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Brown Dwarfs
Until the last few years, brown dwarfs
inhabited the "undiscovered country" between stars and planets. Their mass was
too small for them to be stars and too large for planets. They were expected to
be dim, a thousand times less luminous than the Sun, and relatively cool, with
surface temperatures less than 2500 degrees Celsius.
Because of their intrinsic faintness and low temperature, brown dwarfs were not
discovered until 1995. Today several dozen are known and the number is increasing
rapidly thanks to surveys with telescopes equipped with sensitive infrared detectors. It is now estimated that brown dwarfs
are approximately as numerous as normal stars in our galaxy.
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| The approximate size of a brown dwarf (center) compared to Jupiter left) and the Sun (right). Although brown dwarfs are similar in size to Jupiter, they are much more dense and produce their own light whereas Jupiter shines with reflected light from the Sun.(CXC/K.Kowal)
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According to theory, an object that has a mass of less than about 8 percent of the mass of the Sun
cannot sustain significant nuclear fusion
reactions in its core. This marks the dividing line between red dwarf stars and brown dwarfs. When brown dwarfs are
very young, they generate some energy from the fusion of heavy hydrogen, or deuterium, into helium nuclei, but this supply is used up
in a few tens of millions of years. After that brown dwarfs glow because of the
heat generated by the release of gravitational energy as they slowly
contract.
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