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white dwarf


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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.

Subjects: Astronomy and Astrophysics.


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