title: Mass–energy equivalence

text: As most of the mass which comprises ordinary objects resides in protons and neutrons, converting all the energy of ordinary matter into more useful forms requires that the protons and neutrons be converted to lighter particles, or particles with no mass at all. In the Standard Model of particle physics, the number of protons plus neutrons is nearly exactly conserved. Despite this, Gerard 't Hooft showed that there is a process that converts protons and neutrons to antielectrons and neutrinos. This is the weak SU(2) instanton proposed by the physicists Alexander Belavin, Alexander Markovich Polyakov, Albert Schwarz, and Yu. S. Tyupkin. This process, can in principle destroy matter and convert all the energy of matter into neutrinos and usable energy, but it is normally extraordinarily slow. It was later shown that the process occurs rapidly at extremely high temperatures that would only have been reached shortly after the Big Bang.


title: Mass–energy equivalence

text: Many extensions of the standard model contain magnetic monopoles, and in some models of grand unification, these monopoles catalyze proton decay, a process known as the Callan–Rubakov effect. This process would be an efficient mass–energy conversion at ordinary temperatures, but it requires making monopoles and anti-monopoles, whose production is expected to be inefficient. Another method of completely annihilating matter uses the gravitational field of black holes. The British theoretical physicist Stephen Hawking theorized it is possible to throw matter into a black hole and use the emitted heat to generate power. According to the theory of Hawking radiation, however, larger black holes radiate less than smaller ones, so that usable power can only be produced by small black holes.


title: Mass–energy equivalence

text: Unlike a system's energy in an inertial frame, the relativistic energy (formula_2) of a system depends on both the rest mass (formula_3) and the total momentum of the system. The extension of Einstein's equation to these systems is given by:


title: Mass–energy equivalence

text: where the formula_6 term represents the square of the Euclidean norm (total vector length) of the various momentum vectors in the system, which reduces to the square of the simple momentum magnitude, if only a single particle is considered. This equation is called the energy–momentum relation and reduces to formula_7 when the momentum term is zero. For photons where formula_8, the equation reduces to formula_9.


title: Mass–energy equivalence

text: For speeds much smaller than the speed of light, higher-order terms in this expression get smaller and smaller because is small. For low speeds, all but the first two terms can be ignored:


title: Mass–energy equivalence

text: In classical mechanics, both the term and the high-speed corrections are ignored. The initial value of the energy is arbitrary, as only the change in energy can be measured, so the term is ignored in classical physics. While the higher-order terms become important at higher speeds, the Newtonian equation is a highly accurate low-speed approximation; adding in the third term yields:


title: Mass–energy equivalence

text: The difference between the two approximations is given by formula_13, a number very small for everyday objects. In 2018 NASA announced the Parker Solar Probe was the fastest ever, with a speed of . The difference between the approximations for the Parker Solar Probe in 2018 is formula_14, which accounts for an energy correction of four parts per hundred million. The gravitational constant, in contrast, has a standard relative uncertainty of about formula_15.


title: Mass–energy equivalence

text: The nuclear binding energy is the minimum energy that is required to disassemble the nucleus of an atom into its component parts. The mass of an atom is less than the sum of the masses of its constituents due to the attraction of the strong nuclear force. The difference between the two masses is called the "mass defect" and is related to the binding energy through Einstein's formula. The principle is used in modeling nuclear fission reactions and it implies a great amount of energy can be released by the nuclear fission chain reactions used in both nuclear weapons and nuclear power.


title: Mass–energy equivalence

text: A water molecule weighs a little less than two free hydrogen atoms and an oxygen atom. The minuscule mass difference is the energy needed to split the molecule into three individual atoms (divided by ), which was given off as heat when the molecule formed (this heat had mass). Similarly, a stick of dynamite in theory weighs a little bit more than the fragments after the explosion; in this case the mass difference is the energy and heat that is released when the dynamite explodes. Such a change in mass may only happen when the system is open, and the energy and mass are allowed to escape. Thus, if a stick of dynamite is blown up in a hermetically sealed chamber, the mass of the chamber and fragments, the heat, sound, and light would still be equal to the original mass of the chamber and dynamite. If sitting on a scale, the weight and mass would not change. This would in theory also happen even with a nuclear bomb, if it could be kept in an ideal box of infinite strength, which did not rupture or pass radiation. Thus, a 21.5 kiloton () nuclear bomb produces about one gram of heat and electromagnetic radiation, but the mass of this energy would not be detectable in an exploded bomb in an ideal box sitting on a scale; instead, the contents of the box would be heated to millions of degrees without changing total mass and weight. If a transparent window passing only electromagnetic radiation were opened in such an ideal box after the explosion, and a beam of X-rays and other lower-energy light allowed to escape the box, it would eventually be found to weigh one gram less than it had before the explosion. This weight loss and mass loss would happen as the box was cooled by this process, to room temperature. However, any surrounding mass that absorbed the X-rays (and other "heat") would "gain" this gram of mass from the resulting heating, thus, in this case, the mass "loss" would represent merely its relocation.


title: Mass–energy equivalence

text: Einstein used the centimeter gram second system of units (cgs), but the formula is independent of the system of units. In natural units, the numerical value of the speed of light is set to equal 1, and the formula expresses an equality of numerical values: . In the SI system (expressing the ratio in joules per kilogram using the value of in meters per second):


title: Mass–energy equivalence

text: Any time energy is released, the process can be evaluated from an perspective. For instance, the "Gadget"-style bomb used in the Trinity test and the bombing of Nagasaki had an explosive yield equivalent to 21 kt of TNT. About 1 kg of the approximately 6.15 kg of plutonium in each of these bombs fissioned into lighter elements totaling almost exactly one gram less, after cooling. The electromagnetic radiation and kinetic energy (thermal and blast energy) released in this explosion carried the missing gram of mass.


title: Mass–energy equivalence

text: Whenever energy is added to a system, the system gains mass, as shown when the equation is rearranged:


title: Mass–energy equivalence

text: While Einstein was the first to have correctly deduced the mass–energy equivalence formula, he was not the first to have related energy with mass, though nearly all previous authors thought that the energy that contributes to mass comes only from electromagnetic fields. Once discovered, Einstein's formula was initially written in many different notations, and its interpretation and justification was further developed in several steps.


title: Mass–energy equivalence

text: Eighteenth century theories on the correlation of mass and energy included that devised by the English scientist Isaac Newton in 1717, who speculated that light particles and matter particles were interconvertible in "Query 30" of the "Opticks", where he asks: "Are not the gross bodies and light convertible into one another, and may not bodies receive much of their activity from the particles of light which enter their composition?" Swedish scientist and theologian Emanuel Swedenborg, in his "Principia" of 1734 theorized that all matter is ultimately composed of dimensionless points of "pure and total motion". He described this motion as being without force, direction or speed, but having the potential for force, direction and speed everywhere within it.


title: Mass–energy equivalence

text: During the nineteenth century there were several speculative attempts to show that mass and energy were proportional in various ether theories. In 1873 the Russian physicist and mathematician Nikolay Umov pointed out a relation between mass and energy for ether in the form of , where . The writings of the English engineer Samuel Tolver Preston, and a 1903 paper by the Italian industrialist and geologist Olinto De Pretto, presented a mass–energy relation. Italian mathematician and math historian Umberto Bartocci observed that there were only three degrees of separation linking De Pretto to Einstein, concluding that Einstein was probably aware of De Pretto's work. Preston and De Pretto, following physicist Georges-Louis Le Sage, imagined that the universe was filled with an ether of tiny particles that always move at speed . Each of these particles has a kinetic energy of up to a small numerical factor. The nonrelativistic kinetic energy formula did not always include the traditional factor of , since German polymath Gottfried Leibniz introduced kinetic energy without it, and the is largely conventional in prerelativistic physics. By assuming that every particle has a mass that is the sum of the masses of the ether particles, the authors concluded that all matter contains an amount of kinetic energy either given by or depending on the convention. A particle ether was usually considered unacceptably speculative science at the time, and since these authors did not formulate relativity, their reasoning is completely different from that of Einstein, who used relativity to change frames.


title: Mass–energy equivalence

text: In 1905, and independent of Einstein, French polymath Gustave Le Bon speculated that atoms could release large amounts of latent energy, reasoning from an all-encompassing qualitative philosophy of physics.


title: Mass–energy equivalence

text: There were many attempts in the 19th and the beginning of the 20th century—like those of British physicists J. J. Thomson in 1881 and Oliver Heaviside in 1889, and George Frederick Charles Searle in 1897, German physicists Wilhelm Wien in 1900 and Max Abraham in 1902, and the Dutch physicist Hendrik Antoon Lorentz in 1904—to understand how the mass of a charged object depends on the electrostatic field. This concept was called electromagnetic mass, and was considered as being dependent on velocity and direction as well. Lorentz in 1904 gave the following expressions for longitudinal and transverse electromagnetic mass:


title: Mass–energy equivalence

text: Another way of deriving a type of electromagnetic mass was based on the concept of radiation pressure. In 1900, French polymath Henri Poincaré associated electromagnetic radiation energy with a "fictitious fluid" having momentum and mass


title: Mass–energy equivalence

text: By that, Poincaré tried to save the center of mass theorem in Lorentz's theory, though his treatment led to radiation paradoxes.


title: Mass–energy equivalence

text: Austrian physicist Friedrich Hasenöhrl showed in 1904 that electromagnetic cavity radiation contributes the "apparent mass"


title: Mass–energy equivalence

text: Einstein did not write the exact formula in his 1905 "Annus Mirabilis" paper "Does the Inertia of an object Depend Upon Its Energy Content?"; rather, the paper states that if a body gives off the energy in the form of radiation, its mass diminishes by . This formulation relates only a change in mass to a change in energy without requiring the absolute relationship. The relationship convinced him that mass and energy can be seen as two names for the same underlying, conserved physical quantity. He has stated that the laws of conservation of energy and conservation of mass are "one and the same". Einstein elaborated in a 1946 essay that "the principle of the conservation of mass… proved inadequate in the face of the special theory of relativity. It was therefore merged with the energy conservation principle—just as, about 60 years before, the principle of the conservation of mechanical energy had been combined with the principle of the conservation of heat [thermal energy]. We might say that the principle of the conservation of energy, having previously swallowed up that of the conservation of heat, now proceeded to swallow that of the conservation of mass—and holds the field alone."


