A white dwarf, also called a degenerate dwarf, is a stellar remnant composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored thermal energy. In January 2009, the Research Consortium on Nearby Stars project counted eight white dwarfs among the hundred star systems nearest the Sun. The unusual faintness of white dwarfs was first recognized in 1910 by Henry Norris Russell, Edward Charles Pickering, and Williamina Fleming;, p. 1 the name white dwarf was coined by Willem Luyten in 1922.
White dwarfs are thought to be the final evolutionary state of all stars whose mass is not high enough to become a neutron star—over 97% of the stars in our galaxy., §1. After the hydrogen–fusing lifetime of a main-sequence star of low or medium mass ends, it will expand to a red giant which 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 above 8 solar masses but below 10.5 solar masses, the core temperature suffices to fuse carbon but not neon, in which case an oxygen-neon–magnesium white dwarf may be formed. Also, some helium white dwarfs appear to have been 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, nor is it supported by the heat generated by fusion against gravitational collapse. It 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 solar masses—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 down. 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, since no white dwarf can be older than the age of the Universe (approximately 13.7 billion years), even the oldest white dwarfs still radiate at temperatures of a few thousand kelvins, and no black dwarfs are thought to exist yet.
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... conjectures that have been advanced involve a collision between a white dwarf and an asteroid, or the collision of a white dwarf with a black hole ... four alternative explanations for SCP 06F6, in plausibility order the tidal destruction of a CO white dwarf by an intermediate-mass black hole, a type Ia supernova exploding ...
... star Before accretion of material pushes a white dwarf close to the Chandrasekhar limit, accreted hydrogen-rich material on the surface may ignite in a less destructive type of ... Since the white dwarf's core remains intact, these surface explosions can be repeated as long as accretion continues ... Astronomers have also observed dwarf novae, which have smaller, more frequent luminosity peaks than classical novae ...
... which probably is a mira variable, and one white dwarf, with markedly contrasting spectra and whose proximity and mass characteristics indicate it as a symbiotic star ... its Roche lobe so that matter is transferred to the white dwarf and accumulates until a nova-like outburst occurs, caused by ignition of thermonuclear fusion ... the energy source of novae, but dissimilar to the dwarf novae ...
... If a carbon-oxygen white dwarf accreted enough matter to reach the Chandrasekhar limit of about 1.38 solar masses (for a non-rotating star), it would no longer be able to support the bulk of its plasma ... a substantial fraction of the matter in the white dwarf undergoes nuclear fusion, releasing enough energy (1–2 × 1044 joules) to unbind the star in a supernova explosion ... At this point it becomes a white dwarf star, composed primarily of carbon and oxygen ...
... AM CVn stars with a white-dwarf donor can be formed when a binary consisting of a white dwarf and a low-mass giant evolve through a common-envelope (CE) phase ... The outcome of the CE will be a double white-dwarf binary ... the least-massive (and the largest) of the two white dwarfs will fill its Roche lobe and start mass transfer to its companion ...
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