| definition |
A compact star of high surface temperature, low luminosity, and high density (10^5
- 10^8 g cm^-3), with roughly the mass of the Sun (mean mass ~ 0.6 M_sun) and the
radius of the Earth (R ~ 0.01 R_sun), representing the end-point of the evolution
of all stars with masses less then ~ 5-9 solar masses. A white dwarf is what remains
after the central star of a planetary nebula fades and becomes cool. The Chandrasekhar
limit of 1.43 solar masses is the highest mass that a white dwarf can achieve before
electron degeneracy pressure is unable to support it. In the Hertzsprung-Russell diagram,
white dwarfs form a well-defined sequence around 8 magnitudes fainter than the main
sequence. They are composed of a core of carbon and oxygen nuclei and degenerate electrons
surrounded by a thin shell of helium and an outer skin of hydrogen. White dwarf's
radiation is the leftover heat from the star's past when its core was an active nuclear
reactor. The star slowly cools as heat escapes through the non-degenerate envelope.
The first white dwarf to be discovered was Sirius B, the companion of Sirius. White
dwarfs are divided into several types, according to their spectral features, which
depend on the type of shell burning that dominated as it became a planetary nebula:
DA white dwarf; DB white dwarf; DC white dwarf; DO white dwarf; DZ white dwarf; DQ
white dwarf.
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