title: Mass–energy equivalence

text: He included the second term on the right to make sure that for small velocities the energy would be the same as in classical mechanics, thus satisfying the correspondence principle:


title: Mass–energy equivalence

text: Without this second term, there would be an additional contribution in the energy when the particle is not moving.


title: Mass–energy equivalence

text: Einstein, following Lorentz and Abraham, used velocity- and direction-dependent mass concepts in his 1905 electrodynamics paper and in another paper in 1906. In Einstein's first 1905 paper on , he treated as what would now be called the "rest mass", and it has been noted that in his later years he did not like the idea of "relativistic mass".


title: Mass–energy equivalence

text: In older physics terminology, relativistic energy is used in lieu of relativistic mass and the term "mass" is reserved for the rest mass. Historically, there has been considerable debate over the use of the concept of "relativistic mass" and the connection of "mass" in relativity to "mass" in Newtonian dynamics. One view is that only rest mass is a viable concept and is a property of the particle; while relativistic mass is a conglomeration of particle properties and properties of spacetime. Another view, attributed to Norwegian physicist Kjell Vøyenli, is that the Newtonian concept of mass as a particle property and the relativistic concept of mass have to be viewed as embedded in their own theories and as having no precise connection.


title: Mass–energy equivalence

text: Already in his relativity paper "On the electrodynamics of moving bodies", Einstein derived the correct expression for the kinetic energy of particles:


title: Mass–energy equivalence

text: Now the question remained open as to which formulation applies to bodies at rest. This was tackled by Einstein in his paper "Does the inertia of a body depend upon its energy content?", one of his Annus Mirabilis papers. Here, Einstein used to represent the speed of light in a vacuum and to represent the energy lost by a body in the form of radiation. Consequently, the equation was not originally written as a formula but as a sentence in German saying that "if a body gives off the energy in the form of radiation, its mass diminishes by ." A remark placed above it informed that the equation was approximated by neglecting "magnitudes of fourth and higher orders" of a series expansion. Einstein used a body emitting two light pulses in opposite directions, having energies of before and after the emission as seen in its rest frame. As seen from a moving frame, this becomes and . Einstein obtained, in modern notation:


title: Mass–energy equivalence

text: Neglecting effects higher than third order in after a Taylor series expansion of the right side of this yields:


title: Mass–energy equivalence

text: Einstein concluded that the emission reduces the body's mass by , and that the mass of a body is a measure of its energy content.


title: Mass–energy equivalence

text: The correctness of Einstein's 1905 derivation of was criticized by German theoretical physicist Max Planck in 1907, who argued that it is only valid to first approximation. Another criticism was formulated by American physicist Herbert Ives in 1952 and the Israeli physicist Max Jammer in 1961, asserting that Einstein's derivation is based on begging the question. Other scholars, such as American and Chilean philosophers John Stachel and Roberto Torretti, have argued that Ives' criticism was wrong, and that Einstein's derivation was correct. American physics writer Hans Ohanian, in 2008, agreed with Stachel/Torretti's criticism of Ives, though he argued that Einstein's derivation was wrong for other reasons.


title: Mass–energy equivalence

text: Like Poincaré, Einstein concluded in 1906 that the inertia of electromagnetic energy is a necessary condition for the center-of-mass theorem to hold. On this occasion, Einstein referred to Poincaré's 1900 paper and wrote: "Although the merely formal considerations, which we will need for the proof, are already mostly contained in a work by H. Poincaré, for the sake of clarity I will not rely on that work." In Einstein's more physical, as opposed to formal or mathematical, point of view, there was no need for fictitious masses. He could avoid the "perpetual motion" problem because, on the basis of the mass–energy equivalence, he could show that the transport of inertia that accompanies the emission and absorption of radiation solves the problem. Poincaré's rejection of the principle of action–reaction can be avoided through Einstein's , because mass conservation appears as a special case of the energy conservation law.


title: Mass–energy equivalence

text: There were several further developments in the first decade of the twentieth century. In May 1907, Einstein explained that the expression for energy of a moving mass point assumes the simplest form when its expression for the state of rest is chosen to be (where is the mass), which is in agreement with the "principle of the equivalence of mass and energy". In addition, Einstein used the formula , with being the energy of a system of mass points, to describe the energy and mass increase of that system when the velocity of the differently moving mass points is increased. Max Planck rewrote Einstein's mass–energy relationship as in June 1907, where is the pressure and the volume to express the relation between mass, its latent energy, and thermodynamic energy within the body. Subsequently, in October 1907, this was rewritten as and given a quantum interpretation by German physicist Johannes Stark, who assumed its validity and correctness. In December 1907, Einstein expressed the equivalence in the form and concluded: "A mass is equivalent, as regards inertia, to a quantity of energy . […] It appears far more natural to consider every inertial mass as a store of energy." American physical chemists Gilbert N. Lewis and Richard C. Tolman used two variations of the formula in 1909: and , with being the relativistic energy (the energy of an object when the object is moving), is the rest energy (the energy when not moving), is the relativistic mass (the rest mass and the extra mass gained when moving), and is the rest mass. The same relations in different notation were used by Lorentz in 1913 and 1914, though he placed the energy on the left-hand side: and , with being the total energy (rest energy plus kinetic energy) of a moving material point, its rest energy, the relativistic mass, and the invariant mass.


title: Mass–energy equivalence

text: In 1911, German physicist Max von Laue gave a more comprehensive proof of from the stress–energy tensor, which was later generalized by German mathematician Felix Klein in 1918.


title: Mass–energy equivalence

text: Einstein returned to the topic once again after World War II and this time he wrote in the title of his article intended as an explanation for a general reader by analogy.


title: Mass–energy equivalence

text: An alternative version of Einstein's thought experiment was proposed by American theoretical physicist Fritz Rohrlich in 1990, who based his reasoning on the Doppler effect. Like Einstein, he considered a body at rest with mass . If the body is examined in a frame moving with nonrelativistic velocity , it is no longer at rest and in the moving frame it has momentum . Then he supposed the body emits two pulses of light to the left and to the right, each carrying an equal amount of energy . In its rest frame, the object remains at rest after the emission since the two beams are equal in strength and carry opposite momentum. However, if the same process is considered in a frame that moves with velocity to the left, the pulse moving to the left is redshifted, while the pulse moving to the right is blue shifted. The blue light carries more momentum than the red light, so that the momentum of the light in the moving frame is not balanced: the light is carrying some net momentum to the right. The object has not changed its velocity before or after the emission. Yet in this frame it has lost some right-momentum to the light. The only way it could have lost momentum is by losing mass. This also solves Poincaré's radiation paradox. The velocity is small, so the right-moving light is blueshifted by an amount equal to the nonrelativistic Doppler shift factor . The momentum of the light is its energy divided by , and it is increased by a factor of . So the right-moving light is carrying an extra momentum given by:


title: Mass–energy equivalence

text: The left-moving light carries a little less momentum, by the same amount . So the total right-momentum in both light pulses is twice . This is the right-momentum that the object lost.


title: Mass–energy equivalence

text: So the change in the object's mass is equal to the total energy lost divided by . Since any emission of energy can be carried out by a two-step process, where first the energy is emitted as light and then the light is converted to some other form of energy, any emission of energy is accompanied by a loss of mass. Similarly, by considering absorption, a gain in energy is accompanied by a gain in mass.


title: Mass–energy equivalence

text: It was quickly noted after the discovery of radioactivity in 1897, that the total energy due to radioactive processes is about one "million times" greater than that involved in any known molecular change, raising the question of where the energy comes from. After eliminating the idea of absorption and emission of some sort of Lesagian ether particles, the existence of a huge amount of latent energy, stored within matter, was proposed by New Zealand physicist Ernest Rutherford and British radiochemist Frederick Soddy in 1903. Rutherford also suggested that this internal energy is stored within normal matter as well. He went on to speculate in 1904: "If it were ever found possible to control at will the rate of disintegration of the radio-elements, an enormous amount of energy could be obtained from a small quantity of matter."


title: Mass–energy equivalence

text: Einstein's equation does not explain the large energies released in radioactive decay, but can be used to quantify them. The theoretical explanation for radioactive decay is given by the nuclear forces responsible for holding atoms together, though these forces were still unknown in 1905. The enormous energy released from radioactive decay had previously been measured by Rutherford and was much more easily measured than the small change in the gross mass of materials as a result. Einstein's equation, by theory, can give these energies by measuring mass differences before and after reactions, but in practice, these mass differences in 1905 were still too small to be measured in bulk. Prior to this, the ease of measuring radioactive decay energies with a calorimeter was thought possibly likely to allow measurement of changes in mass difference, as a check on Einstein's equation itself. Einstein mentions in his 1905 paper that mass–energy equivalence might perhaps be tested with radioactive decay, which was known by then to release enough energy to possibly be "weighed," when missing from the system. However, radioactivity seemed to proceed at its own unalterable pace, and even when simple nuclear reactions became possible using proton bombardment, the idea that these great amounts of usable energy could be liberated at will with any practicality, proved difficult to substantiate. Rutherford was reported in 1933 to have declared that this energy could not be exploited efficiently: "Anyone who expects a source of power from the transformation of the atom is talking moonshine."


title: Mass–energy equivalence

text: This outlook changed dramatically in 1932 with the discovery of the neutron and its mass, allowing mass differences for single nuclides and their reactions to be calculated directly, and compared with the sum of masses for the particles that made up their composition. In 1933, the energy released from the reaction of lithium-7 plus protons giving rise to two alpha particles, allowed Einstein's equation to be tested to an error of ±0.5%. However, scientists still did not see such reactions as a practical source of power, due to the energy cost of accelerating reaction particles.


