white dwarf

The white dwarf Sirius B appears as a dot on the lower left portion of this Hubble image. It orbits its bright companion, Sirius, every 50 years. Image: NASA/HE Bond/E Nelan/M Barstow/M Burleigh/JB Holberg/STScI/U Leicester/U Ariz.

A dim, dense, planet-sized star that marks the evolutionary endpoint for all but the most massive stars (see stars, evolution). The first white dwarf to be discovered, in 1862, and the closest to the Sun, is the companion of Sirius.

White dwarfs form from the collapse of stellar cores in which nuclear fusion has stopped, and are exposed to space following the loss of the old star's bloated outer envelope, typically as a planetary nebula. They consist of electron degenerate matter, which provides the pressure needed to prevent further collapse, providing that the mass of the dwarf doesn't exceed the Chandrasekhar limit of about 1.4 solar masses.

Even a fairly large white dwarf, with a mass similar to that of the Sun, is only about as big as Earth. A piece of it the size of a sugar cube would weigh about as the same as a hippopotamus.

Types of white dwarf

Lighter mass stars that never get around to burning carbon in their cores give rise to carbon-oxygen white dwarfs, while stars that start out with a mass of at least 4 solar masses may give rise to neon-oxygen dwarfs. In addition, white dwarfs differ in terms of their spectra which are dictated by the elements that dominate their surfaces. Three varieties – dA, dB and dO (where "d" stands for "degenerate") – have nearly pure surfaces of hydrogen or helium lying atop their cores, whereas PG 1159 stars appear to be partially exposed cores. White dwarfs may also have a mixture of elements on their surfaces, and are named accordingly. For example, dAB stars contain hydrogen and neutral helium, whereas dAO stars have hydrogen and ionized helium.

Internal structure of various white dwarf types

Spectral classification of white dwarfs
type	characteristics
dA	Only Balmer lines: no HeI or metals present
dB	HeI lines: no H or metals present
dC	Continuous spectrum, few or no lines visible
dO	HeII strong: HeI or H present
dZ	Metal lines only: no H or He lines
dQ	Carbon features, either atomic or molecular in any part of the spectrum

White dwarfs and supernovae

White dwarfs that consume matter from their companions can explode as type 1a supernovae, which are used as standard candles to measure cosmic distances. In 1988, these supernovae provided the evidence for the accelerating expansion of the universe when they were found to be dimmer than expected.

Lightest white dwarf

SDSS J091709.55+463821.8, artist's impression

Artist's impression of SDSS J091709.55+463821.8. Credit: David A. Aguilar (CfA)

The lowest-mass white dwarf known is SDSS J091709.55+463821.8, with only 17 percent the mass of the Sun. It lies 7,400 light-years away near the border of the constellations Lynx and Ursa Major. Its discovery was announced in 2007 by a team of American astronomers.

SDSS J091709.55+463821.8 is a member of a binary star system and has evidently lost much of its matter in the past to a companion star, which also appears to be a white dwarf. The two now orbit each other every 7.6 hours at a distance of about 1,040,000 km and with a speed of 536,000 km/h.

Related categories

• TYPES OF STARS
• STELLAR ASTROPHYSICS

Also on this site:

Encyclopedia of Alternative Energy & Sustainable Living
Encyclopedia of History

BACK TO TOP