# Wikijunior:Solar System/The Sun

## What is the Sun?

{{/Coolfacts\|=
1\|!☉︎.svg "☉︎"){width="80"}\|= 2\|Sun Facts\
WARNING: Never look directly at the Sun; it can hurt and damage your
eyes.\|= 3\|

-   If you looked at the Sun in a telescope, you could risk blindness.
-   The Sun is 150,000,000 km (93 million miles) away from Earth.
-   The Sun\'s light takes 8 minutes to reach us on Earth. That means
    that if the Sun blew up, we wouldn\'t see it blow up until 8 minutes
    later!
-   Every second, the Sun turns over 4 million metric tons of gas into
    energy. That\'s 881,849,000,000 pounds!
-   The Sun is as wide as 109 Earths.

}} The Sun is a star --- the closest star to Earth. It is a large ball
of very hot gas in a (plasma) state. The air we breathe and the helium
in a balloon are both gases. It is over 5,500 °C at the surface, and
much hotter at the center, about 15 million °C. The Sun is made of
mostly hydrogen (70%) and helium (28%). It turns many tons of hydrogen
into helium every second, thus creating heat and light.

The Sun makes light and heat that warms the surface of the Earth and
allows plants to grow. We can get food from plants, and we can burn wood
and other parts of plants to cook, warm our houses, and make cars go.
Without the Sun there could not be any life on Earth.

### How big is the Sun?

The Sun is very big --- much, MUCH bigger than the Earth! Even the
mighty planet Jupiter is small by comparison. It is more than a million
km (109 Earths) across and contains about 99.86% of the Solar System\'s
mass. If you could stand on the surface of the Sun, you would weigh 28
times as much as you do on Earth because the Sun has more mass and
therefore more gravitational pull than earth.

More than a million Earths could fit beneath the surface of the Sun. It
doesn\'t look that big from Earth, though. That\'s because the Sun is so
far away. Compared to other stars, the Sun is about average-sized. There
are much bigger stars, and much smaller stars.

A very thin *solar wind* of charged particles blows from the Sun all the
way to the edge of the Solar System. When it gets there, the gases mix
with those coming from other stars.

### What is the surface like?

The Sun doesn\'t have a crust like the Earth that you can stand on. The
whole Sun is made out of gases, fire, and plasma. The gas becomes
thinner as you go farther from the center of the Sun. The outer-part we
see when we look at the Sun is called the photosphere, which means
\"ball of light\". We call it the surface of the Sun because that\'s
where most of the light we see comes from. There is actually a lot of
material from the Sun above the photosphere, and some of the gas is even
blasted away to great distances.

## How does the sun make light and heat?

The Sun is the main source of energy for the Earth. This energy is made
deep inside the Sun in a process called *nuclear fusion*. Four hydrogen
`<b>`{=html}atoms`</b>`{=html} are fused together to make one helium
atom. Some of the leftover matter turns into energy. This is the same
way energy is released in a hydrogen bomb.\
{{  }}

upright=1.5\|thumb\|What is going inside the Sun. The different colors
show the different regions.

**Core**: The center of the Sun is very dense. It\'s about 12 times as
dense as lead. That means that a gallon of the gas from the core of the
Sun would weigh half a ton. It\'s also very hot --- about 15,000,000 °C.
This region is where most of the nuclear reactions are taking place.

**Radiative zone**: In this zone the light and heat produced in the core
fight their way out towards the surface. The gases that make up the zone
are very dense and keep absorbing and emitting the rays. Have you ever
tried to run through water? That\'s what it\'s like for light waves in
this region of the Sun. Light can\'t go very far at all before it runs
into something. Then it bounces off in a different direction. The light
doesn\'t get very far this way. It can take a single ray of light a
million years to get out of this zone.

**Convection zone**: Have you ever seen the air shimmer above a fire?
Perhaps you\'ve been told it\'s because heat rises? Actually, it is the
hot air that is rising. Hot gases get less dense and rise. Cold gases
get denser and sink. In this zone, the gases are less dense. They behave
like air in a fireplace. Gas at the bottom of the zone gets heated up
from below. It rises to the surface, gives off its heat to space, and
sinks again. The gas in the convection zone forms currents like those in
Earth's oceans and atmosphere. The currents are called *convection
cells*.

## What are sunspots?

!The dark areas are
sunspots Sunspots are dark
spots on the Sun, but they are still brighter than lightning. Sunspots
look darker than the rest of the Sun because they are a little cooler.
Even though sunspots are cooler than the rest of the Sun, they are still
hot --- about 4000 °C (7000 °F). Sunspots are caused by changes in the
Sun's magnetic field. The magnetic field stops convection, which causes
the sunspot areas to cool off and become darker. Sunspots usually form
in groups which are carried around the Sun as it rotates.

The number of sunspots we see goes up and down in a cycle of average
length about 11 years.

## What is the Solar Atmosphere like?

Above the photosphere, the Sun's gases are not very dense at all. There
are two layers that we can see with special telescopes. Above that,
gases stream out as *solar wind* that reaches to the edge of the Solar
System.

left\|thumb\|A closeup view of a sunspot and
prominences

### Prominences and solar flares

If you have a telescope with special filters, you can see bumps around
the edge of the Sun. Each one of these is called a *prominence*. They
look like volcanoes erupting. They are hundreds or thousands of
kilometers long. Some are bigger than the Earth. They often seem to come
from sunspots. Sometimes they get so far away from the Sun that they fly
away from it. Then they are called *solar flares*.

### Chromosphere

*Chromosphere* means \"color ball\". It is just above the photosphere.
It is not as bright as the photosphere, and you can\'t normally see it.
But you can see it just before a solar eclipse (only with special
filters!). It looks like a flash of all the colors of light.
Surprisingly, the Chromosphere is even hotter than the photosphere, at
some parts over 20,000 °C.

