 White dwarfs are thought to be the final evolutionary state of stars whose mass is not high enough to become a neutron star, including the Earth's Sun and over 97 % of the other stars in the Milky Way., § 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. 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 a star sheds its outer layers and forms a planetary nebula, it will leave behind this core, which is 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 (M ☉), the core temperature is sufficient to fuse carbon but not neon, in which case an oxygen – neon – magnesium white dwarf may form. Stars of very low mass will not be able to fuse helium, hence, a helium white dwarf may form by mass loss in binary systems.
