- white dwarf star
Any of a class of small, faint stars representing the end point of the evolution of stars without enough mass to become neutron stars or black holes.Named for the white colour of the first ones discovered, they actually occur in a variety of colours depending on their temperature. They are extremely dense, typically containing the mass of the Sun within the volume of the Earth. White dwarfs have exhausted all their nuclear fuel and cannot produce heat by nuclear fusion to counteract their own gravity, which compresses the electrons and nuclei of their atoms until they prevent further gravitational contraction. When a white dwarf's reservoir of thermal energy is exhausted (after several billion years), it stops radiating and becomes a cold, inert stellar remnant, sometimes called a black dwarf. White dwarf stars are predicted to have an upper mass limit, known as the Chandrasekhar limit (see Subrahmanyan Chandrasekhar), of about 1.4 times the Sun's mass. Dying stars that are more massive undergo a supernova explosion. As members of binary stars, white dwarf stars play an essential role in the outbursts of novas.
* * *any of a class of faint stars representing the endpoint of the evolution of intermediate- and low-mass stars. White dwarf stars, so called because of the white colour of the first few that were discovered, are characterized by a low luminosity, a mass on the order of that of the Sun, and a radius comparable to that of the Earth. Because of their large mass and small dimensions, such stars are dense and compact objects with average densities approaching 1,000,000 times that of water.Unlike most other stars that are supported against their own gravitation by normal gas pressure, white dwarf stars are supported by the degeneracy (degenerate gas) pressure of the electron gas in their interior. Degeneracy pressure is the increased resistance exerted by electrons composing the gas, as a result of stellar contraction (see degenerate gas). The application of the so-called Fermi-Dirac statistics and of special relativity to the study of the equilibrium structure of white dwarf stars leads to the existence of a mass-radius relationship through which a unique radius is assigned to a white dwarf of a given mass; the larger the mass, the smaller the radius. Furthermore, the existence of a limiting mass is predicted, above which no stable white dwarf star can exist. This limiting mass, known as the Chandrasekhar limit (q.v.), is on the order of 1.4 solar masses. Both predictions are in excellent agreement with observations of white dwarf stars.The central region of a typical white dwarf star is composed of a mixture of carbon and oxygen. Surrounding this core is a thin envelope of helium and, in most cases, an even thinner layer of hydrogen. A very few white dwarf stars are surrounded by a thin carbon envelope. Only the outermost stellar layers are accessible to astronomical observations.White dwarfs evolve from stars with an initial mass of up to three or four solar masses or even possibly higher. After quiescent phases of hydrogen and helium burning in its core—separated by a first red-giant phase—the star becomes a red giant for a second time. Near the end of this second red-giant phase, the star loses its extended envelope in a catastrophic event, leaving behind a dense, hot, and luminous core surrounded by a glowing spherical shell. This is the planetary-nebula phase. During the entire course of its evolution, which typically takes several billion years, the star will lose a major fraction of its original mass through stellar winds in the giant phases and through its ejected envelope. The hot planetary-nebula nucleus left behind has a mass of 0.5–1.0 solar mass and will eventually cool down to become a white dwarf.White dwarfs have exhausted all their nuclear fuel and so have no residual nuclear energy sources. Their compact structure also prevents further gravitational contraction. The energy radiated away into the interstellar medium is thus provided by the residual thermal energy of the nondegenerate ions composing its core. That energy slowly diffuses outward through the insulating stellar envelope, and the white dwarf slowly cools down. Following the complete exhaustion of this reservoir of thermal energy, a process that takes several additional billion years, the white dwarf stops radiating and has by then reached the final stage of its evolution and becomes a cold and inert stellar remnant. Such an object is sometimes called a black dwarf.Because of their instrinsically low luminosities, white dwarf stars are difficult to observe. They are occasionally found in binary systems, as is the case for the white dwarf companion to the brightest star in the night sky, Sirius (q.v.). White dwarf stars also play an essential role in the outbursts of novae and of other cataclysmic variable stars.
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white dwarf star — noun a faint star of enormous density • Syn: ↑white dwarf • Hypernyms: ↑star … Useful english dictionary
White dwarf — For other uses, see White dwarf (disambiguation). Image of Sirius A and Sirius B taken by the Hubble Space Telescope. Sirius B, which is a white dwarf, can be seen as a faint pinprick of light to the lower left of the much brighter Sirius A … Wikipedia
white dwarf — noun a faint star of enormous density • Syn: ↑white dwarf star • Hypernyms: ↑star * * * noun : a whitish star of high surface temperature and very low intrinsic brightness usually with a mass about comparable to that of the sun but of such small… … Useful english dictionary
dwarf star — Astron. any of the ordinary main sequence stars, as those of spectral types O, B, A, F, G, K, and M. Also called dwarf. Cf. white dwarf. [1910 15] * * * Any star of average or low luminosity, mass, and size, including white dwarf stars and red… … Universalium
White dwarf (disambiguation) — White dwarf, or White Dwarf, has multiple uses:* White dwarf, a compact star that is no longer generating energy through nuclear fusion at its core. * White Dwarf (magazine) is a miniature game magazine published by Games Workshop. * White Dwarf… … Wikipedia
white dwarf — white′ dwarf′ n. astron. a star that is approximately the size of the earth, has undergone gravitational collapse, and is in the final stage of evolution for low mass stars, beginning hot and white and ending cold and dark(black dwarf) •… … From formal English to slang
white dwarf — n. a planet sized, very dense, collapsed star, the fuel of which has been exhausted: initially very bright and hot, it gradually evolves into a black dwarf … English World dictionary
white dwarf — n technical a hot star, near the end of its life, that is more solid but less bright than the sun →↑red giant … Dictionary of contemporary English
white dwarf — noun count TECHNICAL a star that does not shine very brightly and is at the end of its life … Usage of the words and phrases in modern English
Dwarf star — The term dwarf star refers to a variety of distinct classes of stars. Dwarf star alone generally refers to any main sequence star, a star of luminosity class V. Red dwarfs are low mass main sequence stars. Yellow dwarfs are main sequence (dwarf)… … Wikipedia