must supply the vowels. Since that can be difficult, the vowels can be marked as dots called “nikkud” or “tinuah” (plural ”nikudot” and “tinuot” respectively.) In Modern Hebrew, some letters can denote vowels, which are called matres lectionis (mothers of the reading) since they greatly help reading. Vav (or Waw) can make the 'oo' sound (/u/ in IPA) like in food. Yodh (or Yud) can make the 'ee' sound (/i/ in IPA) like in feed.

ensimple/2511.html.txt

@@ -0,0 +1,50 @@
+The mass of an object is a measure of the amount of matter in a body[1]. A mountain has typically more mass than a rock, for instance. Mass should not be confused with the related but quite different concept of weight. A large mass like the Earth will attract a small mass like a human being with enough force to keep the human being from floating away. "Mass attraction" is another word for gravity, a force that exists between all matter.
+
+The unit of mass in the International System of Units is the kilogram, which is represented by the symbol 'kg'. Fractions and multiples of this basic unit include the gram (one thousandth of a kg, symbol 'g') and the tonne (one thousand kg), amongst many others.
+
+In some fields or applications, it is convenient to use different units to simplify the discussions or writings. For instance,
+
+Traditional units are still in encountered in some countries: imperial units such as the ounce or the pound were in widespread use within the British Empire. Some of them are still popular in the United States, which also uses units like the short ton (2,000 pounds, 907 kg) and the long ton (2,240 pounds), not to be confused with the metric tonne (1,000 kg).
+
+Mass is an intrinsic property of the object: it does not depend on its volume, or position in space, for instance. For a long time (at least since the works of Antoine Lavoisier in the second half of the eighteen century), it has been known that the sum of the masses of objects that interact or of the chemicals that react remain conserved throughout these processes. This remains an excellent approximation for everyday life and even most laboratory work.
+
+However, Einstein has shown through his special theory of relativity that the mass m of an object moving at speed v with respect to an observer must be higher than the mass of the same object observed at rest m0 with respect to the observer. The applicable formula is
+
+m
+=
+
+
+
+m
+
+0
+
+
+
+1
+−
+(
+
+v
+
+2
+
+
+
+/
+
+
+c
+
+2
+
+
+)
+
+
+
+
+
+{\displaystyle m={\frac {m_{0}}{\sqrt {1-(v^{2}/c^{2})}}}}
+
+where c stands for the speed of light. This change in mass is only important when the speed of the object with respect to the observer becomes a large fraction of c.

ensimple/2512.html.txt

@@ -0,0 +1,21 @@
+A litre (international spelling) or liter (American spelling) is one of the metric units of volume. It is not an SI unit.
+
+One litre is the volume of 1000 cubic centimetres, that is a cube of 10 × 10 × 10 centimetres (1000 cm3). One litre of water at 4 °C (277 K; 39 °F) has the mass of exactly one kilogram. This results from the definition given in 1795, where the gram was defined as the weight of one cubic centimetre of melting ice.[2]
+
+The symbol for litre is l[1] or ℓ.
+
+For smaller volumes, the decilitre is used: 10 dl = one litre.
+
+For smaller volumes, the centilitre is used: 100 cl = one litre.
+
+For smaller volumes, the millilitre is used: 1000 ml = one litre.
+
+The capital letter "L" is preferred by some people as the small "l" can look like the number one "1".
+
+The metric system was first introduced in France in 1791.[3][4] That system did not have its own unit of capacity or volume because volume can be measured in cubic metres.[4] In 1793 work to make the metric system compulsory in France was started by the Temporary Commission of Republican Weights and Measures. Due to public demand,[4] the commission said that the cubic metre was too big for everyday use. They said that a new unit based on the old cadil should be used instead. One cadil was to be 0.001 cubic metres. This was equivalent to a cube with sides 10 cm.[5] The cadil was also known as the pinte or the litron. The pinte had been an old French unit of measure of capacity.[4] In 1795 the definition was revised. The cadil was given the name litre.[6]
+
+In 1795 the kilogram was defined to be exactly one litre of water at 4 °C. In 1799 the kilogram was redefined. The new definition said that the kilogram was the mass of the kilogram des archives. In 1901 scientists measured the volume of one litre of water at 4 °C very carefully.[Note 1]  They found that it occupied about 1.000028 dm3.[7] The BIPM redefined the litre as being exactly the volume of one kilogram of water at 4 °C.[8]
+
+In 1960 the SI was introduced.  The BIPM changed the definition of the litre back to "one dm3". The litre is not part of SI. The BIPM defined the litre[Note 2] as a "Non-SI unit accepted for use with the SI". This was because it is used in many  countries. The BIPM said that the litre should not be used for very accurate work.[9][10]
+
+According to SI rules, the symbol for the litre should be "l". This is because the litre was not named after somebody whose name was "Litre".[Note 3] However the symbol "l" and the number "1" are easily confused. In 1979 the BIPM made an exception for the symbol for the litre. They said that people could use either "L" or "l" as its symbol.[11]

ensimple/2513.html.txt

@@ -0,0 +1,4 @@
+In Greek mythology, Helen, also known as Helen of Troy (Ancient Greek: Ἑλένη, Helénē), was said to have been the most beautiful woman in the world. The queen of Sparta, Helen was married to Menelaus, but later eloped with Prince Paris of Troy (most often depicted as being kidnapped) and taken to Troy, resulting in the Trojan War as the Achaeans set out to bring her back to Sparta. She was believed to be the daughter of Zeus and Leda, the wife of King Tyndareos of Sparta, and was the sister of twins Castor and Polydeukes, and Clytemnestra.
+
+Media related to Helen at Wikimedia Commons
+

ensimple/2514.html.txt

@@ -0,0 +1,4 @@
+In Greek mythology, Helen, also known as Helen of Troy (Ancient Greek: Ἑλένη, Helénē), was said to have been the most beautiful woman in the world. The queen of Sparta, Helen was married to Menelaus, but later eloped with Prince Paris of Troy (most often depicted as being kidnapped) and taken to Troy, resulting in the Trojan War as the Achaeans set out to bring her back to Sparta. She was believed to be the daughter of Zeus and Leda, the wife of King Tyndareos of Sparta, and was the sister of twins Castor and Polydeukes, and Clytemnestra.
+
+Media related to Helen at Wikimedia Commons
+