white dwarf

white dwarf in astronomy, a type of star that is abnormally faint for its white-hot temperature (see mass-luminosity relation ). Typically, a white dwarf star has the mass of the sun and the radius of the earth but does not emit enough light or other radiation to be easily detected. The existence of white dwarfs is intimately connected with stellar evolution . A white dwarf is the hot core of a star, left over after the star uses up its nuclear fuel and dies. It is made mostly of carbon and is coated by a thin layer of hydrogen and helium gases. The physical conditions inside the star are quite unusual; the central density is about 1 million times that of water.

Astronomers long believed this intense pressure could cause the carbon interiors of white dwarfs to crystallize. In 2004 the discovery of BPM-37093, a star that is located 50 light-years from the earth in the constellation Centaurus and is both pulsating and has sufficient mass to have a crystalline interior. By measuring the pulsations it was possible to study this white dwarf's interior and determine that it had crystallized to form an enormous diamond, some 950 mi (1,500 km) wide. Were it a diamond as we commonly know it, it would weigh some 10 billion trillion trillion carats.

The first white dwarf discovered (1844) was the faint companion in the binary star Sirius. Although invisible to the telescopes of the day, the white dwarf's mass was large enough to produce a noticeable wavy motion in its very bright partner as the two stars revolved around each other. It is believed that white dwarfs could represent as much as a third of the so-called dark matter in the universe.

white dwarf High-density type of star about the size of the Earth, but with a mass about that of the Sun. White dwarfs are of low luminosity and gradually cool down to become cold, dark objects.

white dwarf A small, dense star that is the end-result of the evolution of all but the most massive stars. White dwarfs are thought to form from the collapse of stellar cores once nuclear burning has ceased there. The core is exposed to view when the outer parts of the star are driven off to form a planetary nebula. Such a core contracts under its own gravity until, having reached a size similar to that of the Earth, it has become so dense (5 × 10⁸ kg/m³) that it is supported against further collapse by the pressure of electron degeneracy. White dwarfs are formed with high surface temperatures (above 10 000 K) because of the heat trapped within them, released both by previous nuclear burning and through gravitational contraction. They gradually cool, becoming fainter and redder. White dwarfs may constitute 30 % of the stars in the solar neighbourhood, but because of their low luminosity (typically 10⁻³ to 10⁻⁴ of the Sun's) they are very inconspicuous. The maximum possible mass for a white dwarf is 1.44 solar masses, the Chandrasekhar limit. An object of greater mass would contract further and become either a neutron star or a black hole. See also D star.