A white dwarf (WD) is a star past its main-sequence thermonuclear stage that has expelled its exterior and only the core remains. The remaining mass must be sufficiently low that electron degeneracy prevents further collapse into a neutron star, i.e., it is electron degenerate matter (EDM). As the remains of a star, it is classified as a stellar remnant. They are small and massive, e.g., the size of Earth with the mass of the Sun, and a density on the order of 100,000 times that of Earth:
|Mass range||0.17-1.33 Solar masses|
|Radius range||1300-15000 km|
|Bulk density range||103-107 g/cc|
|Luminosity range||about 0.1 solar for brightest (at the beginning of their life)|
They generally transfer their energy to the surface by conduction (interaction of particles) rather than radiative transfer. Most are oxygen and carbon but under some conditions can have neon, magnesium, or helium. The limit on their mass is about 1.4 solar masses (the Chandrasekhar limit) and if an existing white dwarf grows beyond that, e.g., due to mass transfer from a companion, a type Type Ia supernova can result. White dwarfs begin at the temperature left over from the main sequence, at which time they can be far hotter at the surface than the Sun, after which they cool very slowly. The term black dwarf refers to the theoretical state of a white dwarf cooled to the degree that it emits no appreciable EMR, but the universe is not old enough for any to be near that state (one estimate of the cooling time is 1015 years). Widely different stars are labeled white dwarfs, as long as they are small and hot stellar remnants explainable as electron degenerate matter and they might reasonably be considered many types of objects, given their variety of constituents, structure, mass, and temperature. Types of white dwarfs based upon their observational characteristics:
Some white dwarf spectral classes (e.g., "DBV"):
|DA||Just H spectral lines|
|DB||Just He I lines|
|DO||Includes H II lines|
Optional letter designating other features:
|H||Magnetism but no polarization|
Some white dwarfs show strong magnetic fields (magnetic white dwarfs, MWDs), presumed to have become more pronounced as the progenitor collapsed, analogous to neutron stars.
A pre-white dwarf (PWD) is a star no longer harboring fusion but not yet a white dwarf, i.e., with an intermediate position between asymptotic giant branch and white dwarf on the H-R diagram. They are typically pulsating stars. There are pulsating white dwarfs as well, occurring given certain constituents at certain temperatures.