title: Mass–energy equivalence

text: After the very public demonstration of huge energies released from nuclear fission after the atomic bombings of Hiroshima and Nagasaki in 1945, the equation became directly linked in the public eye with the power and peril of nuclear weapons. The equation was featured on page 2 of the Smyth Report, the official 1945 release by the US government on the development of the atomic bomb, and by 1946 the equation was linked closely enough with Einstein's work that the cover of "Time" magazine prominently featured a picture of Einstein next to an image of a mushroom cloud emblazoned with the equation. Einstein himself had only a minor role in the Manhattan Project: he had cosigned a letter to the U.S. president in 1939 urging funding for research into atomic energy, warning that an atomic bomb was theoretically possible. The letter persuaded Roosevelt to devote a significant portion of the wartime budget to atomic research. Without a security clearance, Einstein's only scientific contribution was an analysis of an isotope separation method in theoretical terms. It was inconsequential, on account of Einstein not being given sufficient information to fully work on the problem.


title: Mass–energy equivalence

text: While is useful for understanding the amount of energy potentially released in a fission reaction, it was not strictly necessary to develop the weapon, once the fission process was known, and its energy measured at 200 MeV (which was directly possible, using a quantitative Geiger counter, at that time). The physicist and Manhattan Project participant Robert Serber noted that somehow "the popular notion took hold long ago that Einstein's theory of relativity, in particular his famous equation , plays some essential role in the theory of fission. Einstein had a part in alerting the United States government to the possibility of building an atomic bomb, but his theory of relativity is not required in discussing fission. The theory of fission is what physicists call a non-relativistic theory, meaning that relativistic effects are too small to affect the dynamics of the fission process significantly." There are other views on the equation's importance to nuclear reactions. In late 1938, the Austrian-Swedish and British physicists Lise Meitner and Otto Robert Frisch—while on a winter walk during which they solved the meaning of Hahn's experimental results and introduced the idea that would be called atomic fission—directly used Einstein's equation to help them understand the quantitative energetics of the reaction that overcame the "surface tension-like" forces that hold the nucleus together, and allowed the fission fragments to separate to a configuration from which their charges could force them into an energetic "fission". To do this, they used "packing fraction", or nuclear binding energy values for elements. These, together with use of allowed them to realize on the spot that the basic fission process was energetically possible.


title: Mass–energy equivalence

text: According to the Einstein Papers Project at the California Institute of Technology and Hebrew University of Jerusalem, there remain only four known copies of this equation as written by Einstein. One of these is a letter written in German to Ludwik Silberstein, which was in Silberstein's archives, and sold at auction for $1.2 million, RR Auction of Boston, Massachusetts said on May 21, 2021.


title: Neutron star

text: A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. white holes, quark stars, and strange stars), neutron stars are the smallest and densest currently known class of stellar objects. Neutron stars have a radius on the order of and a mass of about 1.4 solar masses. They result from the supernova explosion of a massive star, combined with gravitational collapse, that compresses the core past white dwarf star density to that of atomic nuclei.


title: Neutron star

text: Once formed, they no longer actively generate heat, and cool over time; however, they may still evolve further through collision or accretion. Most of the basic models for these objects imply that neutron stars are composed almost entirely of neutrons (subatomic particles with no net electrical charge and with slightly larger mass than protons); the electrons and protons present in normal matter combine to produce neutrons at the conditions in a neutron star. Neutron stars are partially supported against further collapse by neutron degeneracy pressure, a phenomenon described by the Pauli exclusion principle, just as white dwarfs are supported against collapse by electron degeneracy pressure. However, neutron degeneracy pressure is not by itself sufficient to hold up an object beyond 0.7 and repulsive nuclear forces play a larger role in supporting more massive neutron stars. If the remnant star has a mass exceeding the Tolman–Oppenheimer–Volkoff limit of around 2 solar masses, the combination of degeneracy pressure and nuclear forces is insufficient to support the neutron star and it continues collapsing to form a black hole. The most massive neutron star detected so far, PSR J0952–0607, is estimated to be solar masses.


title: Neutron star

text: Neutron stars that can be observed are very hot and typically have a surface temperature of around . Neutron star material is remarkably dense: a normal-sized matchbox containing neutron-star material would have a weight of approximately 3 billion tonnes, the same weight as a 0.5 cubic kilometre chunk of the Earth (a cube with edges of about 800 metres) from Earth's surface. Their magnetic fields are between 10 and 10 (100 million and 1 quadrillion) times stronger than Earth's magnetic field. The gravitational field at the neutron star's surface is about (200 billion) times that of Earth's gravitational field.


title: Neutron star

text: As the star's core collapses, its rotation rate increases as a result of conservation of angular momentum, and newly formed neutron stars hence rotate at up to several hundred times per second. Some neutron stars emit beams of electromagnetic radiation that make them detectable as pulsars. Indeed, the discovery of pulsars by Jocelyn Bell Burnell and Antony Hewish in 1967 was the first observational suggestion that neutron stars exist. The radiation from pulsars is thought to be primarily emitted from regions near their magnetic poles. If the magnetic poles do not coincide with the rotational axis of the neutron star, the emission beam will sweep the sky, and when seen from a distance, if the observer is somewhere in the path of the beam, it will appear as pulses of radiation coming from a fixed point in space (the so-called "lighthouse effect"). The fastest-spinning neutron star known is PSR J1748-2446ad, rotating at a rate of 716 times a second or 43,000 revolutions per minute, giving a linear speed at the surface on the order of (i.e., nearly a quarter the speed of light).


title: Neutron star

text: There are thought to be around one billion neutron stars in the Milky Way, and at a minimum several hundred million, a figure obtained by estimating the number of stars that have undergone supernova explosions. However, most are old and cold and radiate very little; most neutron stars that have been detected occur only in certain situations in which they do radiate, such as if they are a pulsar or part of a binary system. Slow-rotating and non-accreting neutron stars are almost undetectable; however, since the Hubble Space Telescope detection of RX J1856.5−3754 in the 1990s, a few nearby neutron stars that appear to emit only thermal radiation have been detected. Soft gamma repeaters are conjectured to be a type of neutron star with very strong magnetic fields, known as magnetars, or alternatively, neutron stars with fossil disks around them.


title: Neutron star

text: Neutron stars in binary systems can undergo accretion which typically makes the system bright in X-rays while the material falling onto the neutron star can form hotspots that rotate in and out of view in identified X-ray pulsar systems. Additionally, such accretion can "recycle" old pulsars and potentially cause them to gain mass and spin-up to very fast rotation rates, forming the so-called millisecond pulsars. These binary systems will continue to evolve, and eventually the companions can become compact objects such as white dwarfs or neutron stars themselves, though other possibilities include a complete destruction of the companion through ablation or merger. The merger of binary neutron stars may be the source of short-duration gamma-ray bursts and are likely strong sources of gravitational waves. In 2017, a direct detection (GW170817) of the gravitational waves from such an event was observed, and gravitational waves have also been indirectly observed in a system where two neutron stars orbit each other.


title: Neutron star

text: Any main-sequence star with an initial mass of above 8 times the mass of the sun () has the potential to produce a neutron star. As the star evolves away from the main sequence, subsequent nuclear burning produces an iron-rich core. When all nuclear fuel in the core has been exhausted, the core must be supported by degeneracy pressure alone. Further deposits of mass from shell burning cause the core to exceed the Chandrasekhar limit. Electron-degeneracy pressure is overcome and the core collapses further, sending temperatures soaring to over . At these temperatures, photodisintegration (the breaking up of iron nuclei into alpha particles by high-energy gamma rays) occurs. As the temperature climbs even higher, electrons and protons combine to form neutrons via electron capture, releasing a flood of neutrinos. When densities reach nuclear density of , a combination of strong force repulsion and neutron degeneracy pressure halts the contraction. The infalling outer envelope of the star is halted and flung outwards by a flux of neutrinos produced in the creation of the neutrons, becoming a supernova. The remnant left is a neutron star. If the remnant has a mass greater than about , it collapses further to become a black hole.


title: Neutron star

text: As the core of a massive star is compressed during a Type II supernova or a Type Ib or Type Ic supernova, and collapses into a neutron star, it retains most of its angular momentum. But, because it has only a tiny fraction of its parent's radius (and therefore its moment of inertia is sharply reduced), a neutron star is formed with very high rotation speed, and then over a very long period it slows. Neutron stars are known that have rotation periods from about 1.4 ms to 30 s. The neutron star's density also gives it very high surface gravity, with typical values ranging from 10 to 10 m/s (more than 10 times that of Earth). One measure of such immense gravity is the fact that neutron stars have an escape velocity of over half the speed of light. The neutron star's gravity accelerates infalling matter to tremendous speed, and tidal forces near the surface can cause spaghettification. The force of its impact would likely destroy the object's component atoms, rendering all the matter identical, in most respects, to the rest of the neutron star.


title: Neutron star

text: A neutron star has a mass of at least 1.1 solar masses (). The upper limit of mass for a neutron star is called the Tolman–Oppenheimer–Volkoff limit and is generally held to be around , but a recent estimate puts the upper limit at . The maximum observed mass of neutron stars is about for PSR J0740+6620 discovered in September, 2019. Compact stars below the Chandrasekhar limit of are generally white dwarfs whereas compact stars with a mass between and are expected to be neutron stars, but there is an interval of a few tenths of a solar mass where the masses of low-mass neutron stars and high-mass white dwarfs can overlap. It is thought that beyond the stellar remnant will overcome the strong force repulsion and neutron degeneracy pressure so that gravitational collapse will occur to produce a black hole, but the smallest observed mass of a stellar black hole is about . Between and , hypothetical intermediate-mass stars such as quark stars and electroweak stars have been proposed, but none have been shown to exist.


title: Neutron star

text: The temperature inside a newly formed neutron star is from around 10 to 10 kelvin. However, the huge number of neutrinos it emits carry away so much energy that the temperature of an isolated neutron star falls within a few years to around 10 kelvin. At this lower temperature, most of the light generated by a neutron star is in X-rays.


title: Neutron star

text: Some researchers have proposed a neutron star classification system using Roman numerals (not to be confused with the Yerkes luminosity classes for non-degenerate stars) to sort neutron stars by their mass and cooling rates: type I for neutron stars with low mass and cooling rates, type II for neutron stars with higher mass and cooling rates, and a proposed type III for neutron stars with even higher mass, approaching , and with higher cooling rates and possibly candidates for exotic stars.


