A brown dwarf is born when a star of a certain mass burns up the last of its fuel and collapses. The act of collapsing increases the speed and pressure of the atoms, and because of this fusion starts up again when it has collapsed to a certain size. The heat of the fusion causes the star to stop collapsing, and the star can burn faintly like this for a long time. The radiation put off by brown dwarfs is very small, and is indistinguishable from, but probably contributes to, the background radiation from the universe. The light put off by these venerable stars is so faint we cannot see them with even the most powerful of telescopes.

Brown dwarfs are expected to be more numerous than black holes, but there are still very few in the universe. Their total mass can only account for a very small fraction of the dark matter that must be out there.