Editing Stellar evolution (section)

===White and black dwarfs===
{{Main|White dwarf|Black dwarf}}
For a star of {{Solar mass|1}}, the resulting white dwarf is of about {{Solar mass|0.6}}, compressed into approximately the volume of the Earth. White dwarfs are stable because the inward pull of gravity is balanced by the [[degeneracy pressure]] of the star's electrons, a consequence of the [[Pauli exclusion principle]]. Electron degeneracy pressure provides a rather soft limit against further compression; therefore, for a given chemical composition, white dwarfs of higher mass have a smaller volume. With no fuel left to burn, the star radiates its remaining heat into space for billions of years.

A white dwarf is very hot when it first forms, more than 100,000 K at the surface and even hotter in its interior. It is so hot that a lot of its energy is lost in the form of neutrinos for the first 10 million years of its existence and will have lost most of its energy after a billion years.<ref>{{cite web| url = http://www.vectorsite.net/tastgal_05.html| title = Fossil Stars (1): White Dwarfs}}</ref>

The chemical composition of the white dwarf depends upon its mass. A star that has a mass of about 8-12 solar masses will ignite [[Carbon burning process|carbon fusion]] to form magnesium, neon, and smaller amounts of other elements, resulting in a white dwarf composed chiefly of oxygen, neon, and magnesium, provided that it can lose enough mass to get below the [[Chandrasekhar limit]] (see below), and provided that the ignition of carbon is not so violent as to blow the star apart in a supernova.<ref>{{cite journal |author=Ken'ichi Nomoto |title=Evolution of 8–10 {{Solar mass}} stars toward electron capture supernovae. I – Formation of electron-degenerate O + Ne + Mg cores |volume=277 |journal=Astrophysical Journal |date=1984 |pages=791–805 |bibcode=1984ApJ...277..791N |doi=10.1086/161749|doi-access=free }}</ref> A star of mass on the order of magnitude of the Sun will be unable to ignite carbon fusion, and will produce a white dwarf composed chiefly of carbon and oxygen, and of mass too low to collapse unless matter is added to it later (see below). A star of less than about half the mass of the Sun will be unable to ignite helium fusion (as noted earlier), and will produce a white dwarf composed chiefly of helium.

In the end, all that remains is a cold dark mass sometimes called a [[black dwarf]]. However, the universe is not old enough for any black dwarfs to exist yet.

If the white dwarf's mass increases above the [[Chandrasekhar limit]], which is {{Solar mass|1.4}} for a white dwarf composed chiefly of carbon, oxygen, neon, and/or magnesium, then electron degeneracy pressure fails due to [[electron capture]] and the star collapses. Depending upon the chemical composition and pre-collapse temperature in the center, this will lead either to collapse into a [[neutron star]] or runaway ignition of carbon and oxygen. Heavier elements favor continued core collapse, because they require a higher temperature to ignite, because electron capture onto these elements and their fusion products is easier; higher core temperatures favor runaway nuclear reaction, which halts core collapse and leads to a [[Type Ia supernova]].<ref>{{cite journal |author=Ken'ichi Nomoto |author2=Yoji Kondo |name-list-style=amp |title=Conditions for accretion-induced collapse of white dwarfs |journal=Astrophysical Journal |date=1991 |volume=367 |pages=L19–L22 |bibcode=1991ApJ...367L..19N |doi=10.1086/185922}}</ref> These supernovae may be many times brighter than the Type II supernova marking the death of a massive star, even though the latter has the greater total energy release. This instability to collapse means that no white dwarf more massive than approximately {{Solar mass|1.4}} can exist (with a possible minor exception for very rapidly spinning white dwarfs, whose [[centrifugal force]] due to rotation partially counteracts the weight of their matter). Mass transfer in a [[binary system (astronomy)|binary system]] may cause an initially stable white dwarf to surpass the Chandrasekhar limit.

If a white dwarf forms a close binary system with another star, hydrogen from the larger companion may accrete around and onto a white dwarf until it gets hot enough to fuse in a runaway reaction at its surface, although the white dwarf remains below the Chandrasekhar limit. Such an explosion is termed a [[nova]].