title: Neutron star

text: Neutron stars have overall densities of to ( to times the density of the Sun), which is comparable to the approximate density of an atomic nucleus of . The neutron star's density varies from about in the crust—increasing with depth—to about or (denser than an atomic nucleus) deeper inside. A neutron star is so dense that one teaspoon (5 milliliters) of its material would have a mass over , about 900 times the mass of the Great Pyramid of Giza. In the enormous gravitational field of a neutron star, that teaspoon of material would weigh , which is 15 times what the Moon would weigh if it were placed on the surface of the Earth. The entire mass of the Earth at neutron star density would fit into a sphere of 305 m in diameter (the size of the Arecibo Telescope). The pressure increases from to from the inner crust to the center.


title: Neutron star

text: The equation of state of matter at such high densities is not precisely known because of the theoretical difficulties associated with extrapolating the likely behavior of quantum chromodynamics, superconductivity, and superfluidity of matter in such states. The problem is exacerbated by the empirical difficulties of observing the characteristics of any object that is hundreds of parsecs away, or farther.


title: Neutron star

text: A neutron star has some of the properties of an atomic nucleus, including density (within an order of magnitude) and being composed of nucleons. In popular scientific writing, neutron stars are therefore sometimes described as "giant nuclei". However, in other respects, neutron stars and atomic nuclei are quite different. A nucleus is held together by the strong interaction, whereas a neutron star is held together by gravity. The density of a nucleus is uniform, while neutron stars are predicted to consist of multiple layers with varying compositions and densities.


title: Neutron star

text: The magnetic field strength on the surface of neutron stars ranges from  10 to 10 tesla. These are orders of magnitude higher than in any other object: For comparison, a continuous 16 T field has been achieved in the laboratory and is sufficient to levitate a living frog due to diamagnetic levitation. Variations in magnetic field strengths are most likely the main factor that allows different types of neutron stars to be distinguished by their spectra, and explains the periodicity of pulsars.


title: Neutron star

text: The neutron stars known as magnetars have the strongest magnetic fields, in the range of 10 to 10 tesla, and have become the widely accepted hypothesis for neutron star types soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs). The magnetic energy density of a 10 T field is extreme, greatly exceeding the mass-energy density of ordinary matter. Fields of this strength are able to polarize the vacuum to the point that the vacuum becomes birefringent. Photons can merge or split in two, and virtual particle-antiparticle pairs are produced. The field changes electron energy levels and atoms are forced into thin cylinders. Unlike in an ordinary pulsar, magnetar spin-down can be directly powered by its magnetic field, and the magnetic field is strong enough to stress the crust to the point of fracture. Fractures of the crust cause starquakes, observed as extremely luminous millisecond hard gamma ray bursts. The fireball is trapped by the magnetic field, and comes in and out of view when the star rotates, which is observed as a periodic soft gamma repeater (SGR) emission with a period of 5–8 seconds and which lasts for a few minutes.


title: Neutron star

text: The origins of the strong magnetic field are as yet unclear. One hypothesis is that of "flux freezing", or conservation of the original magnetic flux during the formation of the neutron star. If an object has a certain magnetic flux over its surface area, and that area shrinks to a smaller area, but the magnetic flux is conserved, then the magnetic field would correspondingly increase. Likewise, a collapsing star begins with a much larger surface area than the resulting neutron star, and conservation of magnetic flux would result in a far stronger magnetic field. However, this simple explanation does not fully explain magnetic field strengths of neutron stars.


title: Neutron star

text: The gravitational field at a neutron star's surface is about times stronger than on Earth, at around . Such a strong gravitational field acts as a gravitational lens and bends the radiation emitted by the neutron star such that parts of the normally invisible rear surface become visible.


title: Neutron star

text: If the radius of the neutron star is 3"GM"/"c" or less, then the photons may be trapped in an orbit, thus making the whole surface of that neutron star visible "from a single vantage point", along with destabilizing photon orbits at or below the 1 radius distance of the star.


title: Neutron star

text: A fraction of the mass of a star that collapses to form a neutron star is released in the supernova explosion from which it forms (from the law of mass–energy equivalence, ). The energy comes from the gravitational binding energy of a neutron star.


title: Neutron star

text: Hence, the gravitational force of a typical neutron star is huge. If an object were to fall from a height of one meter on a neutron star 12 kilometers in radius, it would reach the ground at around 1400 kilometers per second. However, even before impact, the tidal force would cause spaghettification, breaking any sort of an ordinary object into a stream of material.


title: Neutron star

text: Because of the enormous gravity, time dilation between a neutron star and Earth is significant. For example, eight years could pass on the surface of a neutron star, yet ten years would have passed on Earth, not including the time-dilation effect of the star's very rapid rotation.


title: Neutron star

text: Neutron star relativistic equations of state describe the relation of radius vs. mass for various models. The most likely radii for a given neutron star mass are bracketed by models AP4 (smallest radius) and MS2 (largest radius). "E" is the ratio of gravitational binding energy mass equivalent to the observed neutron star gravitational mass of "M" kilograms with radius "R" meters,


title: Neutron star

text: A neutron star would not be more compact than 10,970 meters radius (AP4 model). Its mass fraction gravitational binding energy would then be 0.187, −18.7% (exothermic). This is not near 0.6/2 = 0.3, −30%.


title: Neutron star

text: The equation of state for a neutron star is not yet known. It is assumed that it differs significantly from that of a white dwarf, whose equation of state is that of a degenerate gas that can be described in close agreement with special relativity. However, with a neutron star the increased effects of general relativity can no longer be ignored. Several equations of state have been proposed (FPS, UU, APR, L, SLy, and others) and current research is still attempting to constrain the theories to make predictions of neutron star matter. This means that the relation between density and mass is not fully known, and this causes uncertainties in radius estimates. For example, a neutron star could have a radius of 10.7, 11.1, 12.1 or 15.1 kilometers (for EOS FPS, UU, APR or L respectively).


title: Neutron star

text: Current understanding of the structure of neutron stars is defined by existing mathematical models, but it might be possible to infer some details through studies of neutron-star oscillations. Asteroseismology, a study applied to ordinary stars, can reveal the inner structure of neutron stars by analyzing observed spectra of stellar oscillations.


title: Neutron star

text: Current models indicate that matter at the surface of a neutron star is composed of ordinary atomic nuclei crushed into a solid lattice with a sea of electrons flowing through the gaps between them. It is possible that the nuclei at the surface are iron, due to iron's high binding energy per nucleon. It is also possible that heavy elements, such as iron, simply sink beneath the surface, leaving only light nuclei like helium and hydrogen. If the surface temperature exceeds 10 kelvins (as in the case of a young pulsar), the surface should be fluid instead of the solid phase that might exist in cooler neutron stars (temperature <10 kelvins).


title: Neutron star

text: The "atmosphere" of a neutron star is hypothesized to be at most several micrometres thick, and its dynamics are fully controlled by the neutron star's magnetic field. Below the atmosphere one encounters a solid "crust". This crust is extremely hard and very smooth (with maximum surface irregularities on the order of millimetres or less), due to the extreme gravitational field.


title: Neutron star

text: Proceeding inward, one encounters nuclei with ever-increasing numbers of neutrons; such nuclei would decay quickly on Earth, but are kept stable by tremendous pressures. As this process continues at increasing depths, the neutron drip becomes overwhelming, and the concentration of free neutrons increases rapidly. In that region, there are nuclei, free electrons, and free neutrons. The nuclei become increasingly small (gravity and pressure overwhelming the strong force) until the core is reached, by definition the point where mostly neutrons exist. The expected hierarchy of phases of nuclear matter in the inner crust has been characterized as "nuclear pasta", with fewer voids and larger structures towards higher pressures.


title: Neutron star

text: The composition of the superdense matter in the core remains uncertain. One model describes the core as superfluid neutron-degenerate matter (mostly neutrons, with some protons and electrons). More exotic forms of matter are possible, including degenerate strange matter (containing strange quarks in addition to up and down quarks), matter containing high-energy pions and kaons in addition to neutrons, or ultra-dense quark-degenerate matter.


title: Neutron star

text: Neutron stars are detected from their electromagnetic radiation. Neutron stars are usually observed to pulse radio waves and other electromagnetic radiation, and neutron stars observed with pulses are called pulsars.


title: Neutron star

text: Pulsars' radiation is thought to be caused by particle acceleration near their magnetic poles, which need not be aligned with the rotational axis of the neutron star. It is thought that a large electrostatic field builds up near the magnetic poles, leading to electron emission. These electrons are magnetically accelerated along the field lines, leading to curvature radiation, with the radiation being strongly polarized towards the plane of curvature. In addition, high-energy photons can interact with lower-energy photons and the magnetic field for electron−positron pair production, which through electron–positron annihilation leads to further high-energy photons.


title: Neutron star

text: The radiation emanating from the magnetic poles of neutron stars can be described as "magnetospheric radiation", in reference to the magnetosphere of the neutron star. It is not to be confused with "magnetic dipole radiation", which is emitted because the magnetic axis is not aligned with the rotational axis, with a radiation frequency the same as the neutron star's rotational frequency.


title: Neutron star

text: If the axis of rotation of the neutron star is different from the magnetic axis, external viewers will only see these beams of radiation whenever the magnetic axis point towards them during the neutron star rotation. Therefore, periodic pulses are observed, at the same rate as the rotation of the neutron star.


title: Neutron star

text: In May 2022, astronomers reported an ultra-long-period radio-emitting neutron star PSR J0901-4046, with spin properties distinct from the known neutron stars. It is unclear how its radio emission is generated, and it challenges the current understanding of how pulsars evolve.


title: Neutron star

text: In addition to pulsars, non-pulsating neutron stars have also been identified, although they may have minor periodic variation in luminosity. This seems to be a characteristic of the X-ray sources known as Central Compact Objects in Supernova remnants (CCOs in SNRs), which are thought to be young, radio-quiet isolated neutron stars.


