As young stars and brown dwarfs contract, their cores grow hotter and denser, and the deuterium nuclei fuse into helium 3 nuclei. (Deuterium fusion can occur in brown dwarfs because it requires a lower temperature--and hence a lower mass--than hydrogen fusion.) The outpouring of energy from these reactions temporarily halts the gravitational contraction and causes the objects to brighten. But after a few million years the deuterium runs out, and the contraction resumes. Lithium fusion occurs next in stars and in brown dwarfs more than 60 times as massive as Jupiter.

During the contraction of a brown dwarf, thermal pressure rises in its core and opposes the gravitational forces. All the electrons are freed from their nuclei by the heat. Because no two electrons can occupy the same quantum state, when the core is very dense the low-energy states are filled, and many electrons are forced to occupy very high energy states. This generates a form of pressure that is insensitive to temperature. Objects supported in this manner are called degenerate. One consequence of this process is that all brown dwarfs are roughly the size of Jupiter--the heavier brown dwarfs are simply denser than the lighter ones.

In stars the cores do not become degenerate. Instead hydrogen fusion provides the pressure that supports the star against its own gravity. Once fusion begins in earnest, the star stops contracting and achieves a steady size, luminosity and temperature. In high-mass brown dwarfs, hydrogen fusion begins but then sputters out. As degeneracy pressure slows the collapse of brown dwarfs, their luminosity from gravitational contraction declines. Although very low mass stars can shine for trillions of years, brown dwarfs fade steadily toward oblivion. This makes them increasingly difficult to find as they age. In the very distant future, when all stars have burned out, brown dwarfs will be the primary repository of hydrogen in the universe. --G.B.