White dwarf

A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: in an Earth-sized volume, it packs a mass that is comparable to the Sun. No nuclear fusion takes place in a white dwarf; what light it radiates is from its residual heat. The nearest known white dwarf is Sirius B, at 8.6 light years, the smaller component of the Sirius binary star. There are currently thought to be eight white dwarfs among the one hundred star systems nearest the Sun. The unusual faintness of white dwarfs was first recognized in 1910. The name white dwarf was coined by Willem Jacob Luyten in 1922. White dwarfs are thought to be the final evolutionary state of stars whose mass is not high enough to become a neutron star or black hole. This includes over 97% of the stars in the Milky Way. After the hydrogen-fusing period of a main-sequence star of low or intermediate mass ends, such a star will expand to a red giant and fuse helium to carbon and oxygen in its core by the triple-alpha process. If a red giant has insufficient mass to generate the core temperatures required to fuse carbon (around 109 K), an inert mass of carbon and oxygen will build up at its center. After such a star sheds its outer layers and forms a planetary nebula, it will leave behind a core, which is the remnant white dwarf. Usually, white dwarfs are composed of carbon and oxygen (CO white dwarf). If the mass of the progenitor is between 7 and 9 solar masses (M☉), the core temperature will be sufficient to fuse carbon but not neon, in which case an oxygen–neon–magnesium (ONeMg or ONe) white dwarf may form. Stars of very low mass will be unable to fuse helium; hence, a helium white dwarf may be formed by mass loss in an interacting binary star system. Because the material in a white dwarf no longer undergoes fusion reactions, it lacks a heat source to support it against gravitational collapse. Instead, it is supported only by electron degeneracy pressure, causing it to be extremely dense. The physics of degeneracy yields a maximum mass for a non-rotating white dwarf, the Chandrasekhar limit— approximately 1.44 times M☉— beyond which electron degeneracy pressure cannot support it. A carbon–oxygen white dwarf which approaches this limit, typically by mass transfer from a companion star, may explode as a Type Ia supernova via a process known as carbon detonation; SN 1006 is a likely example. A white dwarf, very hot when it forms, gradually cools as it radiates its energy. This radiation, which initially has a high color temperature, lessens and reddens over time. Eventually, a white dwarf will cool enough that its material will begin to crystallize into a cold black dwarf. The oldest known white dwarfs still radiate at temperatures of a few thousand kelvins, which establishes an observational limit on the maximum possible age of the universe.

Infobox

Type

Class of stellar remnant.

Mass range

Up to 1.44 M☉

Temperature

Over 798 K

Average luminosity

0.01-0.0001 L☉

Tables

· Composition and structure

Supermassive black hole

Material

Supermassive black hole

Density [kg/m3]

c. 1000

Notes

Critical density of a black hole of around 108 solar masses.

Water (liquid)

Material

Water (liquid)

Density [kg/m3]

1000

Notes

At STP

Osmium

Material

Osmium

Density [kg/m3]

22610

Notes

Near room temperature

The core of the Sun

Material

The core of the Sun

Density [kg/m3]

c. 150000

White dwarf

Material

White dwarf

Density [kg/m3]

1×109

Atomic nuclei

Material

Atomic nuclei

Density [kg/m3]

2.3×1017

Notes

Does not depend strongly on size of nucleus

Neutron star core

Material

Neutron star core

Density [kg/m3]

8.4×1016 – 1×1018

Small black hole

Material

Small black hole

Density [kg/m3]

2×1030

Notes

Critical density of an Earth-mass black hole.

Material	Density [kg/m3]	Notes
Supermassive black hole	c. 1000	Critical density of a black hole of around 108 solar masses.
Water (liquid)	1000	At STP
Osmium	22610	Near room temperature
The core of the Sun	c. 150000
White dwarf	1×109
Atomic nuclei	2.3×1017	Does not depend strongly on size of nucleus
Neutron star core	8.4×1016 – 1×1018
Small black hole	2×1030	Critical density of an Earth-mass black hole.

White dwarf spectral types[24] · Composition and structure › Atmosphere and spectra