title: Neutron star

text: In addition to radio emissions, neutron stars have also been identified in other parts of the electromagnetic spectrum. This includes visible light, near infrared, ultraviolet, X-rays, and gamma rays. Pulsars observed in X-rays are known as X-ray pulsars if accretion-powered, while those identified in visible light are known as optical pulsars. The majority of neutron stars detected, including those identified in optical, X-ray, and gamma rays, also emit radio waves; the Crab Pulsar produces electromagnetic emissions across the spectrum. However, there exist neutron stars called radio-quiet neutron stars, with no radio emissions detected.


title: Neutron star

text: Neutron stars rotate extremely rapidly after their formation due to the conservation of angular momentum; in analogy to spinning ice skaters pulling in their arms, the slow rotation of the original star's core speeds up as it shrinks. A newborn neutron star can rotate many times a second.


title: Neutron star

text: Over time, neutron stars slow, as their rotating magnetic fields in effect radiate energy associated with the rotation; older neutron stars may take several seconds for each revolution. This is called "spin down". The rate at which a neutron star slows its rotation is usually constant and very small.


title: Neutron star

text: The periodic time ("P") is the rotational period, the time for one rotation of a neutron star. The spin-down rate, the rate of slowing of rotation, is then given the symbol formula_8 ("P"-dot), the derivative of "P" with respect to time. It is defined as periodic time increase per unit time; it is a dimensionless quantity, but can be given the units of s⋅s (seconds per second).


title: Neutron star

text: The spin-down rate ("P"-dot) of neutron stars usually falls within the range of 10 to 10 s⋅s, with the shorter period (or faster rotating) observable neutron stars usually having smaller "P"-dot. As a neutron star ages, its rotation slows (as "P" increases); eventually, the rate of rotation will become too slow to power the radio-emission mechanism, and the neutron star can no longer be detected.


title: Neutron star

text: "P" and "P"-dot allow minimum magnetic fields of neutron stars to be estimated. "P" and "P"-dot can be also used to calculate the "characteristic age" of a pulsar, but gives an estimate which is somewhat larger than the true age when it is applied to young pulsars.


title: Neutron star

text: "P" and "P"-dot can also be combined with neutron star's moment of inertia to estimate a quantity called "spin-down luminosity", which is given the symbol formula_9 ("E"-dot). It is not the measured luminosity, but rather the calculated loss rate of rotational energy that would manifest itself as radiation. For neutron stars where the spin-down luminosity is comparable to the actual luminosity, the neutron stars are said to be "rotation powered". The observed luminosity of the Crab Pulsar is comparable to the spin-down luminosity, supporting the model that rotational kinetic energy powers the radiation from it. With neutron stars such as magnetars, where the actual luminosity exceeds the spin-down luminosity by about a factor of one hundred, it is assumed that the luminosity is powered by magnetic dissipation, rather than being rotation powered.


title: Neutron star

text: "P" and "P"-dot can also be plotted for neutron stars to create a "P"–"P"-dot diagram. It encodes a tremendous amount of information about the pulsar population and its properties, and has been likened to the Hertzsprung–Russell diagram in its importance for neutron stars.


title: Neutron star

text: Neutron star rotational speeds can increase, a process known as spin up. Sometimes neutron stars absorb orbiting matter from companion stars, increasing the rotation rate and reshaping the neutron star into an oblate spheroid. This causes an increase in the rate of rotation of the neutron star of over a hundred times per second in the case of millisecond pulsars.


title: Neutron star

text: The most rapidly rotating neutron star currently known, PSR J1748-2446ad, rotates at 716 revolutions per second. A 2007 paper reported the detection of an X-ray burst oscillation, which provides an indirect measure of spin, of 1122 Hz from the neutron star XTE J1739-285, suggesting 1122 rotations a second. However, at present, this signal has only been seen once, and should be regarded as tentative until confirmed in another burst from that star.


title: Neutron star

text: Sometimes a neutron star will undergo a glitch, a sudden small increase of its rotational speed or spin up. Glitches are thought to be the effect of a starquake—as the rotation of the neutron star slows, its shape becomes more spherical. Due to the stiffness of the "neutron" crust, this happens as discrete events when the crust ruptures, creating a starquake similar to earthquakes. After the starquake, the star will have a smaller equatorial radius, and because angular momentum is conserved, its rotational speed has increased.


title: Neutron star

text: Starquakes occurring in magnetars, with a resulting glitch, is the leading hypothesis for the gamma-ray sources known as soft gamma repeaters.


title: Neutron star

text: Recent work, however, suggests that a starquake would not release sufficient energy for a neutron star glitch; it has been suggested that glitches may instead be caused by transitions of vortices in the theoretical superfluid core of the neutron star from one metastable energy state to a lower one, thereby releasing energy that appears as an increase in the rotation rate.


title: Neutron star

text: An "anti-glitch", a sudden small decrease in rotational speed, or spin down, of a neutron star has also been reported. It occurred in the magnetar 1E 2259+586, that in one case produced an X-ray luminosity increase of a factor of 20, and a significant spin-down rate change. Current neutron star models do not predict this behavior. If the cause was internal, it suggests differential rotation of solid outer crust and the superfluid component of the magnetar's inner structure.


title: Neutron star

text: At present, there are about 3,200 known neutron stars in the Milky Way and the Magellanic Clouds, the majority of which have been detected as radio pulsars. Neutron stars are mostly concentrated along the disk of the Milky Way, although the spread perpendicular to the disk is large because the supernova explosion process can impart high translational speeds (400 km/s) to the newly formed neutron star.


title: Neutron star

text: Some of the closest known neutron stars are RX J1856.5−3754, which is about 400 light-years from Earth, and PSR J0108−1431 about 424 light years. RX J1856.5-3754 is a member of a close group of neutron stars called The Magnificent Seven. Another nearby neutron star that was detected transiting the backdrop of the constellation Ursa Minor has been nicknamed Calvera by its Canadian and American discoverers, after the villain in the 1960 film "The Magnificent Seven". This rapidly moving object was discovered using the ROSAT/Bright Source Catalog.


title: Neutron star

text: Neutron stars are only detectable with modern technology during the earliest stages of their lives (almost always less than 1 million years) and are vastly outnumbered by older neutron stars that would only be detectable through their blackbody radiation and gravitational effects on other stars.


title: Neutron star

text: About 5% of all known neutron stars are members of a binary system. The formation and evolution of binary neutron stars and double neutron stars can be a complex process. Neutron stars have been observed in binaries with ordinary main-sequence stars, red giants, white dwarfs, or other neutron stars. According to modern theories of binary evolution, it is expected that neutron stars also exist in binary systems with black hole companions. The merger of binaries containing two neutron stars, or a neutron star and a black hole, has been observed through the emission of gravitational waves.


title: Neutron star

text: Binary systems containing neutron stars often emit X-rays, which are emitted by hot gas as it falls towards the surface of the neutron star. The source of the gas is the companion star, the outer layers of which can be stripped off by the gravitational force of the neutron star if the two stars are sufficiently close. As the neutron star accretes this gas, its mass can increase; if enough mass is accreted, the neutron star may collapse into a black hole.


title: Neutron star

text: The distance between two neutron stars in a close binary system is observed to shrink as gravitational waves are emitted. Ultimately, the neutron stars will come into contact and coalesce.


title: Neutron star

text: The coalescence of binary neutron stars is one of the leading models for the origin of short gamma-ray bursts. Strong evidence for this model came from the observation of a kilonova associated with the short-duration gamma-ray burst GRB 130603B, and finally confirmed by detection of gravitational wave GW170817 and short GRB 170817A by LIGO, Virgo, and 70 observatories covering the electromagnetic spectrum observing the event. The light emitted in the kilonova is believed to come from the radioactive decay of material ejected in the merger of the two neutron stars. This material may be responsible for the production of many of the chemical elements beyond iron, as opposed to the supernova nucleosynthesis theory.


title: Neutron star

text: Neutron stars can host exoplanets. These can be original, circumbinary, captured, or the result of a second round of planet formation. Pulsars can also strip the atmosphere off from a star, leaving a planetary-mass remnant, which may be understood as a chthonian planet or a stellar object depending on interpretation. For pulsars, such pulsar planets can be detected with the pulsar timing method, which allows for high precision and detection of much smaller planets than with other methods. Two systems have been definitively confirmed. The first exoplanets ever to be detected were the three planets Draugr, Poltergeist and Phobetor around PSR B1257+12, discovered in 1992–1994. Of these, Draugr is the smallest exoplanet ever detected, at a mass of twice that of the Moon. Another system is PSR B1620−26, where a circumbinary planet orbits a neutron star-white dwarf binary system. Also, there are several unconfirmed candidates. Pulsar planets receive little visible light, but massive amounts of ionizing radiation and high-energy stellar wind, which makes them rather hostile environments.


title: Neutron star

text: At the meeting of the American Physical Society in December 1933 (the proceedings were published in January 1934), Walter Baade and Fritz Zwicky proposed the existence of neutron stars, less than two years after the discovery of the neutron by James Chadwick. In seeking an explanation for the origin of a supernova, they tentatively proposed that in supernova explosions ordinary stars are turned into stars that consist of extremely closely packed neutrons that they called neutron stars. Baade and Zwicky correctly proposed at that time that the release of the gravitational binding energy of the neutron stars powers the supernova: "In the supernova process, mass in bulk is annihilated". Neutron stars were thought to be too faint to be detectable and little work was done on them until November 1967, when Franco Pacini pointed out that if the neutron stars were spinning and had large magnetic fields, then electromagnetic waves would be emitted. Unbeknown to him, radio astronomer Antony Hewish and his research assistant Jocelyn Bell at Cambridge were shortly to detect radio pulses from stars that are now believed to be highly magnetized, rapidly spinning neutron stars, known as pulsars.


title: Neutron star

text: In 1965, Antony Hewish and Samuel Okoye discovered "an unusual source of high radio brightness temperature in the Crab Nebula". This source turned out to be the Crab Pulsar that resulted from the great supernova of 1054.


title: Neutron star

text: In 1967, Iosif Shklovsky examined the X-ray and optical observations of Scorpius X-1 and correctly concluded that the radiation comes from a neutron star at the stage of accretion.


