White dwarf star

(astronomy) An intrinsically faint star of very small radius and high density; the mass is about 0.6 that of the sun and the average radius is about 5000 miles (8000 kilometers); it is one final stage of stellar evolution with thermonuclear energy sources extinct.

The smallest kind of ordinary star, about the size of the Earth. In most white dwarf stars, an amount of matter equal to 60% of the Sun's mass is compressed into this small volume. One quarter liter (about 1 cup) of white dwarf material has a mass of 600 tons. Several thousand white dwarf stars have been discovered.

Some of the other properties of white dwarf stars reach extreme values. The hottest known stars are either white dwarfs or stars that are just about to become white dwarfs, and have temperatures of a few hundred thousand kelvins. A few white dwarf stars have very strong magnetic fields, with field strengths exceeding 10⁸ gauss (10⁴ teslas), many times stronger than can be generated in laboratories. Some white dwarf stars rotate every few minutes, and some others rotate so slowly that no spin has been detected over several decades. The coolest white dwarf stars emit less energy per second than any other type of visible star. See also High magnetic fields.

White dwarfs are the final stages in the life cycles of low-mass stars like the Sun, the most common types of stars. At present, the Sun is on the main sequence, fusing hydrogen to form helium in its core. Five billion years from now, the hydrogen in the center of the Sun will run out, and the Sun will become a red giant star as it turns to other nuclear reactions to provide its internal heat. Stars like the Sun will become only hot enough inside that they will fuse helium nuclei to form a mixture of carbon and oxygen. They will then reach the end of the nuclear fusion road, and will no longer be able to generate their own sources of energy. At this point in their life cycle, they will be on the way to becoming white dwarf stars, the most common types of stellar remnants. See also Nucleosynthesis; Stellar evolution.

Just before a star becomes a white dwarf star, it will shed its outer layers as its stellar wind strengthens at the very end of the red giant stage of its life cycle. In many and perhaps all cases, the expelled gas will become visible as a glowing gas cloud called a planetary nebula. See also Planetary nebula.

A star becomes a bona fide white dwarf when it contracts to a state where heat pressure plays a very small role in determining its internal structure. The material in white dwarf stars is a form of matter called degenerate matter. The electron clouds in degenerate matter actually touch each other, and this touching gives white dwarf matter a resistance to compression that balances the force of gravity.

White dwarf stars have no nuclear energy sources, so they simply cool. The hottest star to have compacted itself to the white dwarf state has a temperature of 190,000 K (350,000°F), while the coolest white dwarf stars have temperatures near 3800 K (6400°F), still hotter than the coolest main-sequence stars. The internal structure of these two stars is virtually the same.

However, the surface layers of white dwarfs are much more varied than is the case of other types of stars. For example, white dwarf stars come in two basic flavors, hydrogen-rich stars, whose hydrogen/helium ratios can exceed 10⁷, and helium-rich stars, whose hydrogen/helium ratio can be less than 10⁻⁷. (In virtually every other type of star, the hydrogen/helium ratio is near its cosmic value of approximately 10:1.) At least part of this extraordinary variation (over 14 orders of magnitude) probably comes from subtle differences in the way that white dwarfs form in the late stages of red giant life cycles.