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A white dwarf is the kind of star which a main-sequence star of low or medium mass will become in the last stage of its evolution. When such a star has become a red giant during its helium-burning phase, in which helium is fused to carbon and oxygen by the triple-alpha process, it generally has insufficient mass to generate the core temperatures required to fuse carbon. After shedding its outer layers to form a planetary nebula, it will leave behind an inert core, which forms the remnant white dwarf. Usually, therefore, white dwarfs are composed of carbon and oxygen, although some helium white dwarfs appear to have been formed by mass loss in binary systems. It is also possible that core temperatures suffice to fuse carbon but not neon, in which case an oxygen-neon-magnesium white dwarf may be formed.
The material in a white dwarf no longer undergoes fusion reactions, so the star has no source of energy, nor is it supported against gravitational collapse by the heat generated by fusion. It is supported only by electron degeneracy pressure and is therefore extremely dense, with a typical mass on the order of the Sun's contained in a volume on the order of the Earth's. The physics of degeneracy yields a maximum mass for a white dwarf, the Chandrasekhar limit—approximately 1.4 solar masses—beyond which it cannot be supported by degeneracy pressure. A carbon-oxygen white dwarf which 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.
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 is the source of its faint luminosity, which may however have a high color temperature. Over a very long period of time, it will cool to temperatures at which it is no longer visible. 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.
As a class, white dwarfs are fairly common; they comprise roughly 6% of all stars in the solar neighborhood