### Corona

!The Sun\'s corona during an
eclipse
*Corona* means \"crown.\" That is what pictures of the corona look like.
It is just above the chromosphere. It is hotter than the photosphere,
and it glows. It is made of thin gases, and blows away as solar wind. It
shifts and changes, but it is hard to see, even with special telescopes.

### Solar wind

At the top of the corona, some of the gas blows out as solar wind. It
blows fast --- about 60 km per second (more than 100,000 miles per
hour). But there isn\'t very much of it. The solar wind is strong enough
to push dust and gas away from a comet to make a tail.

The solar wind can even push big things. In 1960, the satellite Echo I
was put into orbit. It was a large balloon. Since it was so large and
light, the solar wind pushed it around in its orbit. In the future, some
space craft may use the solar wind to travel between planets using
*solar sails* similar to the way sailboats use the Earth\'s wind in
their sails to cross the ocean.

### Heliopause

thumb\|left\|Heliopause: where the solar wind hits the edge of the
Solar System *Heliopause*
is where the solar wind hits the wind from other stars. Near here, the
solar wind slows down suddenly. In May 2005, the Voyager I spacecraft
went through this region and felt a big bump. It is now just inside the
heliopause. Because this happens so far from Earth, it is hard to
study!\

## What is solar weather?

Did you know the Sun has weather? Earth weather is what is going on in
Earth's atmosphere. *Solar weather* is what\'s going on in the Sun's
atmosphere. Solar weather affects us on Earth. Solar weather (also
called space weather) includes sunlight and the solar wind.

Solar flares shoot a lot of very hot gas out from the Sun. If a solar
flare is aimed towards Earth, protons --- subatomic particles with
positive electric charge --- might be shot at Earth at high speed, and a
*solar storm* could result. That could cause electrical blackouts or
block radio signals. It could damage satellites in orbit. Radiation from
a bad solar storm could be very dangerous for astronauts, so they must
be protected. The Earth's magnetic field and atmosphere usually protect
us from flares.

!Northern Lights Solar flares
can also cause an *aurora*. Auroras look like beautiful curtains of
shimmering light. They are called Northern Lights (Aurora Borealis) if
they are near the North Pole. They are called Southern Lights (Aurora
Australis) if they are near the South Pole. Solar weather affects other
planets, too. We have pictures of auroras on every planet except Mercury
and Pluto.

Just like we can get Earth weather forecasts, we can get Solar weather
forecasts. Forecasters study the Sun to figure out when flares will
happen. They try to tell when solar storms will hit Earth. They also try
to tell when solar storms will go to other parts of the Solar System.

Next Topic: Mercury

## The life cycle of a star

upright=3\|thumb\|center\|Life Cycle Of Our
Sun A forming sun\'s life starts
out as a nebula; this is a cloud of gas made up mostly of hydrogen. Over
time the nebula contracts; its core becomes more and more compact and
hotter until it can fuse hydrogen to helium. This fusion becomes its
power source. After this, the star will look very much like our sun.
They fuse hydrogen atoms together to make helium atoms.

!A comparison between the sizes of the Sun as it is now, and how it
will be as a red
giant.

After billions of years, this star will die. The star will exhaust its
supply of hydrogen in its core, so that there will no longer be any
source of heat to support the core against gravity. The core shrinks,
and the hydrogen starts to burn in a shell around the core. The outer
layers of the star swell and cool, creating a red giant. This is all
with an ordinary star and will be the life cycle of our sun, although
the process may vary from star to star. When our sun becomes a red giant
it will consume Mercury and Venus, and possibly the Earth. The sun will
then begin a cycle of expansion and contraction, puffing away its outer
atmosphere as the solar system becomes a planetary nebula. Once the sun
can not longer sustain fusion, it will collapse down and condense into a
white dwarf star. At this stage the light it emits is the left over
energy of our dead sun, radiating the residual heat until eventually the
white dwarf cools and darkens.

## Exploration of the Sun

Early space probes designed to collect information about the sun were
NASA\'s Pioneers 5 through 9 and the Helios 1 and 2. Those were in the
1950s, 60s, and 70s. They collected a lot of data about the Sun. Other
projects observed the Sun from Earth orbit, such as a Japanese satellite
launched in 1991 called Yohkoh, which means *Sunbeam* in English. It
told scientists more about solar flares, and activity on the sun\'s
surface. Thanks to Yohkoh, they knew more about how to classify solar
flares, and whether or not they will cause electrical disturbance on
Earth or not. Two important missions to study Sun are called the *Solar
and Heliospheric Observatory* (SHO) and the *Solar Dynamics Observatory*
(SDO). They have taken many pictures of the Sun, along with discovering
many comets near the Sun. All of these observations were across the
Sun\'s equator. The first space probe to observe the Sun\'s poles was
named *Ulysses*, after a famous Greek king who traveled on a very, very
long voyage. The Ulysses probe went all the way to Jupiter before
approaching the Sun. It took a famous picture of the comet
*Shoemaker-Levy-9* colliding with Jupiter.

!The Shoemaker-Levy 9 fragments falling back on
Jupiter.

## References

NASA. The Chromosphere ---
1\
Col, Jeananda. Enchanted Learning/Zoom Astronomy
2
1998--2005\
`<i>`{=html}Usborne Internet-Linked Science Encyclopedia`</i>`{=html},
Usborne Publishing Ltd.
3\
Dickinson, Terrence. `<i>`{=html}The Universe and Beyond`</i>`{=html} .
Firefly Books \
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