Brown Dwarfs III 
The universe is populated with countless balls of gas. The lightest of these gas balls are planets, which range from a few to perhaps a couple of dozen times the diameter of Earth. The heaviest gas balls are stars, which range from planet-sized up to solar system-filling supergiants. And the ones in the middle are brown dwarfs, which are one-size-fits all.
Brown dwarfs are more massive than planets, but not massive enough to become true stars. There may be tens of billions of them or more in our galaxy alone. Yet they’re so faint that astronomers have confirmed only a few dozen.
Because brown dwarfs are so small and far away, it’s impossible to directly measure how big they are. But models of how they work show that they should all be about the same size -- no bigger than the planet Jupiter, even though they may be up to about 75 times more massive than Jupiter.
That’s because as a brown dwarf’s mass goes up, so does the strength of its gravity. Stronger gravity squeezes more tightly, pulling the outer layers toward the center. The net effect is that all brown dwarfs stay about the same size, regardless of mass.
That’s not the case with stars, though. More-massive stars do have stronger gravity, but the nuclear reactions in their cores produce pressure that pushes outward. As a star’s mass increases, so does the pressure. So in general, heavy stars are bigger than lightweight stars -- the biggest balls of gas in the universe.
Script by Damond Benningfield, Copyright 2010