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Brown dwarfs are intriguing objects, intermediate between stars and planets.
Stars can shine constantly for billions of years because they generate nuclear energy from the fusion of hydrogen into helium. But brown dwarfs cannot sustain nuclear power production. After a modest initial flush, they cool off and become progressively fainter. Young brown dwarfs are now known to exist in the hundreds in the Sun's neighbourhood. Over most of this range their appearances are similar to cool stars of the same temperature.