title: Neutron star

text: In 1967, Jocelyn Bell Burnell and Antony Hewish discovered regular radio pulses from PSR B1919+21. This pulsar was later interpreted as an isolated, rotating neutron star. The energy source of the pulsar is the rotational energy of the neutron star. The majority of known neutron stars (about 2000, as of 2010) have been discovered as pulsars, emitting regular radio pulses.


title: Neutron star

text: In 1968, Richard V. E. Lovelace and collaborators discovered period formula_10 ms of the Crab pulsar using Arecibo Observatory. After this discovery, scientists concluded that pulsars were rotating neutron stars. Before that, many scientists believed that pulsars were pulsating white dwarfs.


title: Neutron star

text: In 1971, Riccardo Giacconi, Herbert Gursky, Ed Kellogg, R. Levinson, E. Schreier, and H. Tananbaum discovered 4.8 second pulsations in an X-ray source in the constellation Centaurus, Cen X-3. They interpreted this as resulting from a rotating hot neutron star. The energy source is gravitational and results from a rain of gas falling onto the surface of the neutron star from a companion star or the interstellar medium.


title: Neutron star

text: In 1974, Antony Hewish was awarded the Nobel Prize in Physics "for his decisive role in the discovery of pulsars" without Jocelyn Bell who shared in the discovery.


title: Neutron star

text: In 1974, Joseph Taylor and Russell Hulse discovered the first binary pulsar, PSR B1913+16, which consists of two neutron stars (one seen as a pulsar) orbiting around their center of mass. Albert Einstein's general theory of relativity predicts that massive objects in short binary orbits should emit gravitational waves, and thus that their orbit should decay with time. This was indeed observed, precisely as general relativity predicts, and in 1993, Taylor and Hulse were awarded the Nobel Prize in Physics for this discovery.


title: Neutron star

text: In 1982, Don Backer and colleagues discovered the first millisecond pulsar, PSR B1937+21. This object spins 642 times per second, a value that placed fundamental constraints on the mass and radius of neutron stars. Many millisecond pulsars were later discovered, but PSR B1937+21 remained the fastest-spinning known pulsar for 24 years, until PSR J1748-2446ad (which spins ~716 times a second) was discovered.


title: Neutron star

text: In 2003, Marta Burgay and colleagues discovered the first double neutron star system where both components are detectable as pulsars, PSR J0737−3039. The discovery of this system allows a total of 5 different tests of general relativity, some of these with unprecedented precision.


title: Neutron star

text: In 2010, Paul Demorest and colleagues measured the mass of the millisecond pulsar PSR J1614−2230 to be , using Shapiro delay. This was substantially higher than any previously measured neutron star mass (, see PSR J1903+0327), and places strong constraints on the interior composition of neutron stars.


title: Neutron star

text: In 2013, John Antoniadis and colleagues measured the mass of PSR J0348+0432 to be , using white dwarf spectroscopy. This confirmed the existence of such massive stars using a different method. Furthermore, this allowed, for the first time, a test of general relativity using such a massive neutron star.


title: Neutron star

text: In August 2017, LIGO and Virgo made first detection of gravitational waves produced by colliding neutron stars.


title: Neutron star

text: In October 2018, astronomers reported that GRB 150101B, a gamma-ray burst event detected in 2015, may be directly related to the historic GW170817 and associated with the merger of two neutron stars. The similarities between the two events, in terms of gamma ray, optical and x-ray emissions, as well as to the nature of the associated host galaxies, are "striking", suggesting the two separate events may both be the result of the merger of neutron stars, and both may be a kilonova, which may be more common in the universe than previously understood, according to the researchers.


title: Neutron star

text: In July 2019, astronomers reported that a new method to determine the Hubble constant, and resolve the discrepancy of earlier methods, has been proposed based on the mergers of pairs of neutron stars, following the detection of the neutron star merger of GW170817. Their measurement of the Hubble constant is (km/s)/Mpc.


title: Neutron star

text: A 2020 study by University of Southampton PhD student Fabian Gittins suggested that surface irregularities ("mountains") may only be fractions of a millimeter tall (about 0.000003% of the neutron star's diameter), hundreds of times smaller than previously predicted, a result bearing implications for the non-detection of gravitational waves from spinning neutron stars.


title: Saint Patrick's Day

text: Saint Patrick's Day, or the Feast of Saint Patrick (), is a cultural and religious celebration held on 17 March, the traditional death date of Saint Patrick (), the foremost patron saint of Ireland.


title: Saint Patrick's Day

text: Saint Patrick's Day was made an official Christian feast day in the early 17th century and is observed by the Catholic Church, the Anglican Communion (especially the Church of Ireland), the Eastern Orthodox Church, and the Lutheran Church. The day commemorates Saint Patrick and the arrival of Christianity in Ireland, and celebrates the heritage and culture of the Irish in general. Celebrations generally involve public parades and festivals, céilithe, and the wearing of green attire or shamrocks. Christians who belong to liturgical denominations also attend church services and historically the Lenten restrictions on eating and drinking alcohol were lifted for the day, which has encouraged and propagated the holiday's tradition of alcohol consumption.


title: Saint Patrick's Day

text: Saint Patrick's Day is a public holiday in the Republic of Ireland, Northern Ireland, the Canadian province of Newfoundland and Labrador (for provincial government employees), and the British Overseas Territory of Montserrat. It is also widely celebrated in the United Kingdom, Canada, United States, Argentina, Australia and New Zealand, especially amongst Irish diaspora. Saint Patrick's Day is celebrated in more countries than any other national festival. Modern celebrations have been greatly influenced by those of the Irish diaspora, particularly those that developed in North America. However, there has been criticism of Saint Patrick's Day celebrations for having become too commercialised and for fostering negative stereotypes of the Irish people.


title: Saint Patrick's Day

text: Saint Patrick was a 5th-century Romano-British Christian missionary and Bishop in Ireland. Much of what is known about Saint Patrick comes from the "Declaration", which was allegedly written by Patrick himself. It is believed that he was born in Roman Britain in the fourth century, into a wealthy Romano-British family. His father was a deacon and his grandfather was a priest in the Christian church. According to the "Declaration", at the age of sixteen, he was kidnapped by Irish raiders and taken as a slave to Gaelic Ireland. It says that he spent six years there working as a shepherd and that during this time he found God. The "Declaration" says that God told Patrick to flee to the coast, where a ship would be waiting to take him home. After making his way home, Patrick went on to become a priest.


title: Saint Patrick's Day

text: According to tradition, Patrick returned to Ireland to convert the pagan Irish to Christianity. The "Declaration" says that he spent many years evangelising in the northern half of Ireland and converted thousands.


title: Saint Patrick's Day

text: Patrick's efforts were eventually turned into an allegory in which he drove "snakes" out of Ireland, despite the fact that snakes were not known to inhabit the region.


title: Saint Patrick's Day

text: Tradition holds that he died on 17 March and was buried at Downpatrick. Over the following centuries, many legends grew up around Patrick and he became Ireland's foremost saint.


title: Saint Patrick's Day

text: Today's Saint Patrick's Day celebrations have been greatly influenced by those that developed among the Irish diaspora, especially in North America. Until the late 20th century, Saint Patrick's Day was often a bigger celebration among the diaspora than it was in Ireland.


title: Saint Patrick's Day

text: Celebrations generally involve public parades and festivals, Irish traditional music sessions (céilithe), and the wearing of green attire or shamrocks. There are also formal gatherings such as banquets and dances, although these were more common in the past. Saint Patrick's Day parades began in North America in the 18th century but did not spread to Ireland until the 20th century. The participants generally include marching bands, the military, fire brigades, cultural organisations, charitable organisations, voluntary associations, youth groups, fraternities, and so on. However, over time, many of the parades have become more akin to a carnival. More effort is made to use the Irish language, especially in Ireland, where 1 March to St Patrick's Day on 17 March is Seachtain na Gaeilge ("Irish language week").


title: Saint Patrick's Day

text: Since 2010, famous landmarks have been lit up in green on Saint Patrick's Day as part of Tourism Ireland's "Global Greening Initiative" or "Going Green for St Patrick's Day". The Sydney Opera House and the Sky Tower in Auckland were the first landmarks to participate and since then over 300 landmarks in fifty countries across the globe have gone green for Saint Patricks day.


title: Saint Patrick's Day

text: Christians may also attend church services, and the Lenten restrictions on eating and drinking alcohol are lifted for the day. Perhaps because of this, drinking alcohol – particularly Irish whiskey, beer, or cider – has become an integral part of the celebrations. The Saint Patrick's Day custom of "drowning the shamrock" or "wetting the shamrock" was historically popular. At the end of the celebrations, especially in Ireland, a shamrock is put into the bottom of a cup, which is then filled with whiskey, beer, or cider. It is then drunk as a toast to Saint Patrick, Ireland, or those present. The shamrock would either be swallowed with the drink or taken out and tossed over the shoulder for good luck.


title: Saint Patrick's Day

text: Irish Government Ministers travel abroad on official visits to various countries around the globe to celebrate Saint Patrick's Day and promote Ireland. The most prominent of these is the visit of the Irish Taoiseach (Irish Prime Minister) with the U.S. President which happens on or around Saint Patrick's Day. Traditionally the Taoiseach presents the U.S. President a Waterford Crystal bowl filled with shamrocks. This tradition began when in 1952, Irish Ambassador to the U.S. John Hearne sent a box of shamrocks to President Harry S. Truman. From then on it became an annual tradition of the Irish ambassador to the U.S. to present the Saint Patrick's Day shamrock to an official in the U.S. President's administration, although on some occasions the shamrock presentation was made by the Irish Taoiseach or Irish President to the U.S. President personally in Washington, such as when President Dwight D. Eisenhower met Taoiseach John A. Costello in 1956 and President Seán T. O'Kelly in 1959 or when President Ronald Reagan met Taoiseach Garret FitzGerald in 1986 and Taoiseach Charles Haughey in 1987. However it was only after the meeting between Taoiseach Albert Reynolds and President Bill Clinton in 1994 that the presenting of the shamrock ceremony became an annual event for the leaders of both countries for Saint Patrick's Day. The presenting of the Shamrock ceremony was cancelled in 2020 due to the severity of the COVID-19 pandemic.


