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White Dwarfs

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White dwarfs develop when a star from maximum 1.4 solar masses dies. These stars are not heavy enough to generate the core temperatures required to fuse carbon in nucleosynthesis reactions and after they have become a red giant during their helium-burning phase, they will shed their outer layers to form a planetary nebula, leaving behind an inert core consisting mostly of carbon and oxygen.

This core has no further source of energy, and so will gradually radiate away its energy and cool down. The core, no longer supported against gravitational collapse by fusion reactions, becomes extremely dense, with a typical mass of about half that of the sun contained in a volume about equal to that of the Earth. The white dwarf is supported only by electron degeneracy pressure and this makes white dwarfs one of the densest forms of matter, surpassed only by neutron stars.

The oldest white dwarfs still radiate temperatures of a few thousand kelvins.The present universe is not old enough to cool the white dwarfs to temperatures at which they are no longer visible. This hypothetical typ of astronomical object called black dwarf.

History

The first white dwarf was discovered 1863 in the Sirius system in the constellation Canis Major, a dark companion of the brightest star in the sky. This companion was called Sirius B and had a surface temperature of about 25,000 K but is 10,000 times fainter than the main star in this system.

The second white dwarf was discovered 1917 and called after his founder (Adriaan Van Maanen) Van Maanen's Star, 14.4 light years away in the constellation Pisces.

Subrahmanyan Chandrasekhar discovered in 1930 the Chandrasekhar limit (the maximum mass of a white dwarf) and received the Nobel prize in 1983.

Recent developments

August 1995 - The Hubble space telecope observed more than 75 white dwarfs in the globular cluster M4 in the constellation Scorpius. M4 is located 7,000 light years away, but is the nearest globular cluster to Earth. It is also approximately 14 billion years old, which is why so many of its stars are near the end of their lives.

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