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BROWN DWARF

Brown dwarf is a heavenly body that has more mass than a planet but less mass than a star. Brown dwarfs are about same size as the planet Jupiter, but they have about 13 to 75 times as much mass as Jupiter. Brown dwarfs are difficult to detect as they are so dim. Stars and brown dwarfs form in the same way.A cloud of gas and dust shrinks under the force of gravity. A ball of gas forms the clouds' center. As the cloud continues to shrink, the ball heats up. When the core of the ball becomes hot enough, hydrogen atoms begin to fuse. If the ball has enough mass- more than about the 75 times the mass of Jupiter-fusion will continue. The object the becomes a star. If the ball has less mass little fusion occurs. the object then becomes a brown dwarf.As the brown dwarf continues to shrink, the electrons in its core push against more strongly.This pushing creates pressure that works against the force of gravity.Eventually, the pressure equals the force of gravity. The brown dwarf stops shrinking and begins to cool.As it cools, fusion ceases. Its temperature continues to drop and its glow slowly fades.

The more mass a brown dwarf has, the hotter and brighter it begins. the hottest and brightest brown dwarfs glow a dull red.They resemble low mass stars called red dwarfs. To distinguish between a red dwarf and a brown dwarf, astronomers analyse the light given off by the object for evidence of the presence of the element lithium. Stars destroy lithium while brown dwarfs do not. The presence of lithium indicates that the object is a brown dwarf. As the brown dwarf cools it less closely resembles a star. Once the brown dwarf's surface cools below a star like temperature astronomers can easily distinguish it from a star.Heavier elements such as iron and titanium begin to form dust grain in the brown dwarf's atmosphere.

As the brown dwarf cools, it begins to more closely resemble a gas giant planet like Jupiter.Brown dwarfs can appear alone or in groups of stars called star clusters. Like Jupiter Brown dwarf rotate rapidly, spinning completely around in just a few hours. scientists believe the atmosphere of cooler brown dwarf may be marked by band of swirling gas much like the atmosphere of Jupiter.

source: world book