title: Saint Patrick's Day

text: On Saint Patrick's Day, it is customary to wear shamrocks, green clothing or green accessories. Saint Patrick is said to have used the shamrock, a three-leaved plant, to explain the Holy Trinity to the pagan Irish. This story first appears in writing in 1726, though it may be older. In pagan Ireland, three was a significant number and the Irish had many triple deities, which may have aided St Patrick in his evangelisation efforts. Roger Homan writes, "We can perhaps see St Patrick drawing upon the visual concept of the "triskele" when he uses the shamrock to explain the Trinity". Patricia Monaghan says there is no evidence the shamrock was sacred to the pagan Irish. Jack Santino speculates that it may have represented the regenerative powers of nature, and was recast in a Christian contexticons of St Patrick often depict the saint "with a cross in one hand and a sprig of shamrocks in the other".


title: Saint Patrick's Day

text: The first association of the colour green with Ireland is from a legend in the 11th century "Lebor Gabála Érenn" (The Book of the Taking of Ireland). It tells of Goídel Glas (Goídel the green), the eponymous ancestor of the Gaels and creator of the Goidelic languages (Irish, Scottish Gaelic, Manx). Goídel is bitten by a venomous snake but saved from death by Moses placing his staff on the snakebite, leaving him with a green mark. His descendants settle in Ireland, a land free of snakes. One of these, Íth, climbs the Tower of Hercules and is so captivated by the sight of a beautiful green island in the distance that he must set sail immediately.


title: Saint Patrick's Day

text: The colour green was further associated with Ireland from the 1640s, when the green harp flag was used by the Irish Catholic Confederation. Later, James Connolly described this flag as representing "the sacred emblem of Ireland's unconquered soul". Green ribbons and shamrocks have been worn on St Patrick's Day since at least the 1680s. Since then, the colour green and its association with St Patrick's Day have grown. The Friendly Brothers of St Patrick, an Irish fraternity founded in about 1750, adopted green as its colour. The Order of St Patrick, an Anglo-Irish chivalric order founded in 1783, instead adopted blue as its colour, which led to blue being associated with St Patrick. In the 1790s, the colour green was adopted by the United Irishmen. This was a republican organisation—led mostly by Protestants but with many Catholic members—who launched a rebellion in 1798 against British rule. Ireland was first called "the Emerald Isle" in "When Erin First Rose" (1795), a poem by a co-founder of the United Irishmen, William Drennan, which stresses the historical importance of green to the Irish. The phrase "wearing of the green" comes from a song of the same name about United Irishmen being persecuted for wearing green. The flags of the 1916 Easter Rising featured green, such as the Starry Plough banner and the Proclamation Flag of the Irish Republic. When the Irish Free State was founded in 1922, the government ordered all post boxes be painted green, under the slogan "green paint for a green people"; in 1924, the government introduced a green Irish passport.


title: Saint Patrick's Day

text: The wearing of the 'St Patrick's Day Cross' was also a popular custom in Ireland until the early 20th century. These were a Celtic Christian cross made of paper that was "covered with silk or ribbon of different colours, and a bunch or rosette of green silk in the centre".


title: Saint Patrick's Day

text: Saint Patrick's feast day, as a kind of national day, was already being celebrated by the Irish in Europe in the ninth and tenth centuries. In later times, he became more and more widely seen as the patron of Ireland. Saint Patrick's feast day was finally placed on the universal liturgical calendar in the Catholic Church due to the influence of Waterford-born Franciscan scholar Luke Wadding in the early 1600s. Saint Patrick's Day thus became a holy day of obligation for Roman Catholics in Ireland. It is also a feast day in the Church of Ireland, which is part of the worldwide Anglican Communion. The church calendar avoids the observance of saints' feasts during certain solemnities, moving the saint's day to a time outside those periods. St Patrick's Day is occasionally affected by this requirement, when 17 March falls during Holy Week. This happened in 1940, when Saint Patrick's Day was observed on 3 April to avoid it coinciding with Palm Sunday, and again in 2008, where it was officially observed on 15 March. St Patrick's Day will not fall within Holy Week again until 2160. However, the popular festivities may still be held on 17 March or on a weekend near to the feast day.


title: Saint Patrick's Day

text: In 1903, St Patrick's Day became an official public holiday in Ireland. This was thanks to the Bank Holiday (Ireland) Act 1903, an act of the United Kingdom Parliament introduced by Irish Member of Parliament James O'Mara.


title: Saint Patrick's Day

text: The first St Patrick's Day parade in Ireland was held in Waterford in 1903. The week of St Patrick's Day 1903 had been declared Irish Language Week by the Gaelic League and in Waterford they opted to have a procession on Sunday 15 March. The procession comprised the Mayor and members of Waterford Corporation, the Trades Hall, the various trade unions and bands who included the 'Barrack St Band' and the 'Thomas Francis Meagher Band'. The parade began at the premises of the Gaelic League in George's St and finished in the Peoples Park, where the public were addressed by the Mayor and other dignitaries. On Tuesday 17 March, most Waterford businesses—including public houses—were closed and marching bands paraded as they had two days previously. The Waterford Trades Hall had been emphatic that the National Holiday be observed.


title: Saint Patrick's Day

text: On St Patrick's Day 1916, the Irish Volunteers—an Irish nationalist paramilitary organisation—held parades throughout Ireland. The authorities recorded 38 St Patrick's Day parades, involving 6,000 marchers, almost half of whom were said to be armed. The following month, the Irish Volunteers launched the Easter Rising against British rule. This marked the beginning of the Irish revolutionary period and led to the Irish War of Independence and Civil War. During this time, St Patrick's Day celebrations in Ireland were muted, although the day was sometimes chosen to hold large political rallies. The celebrations remained low-key after the creation of the Irish Free State; the only state-organized observance was a military procession and trooping of the colours, and an Irish-language mass attended by government ministers. In 1927, the Irish Free State government banned the selling of alcohol on St Patrick's Day, although it remained legal in Northern Ireland. The ban was not repealed until 1961.


title: Saint Patrick's Day

text: The first official, state-sponsored St Patrick's Day parade in Dublin took place in 1931. On three occasions, parades across the Republic of Ireland have been cancelled from taking place on St Patrick's Day, with all years involving health and safety reasons. In 2001, as a precaution to the foot-and-mouth outbreak, St Patrick's Day celebrations were postponed to May and in 2020 and 2021, as a consequence to the severity of the COVID-19 pandemic, the St Patrick's Day Parade was cancelled outright. Organisers of the St Patrick's Day Festival 2021 will instead host virtual events around Ireland on their SPF TV online channel.


title: Saint Patrick's Day

text: In Northern Ireland, the celebration of St Patrick's Day was affected by sectarian divisions. A majority of the population were Protestant Ulster unionists who saw themselves as British, while a substantial minority were Catholic Irish nationalists who saw themselves as Irish. Although it was a public holiday, Northern Ireland's unionist government did not officially observe St Patrick's Day. During the conflict known as the Troubles (late 1960s–late 1990s), public St Patrick's Day celebrations were rare and tended to be associated with the Catholic community. In 1976, loyalists detonated a car bomb outside a pub crowded with Catholics celebrating St Patrick's Day in Dungannon; four civilians were killed and many injured. However, some Protestant unionists attempted to 're-claim' the festival, and in 1985 the Orange Order held its own St Patrick's Day parade. Since the end of the conflict in 1998 there have been cross-community St Patrick's Day parades in towns throughout Northern Ireland, which have attracted thousands of spectators.


title: Saint Patrick's Day

text: In the mid-1990s the government of the Republic of Ireland began a campaign to use St Patrick's Day to showcase Ireland and its culture. The government set up a group called St Patrick's Festival, with the aims:


title: Saint Patrick's Day

text: The first St Patrick's Festival was held on 17 March 1996. In 1997, it became a three-day event, and by 2000 it was a four-day event. By 2006, the festival was five days long; more than 675,000 people attended the 2009 parade. Overall 2009's five-day festival saw almost 1 million visitors, who took part in festivities that included concerts, outdoor theatre performances, and fireworks. The Skyfest which ran from 2006 to 2012 formed the centrepiece of the St Patrick's festival.


title: Saint Patrick's Day

text: The topic of the 2004 St Patrick's Symposium was "Talking Irish", during which the nature of Irish identity, economic success, and the future were discussed. Since 1996, there has been a greater emphasis on celebrating and projecting a fluid and inclusive notion of "Irishness" rather than an identity based around traditional religious or ethnic allegiance. The week around St Patrick's Day usually involves Irish language speakers using more Irish during "Seachtain na Gaeilge" ("Irish Language Week").


title: Saint Patrick's Day

text: Christian leaders in Ireland have expressed concern about the secularisation of St Patrick's Day. In "The Word" magazine's March 2007 issue, Fr Vincent Twomey wrote, "It is time to reclaim St Patrick's Day as a church festival". He questioned the need for "mindless alcohol-fuelled revelry" and concluded that "it is time to bring the piety and the fun together".


title: Saint Patrick's Day

text: The biggest celebrations outside the cities are in Downpatrick, County Down, where Saint Patrick is said to be buried. The shortest St. Patrick's Day parade in the world formerly took place in Dripsey, County Cork. The parade lasted just 23.4 metres and traveled between the village's two pubs. The annual event began in 1999, but ceased after five years when one of the two pubs closed.


title: Saint Patrick's Day

text: In England, the British Royals traditionally present bowls of shamrock to members of the Irish Guards, a regiment in the British Army, following Queen Alexandra introducing the tradition in 1901. Since 2012 the Duchess of Cambridge has presented the bowls of shamrock to the Irish Guards. While female royals are often tasked with presenting the bowls of shamrock, male royals have also undertaken the role, such as King George VI in 1950 to mark the 50th anniversary of the formation of the Irish Guards, and in 2016 the Duke of Cambridge in place of his wife. Fresh Shamrocks are presented to the Irish Guards, regardless of where they are stationed, and are flown in from Ireland.


title: Saint Patrick's Day

text: While some Saint Patrick's Day celebrations could be conducted openly in Britain pre 1960s, this would change following the commencement by the IRA's bombing campaign on mainland Britain and as a consequence this resulted in a suspicion of all things Irish and those who supported them which led to people of Irish descent wearing a sprig of shamrock on Saint Patrick's day in private or attending specific events. Today after many years following the Good Friday Agreement, people of Irish descent openly wear a sprig of shamrock to celebrate their Irishness.


