White dwarf is a kind of star that has run out of fuel. A typical white dwarf has about 60 percent as much mass (amount of matter) as a medium-sized star, such as the sun, but is no larger than a small planet, such as Earth. Thus, its density is about 18 tons per cubic inch (1 metric ton per cubic centimeter). A white dwarf is the last stage in the life of most stars—those not massive enough to end in powerful explosions called supernovae. The sun itself will eventually become a white dwarf.
A white dwarf is not actually white. Its color depends on its temperature. The hottest white dwarfs are violet, and the coolest are a deep red. A white dwarf sends out only about 1/1,000 as much light as the sun. As a result, no white dwarf is visible to the unaided eye.
During most of a star’s early lifetime, it produces energy by means of nuclear fusion, a joining of two atomic nuclei to produce a single nucleus. Fusion takes place in the star’s core and occurs in several steps, some of which release energy. The main fusion fuels are hydrogen and helium nuclei. When most of the fuel has fused, the star may swell to many times its original size, becoming a red giant. Later, it throws off its outer layers, leaving only the core—the white dwarf. The white dwarf then cools and changes color. After billions of years, it stops glowing, ending its life as a black dwarf.
See also Star (Evolution of stars).
