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A neutron star is a type of compact star that can result from the gravitational collapse of a massive star after a supernova. Neutron stars are the densest and smallest stars known to exist in the Universe; with a radius of only about 11–11.5 km (7 mi), they can have a mass of about twice that of the Sun. Neutron stars are composed almost entirely of neutrons, which are subatomic particles without net electrical charge and with slightly larger mass than protons. Neutron stars are very hot and are supported against further collapse by quantum degeneracy pressure due to the phenomenon described by the Pauli exclusion principle, which states that no two neutrons (or any other fermionic particles) can occupy the same place and quantum state simultaneously. A neutron star has a mass of at least 1.1 and perhaps up to 3 solar masses (), though the highest observed mass is Neutron stars typically have a surface temperature around ~.〔(Neutron star mass measurements )〕〔A neutron star's density increases as its mass increases, and its radius decreases non-linearly. ((NASA mass radius graph ))〕 Neutron stars have overall densities of to ( to times the density of the Sun),〔 derives from mass 2.68 kg / volume of star of radius 12 km; derives from mass per volume of star radius 11.9 km〕 which is comparable to the approximate density of an atomic nucleus of .〔(【引用サイトリンク】 title=Calculating a Neutron Star's Density ) NB 3 kg/m3 is 〕 The neutron star's density varies from below in the crust—increasing with depth—to above or deeper inside (denser than an atomic nucleus).〔(【引用サイトリンク】 title=Introduction to neutron stars )〕 A normal-sized matchbox containing neutron-star material would have a mass of approximately 5 trillion tons or ~1000 km3 of Earth rock. In general, compact stars of less than (the Chandrasekhar limit) are white dwarfs, whereas compact stars with a mass between and (the Tolman–Oppenheimer–Volkoff limit) should be neutron stars. The maximum observed mass of neutron stars is about . The smallest observed mass of a stellar black hole is about , though compact stars with more than will overcome the neutron degeneracy pressure and gravitational collapse will usually occur to produce a black hole.〔(), a star will collapse into a black hole.〕 Between and , hypothetical intermediate-mass stars such as quark stars and electroweak stars have been proposed, but none have been shown to exist. The equations of state of matter at such high densities are not precisely known because of the theoretical and empirical difficulties. Some neutron stars rotate very rapidly (up to 716 times a second, or approximately 43,000 revolutions per minute) and emit beams of electromagnetic radiation as pulsars. Indeed, the discovery of pulsars in 1967 first suggested that neutron stars exist. Gamma-ray bursts may be produced from rapidly rotating, high-mass stars that collapse to form a neutron star, or from the merger of binary neutron stars. There are thought to be around 100 million neutron stars in the galaxy, but they can only be easily detected in certain instances, such as if they are a pulsar or part of a binary system. Non-rotating and non-accreting neutron stars are virtually undetectable; however, the Hubble Space Telescope has observed one thermally radiating neutron star, called RX J185635-3754. ==Formation== Any main-sequence star with an initial mass of above has the potential to become 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 material 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 , neutron degeneracy pressure halts the contraction. The infalling outer atmosphere of the star is flung outwards, becoming a Type II or Type Ib supernova. The remnant left is a neutron star. If it has a mass greater than about , it collapses further to become a black hole. Other neutron stars are formed within close binaries. As the core of a massive star is compressed during a Type II, Type Ib or Type Ic supernova, and collapses into a neutron star, it retains most of its angular momentum. 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 gradually slows down. 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 1012 to 1013 m/s2 (more than 1011 times of that of Earth).〔 One measure of such immense gravity is the fact that neutron stars have an escape velocity ranging from 100,000 km/s to 150,000 km/s, that is, from a third to half the speed of light. Matter falling onto the surface of a neutron star would be accelerated to tremendous speed by the star's gravity. The force of impact would likely destroy the object's component atoms, rendering all its matter identical, in most respects, to the rest of the star. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Neutron star」の詳細全文を読む スポンサード リンク
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