title: Saint Patrick's Day

text: Christian denominations in Great Britain observing his feast day include The Church of England and the Roman Catholic Church.


title: Saint Patrick's Day

text: Birmingham holds the largest Saint Patrick's Day parade in Britain with a city centre parade over a two-mile (3 km) route through the city centre. The organisers describe it as the third biggest parade in the world after Dublin and New York.


title: Saint Patrick's Day

text: London, since 2002, has had an annual Saint Patrick's Day parade which takes place on weekends around the 17th, usually in Trafalgar Square. In 2008 the water in the Trafalgar Square fountains was dyed green. In 2020 the Parade was cancelled due to the COVID-19 pandemic.


title: Saint Patrick's Day

text: Liverpool has the highest proportion of residents with Irish ancestry of any English city. This has led to a long-standing celebration on St Patrick's Day in terms of music, cultural events and the parade.


title: Saint Patrick's Day

text: Manchester hosts a two-week Irish festival in the weeks prior to Saint Patrick's Day. The festival includes an Irish Market based at the city's town hall which flies the Irish tricolour opposite the Union Flag, a large parade as well as a large number of cultural and learning events throughout the two-week period.


title: Saint Patrick's Day

text: The first Saint Patrick's Day celebrations in Malta took place in the early 20th century by soldiers of the Royal Dublin Fusiliers who were stationed in Floriana. Celebrations were held in the Balzunetta area of the town, which contained a number of bars and was located close to the barracks. The Irish diaspora in Malta continued to celebrate the feast annually.


title: Saint Patrick's Day

text: Today, Saint Patrick's Day is mainly celebrated in Spinola Bay and Paceville areas of St Julian's, although other celebrations still occur at Floriana and other locations. Thousands of Maltese attend the celebrations, "which are more associated with drinking beer than traditional Irish culture."


title: Saint Patrick's Day

text: Norway has had a St. Patrick's Day parade in Oslo since 2000, first organized by Irish expatriates living in Norway, and partially coordinated with the Irish embassy in Oslo.


title: Saint Patrick's Day

text: The first Saint Patrick's Day parade in Russia took place in 1992. Since 1999, there has been a yearly "Saint Patrick's Day" festival in Moscow and other Russian cities. The official part of the Moscow parade is a military-style parade and is held in collaboration with the Moscow government and the Irish embassy in Moscow. The unofficial parade is held by volunteers and resembles a carnival. In 2014, Moscow Irish Week was celebrated from 12 to 23 March, which includes Saint Patrick's Day on 17 March. Over 70 events celebrating Irish culture in Moscow, St Petersburg, Yekaterinburg, Voronezh, and Volgograd were sponsored by the Irish Embassy, the Moscow City Government, and other organisations.


title: Saint Patrick's Day

text: In 2017, the Russian Orthodox Church added the feast day of Saint Patrick to its liturgical calendar, to be celebrated on .


title: Saint Patrick's Day

text: Sarajevo, the capital city of Bosnia and Herzegovina has a large Irish expatriate community. The community established the Sarajevo Irish Festival in 2015, which is held for three days around and including Saint Patrick's Day. The festival organizes an annual a parade, hosts Irish theatre companies, screens Irish films and organizes concerts of Irish folk musicians. The festival has hosted numerous Irish artists, filmmakers, theatre directors and musicians such as Conor Horgan, Ailis Ni Riain, Dermot Dunne, Mick Moloney, Chloë Agnew and others.


title: Saint Patrick's Day

text: The Scottish town of Coatbridge, where the majority of the town's population are of Irish descent, also has a Saint Patrick's Day Festival which includes celebrations and parades in the town centre.


title: Saint Patrick's Day

text: Glasgow has a considerably large Irish population; due, for the most part, to the Irish immigration during the 19th century. This immigration was the main cause in raising the population of Glasgow by over 100,000 people. Due to this large Irish population, there are many Irish-themed pubs and Irish interest groups who hold yearly celebrations on Saint Patrick's day in Glasgow. Glasgow has held a yearly Saint Patrick's Day parade and festival since 2007.


title: Saint Patrick's Day

text: While Saint Patrick's Day in Switzerland is commonly celebrated on 17 March with festivities similar to those in neighbouring central European countries, it is not unusual for Swiss students to organise celebrations in their own living spaces on Saint Patrick's Eve. Most popular are usually those in Zurich's Kreis 4. Traditionally, guests also contribute with beverages and dress in green.


title: Saint Patrick's Day

text: Although it is not a national holiday in Lithuania, the Vilnia River is dyed green every year on the Saint Patrick's Day in the capital Vilnius.


title: Saint Patrick's Day

text: One of the longest-running and largest Saint Patrick's Day () parades in North America occurs each year in Montreal, whose city flag includes a shamrock in its lower-right quadrant. The yearly celebration has been organised by the United Irish Societies of Montreal since 1929. The parade has been held yearly without interruption since 1824. St Patrick's Day itself, however, has been celebrated in Montreal since as far back as 1759 by Irish soldiers in the Montreal Garrison following the British conquest of New France.


title: Saint Patrick's Day

text: In Saint John, New Brunswick Saint Patrick's Day is celebrated as a week-long celebration. Shortly after the JP Collins Celtic Festival is an Irish festival celebrating Saint John's Irish heritage. The festival is named for a young Irish doctor James Patrick Collins who worked on Partridge Island (Saint John County) quarantine station tending to sick Irish immigrants before he died there himself.


title: Saint Patrick's Day

text: In Manitoba, the Irish Association of Manitoba runs a yearly three-day festival of music and culture based around St Patrick's Day.


title: Saint Patrick's Day

text: In 2004, the CelticFest Vancouver Society organised its first yearly festival in downtown Vancouver to celebrate the Celtic Nations and their cultures. This event, which includes a parade, occurs each year during the weekend nearest St Patrick's Day.


title: Saint Patrick's Day

text: In Quebec City, there was a parade from 1837 to 1926. The Quebec City St-Patrick Parade returned in 2010 after more than 84 years. For the occasion, a portion of the New York Police Department Pipes and Drums were present as special guests.


title: Saint Patrick's Day

text: The Toronto Maple Leafs hockey team was known as the Toronto St. Patricks from 1919 to 1927, and wore green jerseys. In 1999, when the Maple Leafs played on St Patrick's Day, they wore green St Patrick's retro uniforms.


title: Saint Patrick's Day

text: In March 2009, the Calgary Tower changed its top exterior lights to new green CFL bulbs just in time for St Patrick's Day. Part of an environmental non-profit organisation's campaign (Project Porchlight), the green represented environmental concerns. Approximately 210 lights were changed in time for Saint Patrick's Day, and resembled a Leprechaun's hat. After a week, white CFLs took their place. The change was estimated to save the Calgary Tower some $12,000 and reduce greenhouse gas emissions by 104 tonnes.


title: Saint Patrick's Day

text: Saint Patrick's Day, while not a legal holiday in the United States, is nonetheless widely recognised and observed throughout the country as a celebration of Irish and Irish-American culture. Celebrations include prominent displays of the colour green, religious observances, numerous parades, and copious consumption of alcohol. The holiday has been celebrated in what is now the U.S since 1601.


title: Saint Patrick's Day

text: In 2020, for the first time in over 250 years, the parade in New York City, the largest in the world, was postponed due to concerns about the COVID-19 pandemic.


title: Saint Patrick's Day

text: In Buenos Aires, a party is held in the downtown street of Reconquista, where there are several Irish pubs; in 2006, there were 50,000 people in this street and the pubs nearby. Neither the Catholic Church nor the Irish community, the fifth largest in the world outside Ireland, take part in the organisation of the parties.


title: Saint Patrick's Day

text: The island of Montserrat is known as the "Emerald Island of the Caribbean" because of its founding by Irish refugees from Saint Kitts and Nevis. Montserrat is one of three places where Saint Patrick's Day is a public holiday, along with Ireland and the Canadian province of Newfoundland and Labrador. The holiday in Montserrat also commemorates a failed slave uprising that occurred on 17 March 1768.


title: Saint Patrick's Day

text: St Patrick's Day is not a national holiday in Australia, although it is celebrated each year across the country's states and territories. Festivals and parades are often held on weekends around 17 March in cities such as Sydney, Brisbane, Adelaide, and Melbourne. On occasion, festivals and parades are cancelled. For instance, Melbourne's 2006 and 2007 St Patrick's Day festivals and parades were cancelled due to sporting events (Commonwealth Games and Australian Grand Prix) being booked on and around the planned St Patrick's Day festivals and parades in the city. In Sydney the parade and family day was cancelled in 2016 due to financial problems. However, Brisbane's St Patrick's Day parade, which was cancelled at the outbreak of World War II and wasn't revived until 1990, was not called off in 2020 as precaution for the COVID-19 pandemic, in contrast to many other St Patrick's Day parades around the world.


title: Saint Patrick's Day

text: The first mention of St Patrick's Day being celebrated in Australia was in 1795, when Irish convicts and administrators, Catholic and Protestant, in the penal colony came together to celebrate the day as a national holiday, despite a ban against assemblies being in place at the time. This unified day of Irish nationalist observance would soon dissipate over time, with celebrations on St Patrick's Day becoming divisive between religions and social classes, representative more of Australianness than of Irishness and held intermittingly throughout the years. Historian Patrick O'Farrell credits the 1916 Easter Rising in Dublin and Archbishop Daniel Mannix of Melbourne for re-igniting St Patrick's Day celebrations in Australia and reviving the sense of Irishness amongst those with Irish heritage. The organisers of the St Patrick's festivities in the past were, more often than not, the Catholic clergy which often courted controversy. Bishop Patrick Phelan of Sale described in 1921 how the authorities in Victoria had ordered that a Union Jack be flown at the front of the St Patrick's Day parade and following the refusal by Irishmen and Irish-Australians to do so, the authorities paid for an individual to carry the flag at the head of the parade. This individual was later assaulted by two men who were later fined in court.