White dwarf

Words to Know

Black dwarf: Cooling remnants of a white dwarf that has ceased to glow.

Nebula: Cloud of interstellar gas and dust.

Nuclear fusion: Reaction involving the merging of two hydrogen nuclei into one helium nucleus, releasing a tremendous amount of energy in the process.

Red giant: Stage in which an average-sized star spends the final 10 percent of its lifetime; its surface temperature drops and its diameter expands to 10 to 1,000 times that of the Sun.

A white dwarf is the fate awaiting the Sun and other average-sized stars. It is the core of a dead star left to cool for eternity.

Nuclear fusion is the merging of two hydrogen nuclei into one helium nucleus, with the release of a tremendous amount of energy in the process. This occurs in the early stages of every star's life. It fuels the star and provides an outward pressure that acts as a balance to the star's tremendous gravity. In the absence of fusion, gravity takes over and causes a star to collapse upon itself. The larger the original star, the smaller a white dwarf it becomes. The reason for this pattern is that larger stars have stronger gravitational fields, which produce a more complete collapse.

An average-sized star like the Sun will spend the final 10 percent of its life as a red giant. In this phase of a star's evolution, the star's surface temperature drops to between 3,140 and 6,741°F (1,727 and 3,727°C) and its diameter expands to 10 to 1,000 times that of the Sun. The star takes on a reddish color, which is what gives it its name.

Buried deep inside the star is a hot, dense core, about the size of Earth. The core makes up about 1 percent of the star's diameter. The helium left burning at the core eventually ejects the star's atmosphere, which explodes off into space as a planetary nebula (gas and dust cloud). All that remains of the star is a glowing core, a white dwarf.

The term white dwarf is a bit misleading. The core starts out white, but as it cools it displays a range of colors—from yellow to red. When all heat within the core has escaped, the body ceases to glow and becomes a black dwarf. Billions of white dwarfs exist within our galaxy, many of them now in the form of black dwarfs. These cold, dark globes, however, are next to impossible to detect.

[See also Red giant; Star ]

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 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.

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.