After such a star sheds its outer layers and forms a planetary nebula, it will leave behind a core, which is the remnant white dwarf. 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.
: §1 After the hydrogen- fusing period of a main-sequence star of low or medium mass ends, such 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. This includes over 97% of the other stars in the Milky Way. White dwarfs are thought to be the final evolutionary state of stars whose mass is not high enough to become a neutron star or black hole. : 1 The name white dwarf was coined by Willem Luyten in 1922. The unusual faintness of white dwarfs was first recognized in 1910. There are currently thought to be eight white dwarfs among the hundred star systems nearest the Sun. The nearest known white dwarf is Sirius B, at 8.6 light years, the smaller component of the Sirius binary star. A white dwarf's faint luminosity comes from the emission of residual thermal energy no fusion takes place in a white dwarf. A white dwarf is very dense: Its mass is comparable to that of the Sun, while its volume is comparable to that of Earth. Sirius B, which is a white dwarf, can be seen as a faint point of light to the lower left of the much brighter Sirius A.Ī white dwarf, also called a degenerate dwarf, is a stellar core remnant composed mostly of electron-degenerate matter. Image of Sirius A and Sirius B taken by the Hubble Space Telescope.
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