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A white dwarf (WD) is a star past its main-sequence thermonuclear stage that has expelled its exterior and only the core remains. The mass of this remaining material must be sufficiently low that electron degeneracy prevents further collapse into a neutron star, i.e., it remains as 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, for 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 WD life) |
Interestingly, the more massive the white dwarf, the smaller its radius, which implies a considerable range of densities (over three orders-of-magnitude).
They generally transfer their energy to the surface by conduction (interaction of particles) rather than radiative transfer. Their luminosity is quite low, produced entirely by their very slow cooling, and only somewhat-nearby white dwarfs have been observed. Most are oxygen and carbon but under some conditions can include 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 binary companion, it cannot remain stable: it must collapse into a neutron star (or stellar-mass black hole), but it is presumed that often near that limit, presumably before sufficient density is achieved, a Type Ia supernova results (and no remnant remains). White dwarfs begin at the temperature left over from earlier fusion plus the effects of the subsequent gravitational collapse (increasing the temperature through the Kelvin-Helmholtz mechanism), at which time they have a far higher surface temperature than the Sun, given their much smaller surface. The term black dwarf refers to the theoretical state of a white dwarf cooled to the degree that it emits no appreciable EMR, but with their very slow cooling, 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 could reasonably be classified into a number of distinct types of objects, given their variety of constituents, structure, mass, density, 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 |
| DC | No lines |
| DO | Includes H II lines |
| DZ | metal lines |
| DQ | Carbon lines |
| DX | Unclear |
Some optional letters designating other features:
| P | Polarized |
| H | Magnetism but no polarization |
| E | Emission lines |
| V | Variable |
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, which can occur with certain constituents at certain temperatures.