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A white dwarf, also called a degenerate dwarf, is a stellar remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to that of the Sun, and its volume is comparable to that of Earth. A white dwarf's faint luminosity comes from the emission of stored thermal energy.〔 The nearest known white dwarf is Sirius B, at 8.6 light years, the smaller component of the Sirius binary star. There are currently thought to be eight white dwarfs among the hundred star systems nearest the Sun.〔 〕 The unusual faintness of white dwarfs was first recognized in 1910.〔 The name ''white dwarf'' was coined by Willem Luyten in 1922.〔 White dwarfs are thought to be the final evolutionary state of stars (including our Sun) whose mass is not high enough to become a neutron star—over 97% of the stars in the Milky Way.〔, §1. After the hydrogen–fusing period of a main-sequence star of low or medium mass ends, a star will expand to a red giant during which it fuses helium to carbon and oxygen in its core by the triple-alpha process. If a red giant has insufficient mass to generate the core temperatures required to fuse carbon, around 1 billion K, an inert mass of carbon and oxygen will build up at its center. After shedding its outer layers to form a planetary nebula, it will leave behind this core, which forms the remnant white dwarf.〔 〕 Usually, therefore, white dwarfs are composed of carbon and oxygen. If the mass of the progenitor is between 8 and 10.5 solar masses (), the core temperature is sufficient to fuse carbon but not neon, in which case an oxygen-neon–magnesium white dwarf may be formed.〔 〕 Stars of very low mass will not be able to fuse helium, hence, a helium white dwarf〔 〕〔 〕 may be formed by mass loss in binary systems. The material in a white dwarf no longer undergoes fusion reactions, so the star has no source of energy. As a result, it cannot support itself by the heat generated by fusion against gravitational collapse, but is supported only by electron degeneracy pressure, causing it to be extremely dense. The physics of degeneracy yields a maximum mass for a non-rotating white dwarf, the Chandrasekhar limit—approximately 1.4 —beyond which it cannot be supported by electron degeneracy pressure. A carbon-oxygen white dwarf that approaches this mass limit, typically by mass transfer from a companion star, may explode as a Type Ia supernova via a process known as carbon detonation.〔 〕〔 (SN 1006 is thought to be a famous example.) A white dwarf is very hot when it is formed, but since it has no source of energy, it will gradually radiate away its energy and cool. This means that its radiation, which initially has a high color temperature, will lessen and redden with time. Over a very long time, a white dwarf will cool to temperatures at which it will no longer emit significant heat or light, and it will become a cold ''black dwarf''.〔 However, the length of time it takes for a white dwarf to reach this state is calculated to be longer than the current age of the universe (approximately 13.8 billion years),〔 〕 and since no white dwarf can be older than the age of the universe, it is thought that no black dwarfs yet exist.〔〔 〕 The oldest white dwarfs still radiate at temperatures of a few thousand kelvins. == Discovery == The first white dwarf discovered was in the triple star system of 40 Eridani, which contains the relatively bright main sequence star 40 Eridani A, orbited at a distance by the closer binary system of the white dwarf 40 Eridani B and the main sequence red dwarf 40 Eridani C. The pair 40 Eridani B/C was discovered by William Herschel on 31 January 1783;〔 〕, p. 73 it was again observed by Friedrich Georg Wilhelm Struve in 1825 and by Otto Wilhelm von Struve in 1851.〔 〕〔 〕 In 1910, Henry Norris Russell, Edward Charles Pickering and Williamina Fleming discovered that, despite being a dim star, 40 Eridani B was of spectral type A, or white.〔 〕 In 1939, Russell looked back on the discovery:〔''White Dwarfs'', E. Schatzman, Amsterdam: North-Holland, 1958.〕, p. 1 I was visiting my friend and generous benefactor, Prof. Edward C. Pickering. With characteristic kindness, he had volunteered to have the spectra observed for all the stars—including comparison stars—which had been observed in the observations for stellar parallax which Hinks and I made at Cambridge, and I discussed. This piece of apparently routine work proved very fruitful—it led to the discovery that all the stars of very faint absolute magnitude were of spectral class M. In conversation on this subject (as I recall it), I asked Pickering about certain other faint stars, not on my list, mentioning in particular 40 Eridani B. Characteristically, he sent a note to the Observatory office and before long the answer came (I think from Mrs Fleming) that the spectrum of this star was A. I knew enough about it, even in these paleozoic days, to realize at once that there was an extreme inconsistency between what we would then have called "possible" values of the surface brightness and density. I must have shown that I was not only puzzled but crestfallen, at this exception to what looked like a very pretty rule of stellar characteristics; but Pickering smiled upon me, and said: "It is just these exceptions that lead to an advance in our knowledge", and so the white dwarfs entered the realm of study! The spectral type of 40 Eridani B was officially described in 1914 by Walter Adams.〔 〕 The white dwarf companion of Sirius, Sirius B, was next to be discovered. During the nineteenth century, positional measurements of some stars became precise enough to measure small changes in their location. Friedrich Bessel used position measurements to determine that the stars Sirius (α Canis Majoris) and Procyon (α Canis Minoris) were changing their positions periodically. In 1844 he predicted that both stars had unseen companions:〔 〕 If we were to regard ''Sirius'' and ''Procyon'' as double stars, the change of their motions would not surprise us; we should acknowledge them as necessary, and have only to investigate their amount by observation. But light is no real property of mass. The existence of numberless visible stars can prove nothing against the existence of numberless invisible ones. Bessel roughly estimated the period of the companion of Sirius to be about half a century;〔 C. A. F. Peters computed an orbit for it in 1851.〔 〕 It was not until 31 January 1862 that Alvan Graham Clark observed a previously unseen star close to Sirius, later identified as the predicted companion.〔 Walter Adams announced in 1915 that he had found the spectrum of Sirius B to be similar to that of Sirius.〔 〕 In 1917, Adriaan van Maanen discovered Van Maanen's Star, an isolated white dwarf.〔 〕 These three white dwarfs, the first discovered, are the so-called ''classical white dwarfs''.〔, p. 2 Eventually, many faint white stars were found which had high proper motion, indicating that they could be suspected to be low-luminosity stars close to the Earth, and hence white dwarfs. Willem Luyten appears to have been the first to use the term ''white dwarf'' when he examined this class of stars in 1922;〔〔 〕〔 〕〔 〕〔 〕 the term was later popularized by Arthur Stanley Eddington.〔〔 Despite these suspicions, the first non-classical white dwarf was not definitely identified until the 1930s. 18 white dwarfs had been discovered by 1939.〔, p. 3 Luyten and others continued to search for white dwarfs in the 1940s. By 1950, over a hundred were known,〔 〕 and by 1999, over 2,000 were known.〔 〕 Since then the Sloan Digital Sky Survey has found over 9,000 white dwarfs, mostly new.〔 〕 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「White dwarf」の詳細全文を読む スポンサード リンク
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