An Etymological Dictionary of Astronomy and Astrophysics
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A → white dwarf whose spectrum shows the → Balmer lines of hydrogen only, with no helium or metals.

D short for → dwarf; A a convention; → white.

A → white dwarf whose spectrum shows strong He I in the absence of hydrogen or metal lines.

D short for → dwarf; B a convention; → white.

A → white dwarf showing a continuous spectrum with no readily apparent lines.

D short for → dwarf; C a convention; → white.

A → white dwarf whose spectrum shows strong lines of singly ionized helium He II; He I or H may be present. As a DO star cools, the He II will recombine with free electrons to form He I, eventually changing the DO type into a DB white dwarf.

D short for → dwarf; O a convention; → white.

A → double-lined binary with two → white dwarf components. Short-period double white dwarfs can lose → orbital angular momentum by emitting → gravitational radiation and if the total mass of the binary exceeds the → Chandrasekhar limit, their eventual → merger might produce a → Type Ia supernova.

→ double; → white; → dwarf.

A → white dwarf whose spectrum shows carbon features of any kind.

D short for → dwarf; Q a convention; → white.

A → white dwarf whose spectrum shows metal lines only; no H or He.

D short for → dwarf; Z a convention; → white.

A compact star of high surface temperature, low luminosity, and high density (10⁵-10⁸ 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. → white dwarf crystallization; → cooling time; → Mestel theory. 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. For a review see Kepler and Brdaley (1995, Baltic Astron. 4, 166).

The term white dwarf was coined by the Dutch-American astronomer Willem Luyten (1899-1994) in 1922, from → white + → dwarf.

In the → Hertzsprung-Russell diagram, the evolutionary track followed by a → low-mass or an → intermediate-mass star when it can no longer produce thermonuclear energy. The track starts at the end of the → horizontal branch to lead the star to a → white dwarf phase.

→ white; → dwarf; → cooling; → track.

The most important phenomenon occurring during → white dwarf evolution, which results from its cooling. Crystallization is a → phase transition whereby → latent heat is released. At the cooler end of a white dwarf's life (→ cooling time), the → thermal energy of nuclei, which are positively charged ions, becomes small and the effects of electrostatic interaction on the motion of ions become important. The ions repel each other and their distribution will be such that the → Coulomb energy per ion is a minimum. This will cause the ions to form crystal-like lattice structures. As the star cystallizes it releases latent heat, providing an additional energy source that slows the cooling process compared to the → Mestel theory. Once the bulk of the white dwarf is crystalline, heat can travel through the star more easily and the white dwarf cools faster.

→ white; → dwarf; → crystallization.