Brown Dwarf

  • browndwarf1.jpg
    Located 19 light-years from the sun in the constellation of Lepus, the brown dwarf star Gliese 229B orbits the star red dwarf Gliese 229. 100,000 times dimmer than the Sun, Gliese 229B is the faintest stellar object ever seen orbiting another star.
    Credit: STScI, T. Nakajima, S. Kulkarni, S. Durrance, D. Golimowski, NASA.

    Brown dwarfs are are extremely low mass stars that do not produce energy by nuclear fusion. Rather, the small amount of energy emitted by these stars comes almost exclusively from the heat stored by the star during the collapse of its parent gas cloud. Brown dwarfs therefore gradually cool and fade with cosmological time.

    Brown dwarfs are sometimes referred to as 'failed stars' since they are are more massive than planets but have insufficient mass to sustain nuclear fusion in their cores. According to the accepted theories, the mass required in order to sustain fusion is about 1/12th of a solar mass. This therefore sets the upper mass limit for brown dwarfs. The lower limit for classification as a brown dwarf is somewhat more arbitrary, but, generally, a mass greater than 1/200th of a solar mass is required for an object to be classified as a brown dwarf and not a planet.

    The core temperatures of brown dwarfs must be below about 3 million degrees, as at this temperature fusion becomes sustainable. Surface temperatures are about 1,000 degrees Kelvin. At such temperatures the atmospheres of brown dwarfs contain many molecules - including water!

    Because of their low temperatures and small sizes, brown dwarfs have extremely low luminosities (about 1/100,000th of the solar luminosity). This makes them extremely difficult to observe. Indeed, even with modern telescopes it is almost impossible to observe brown dwarfs more distant than two or three hundred light years. Consequently, it was only in 1995 that the first confirmed brown dwarf was observed.

    Due to their extremely low luminosities, brown dwarfs are are one of the candidates for baryonic dark matter. However, currently observations suggest that there are insufficient numbers of brown dwarfs to account for a significant fraction of the missing mass of the Milky Way.


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