Observations of brown dwarfs

The brown dwarfs occupy the gap between the least massive star and the most massive planet. They begin as dimly stellar in appearance and experience f usion (of at least deuterium) in their interiors. But they are never able t o stabilize their luminosity or temperature and grow ever fainter and coole r with time. For that reason, they can be viewed as a constituent of baryon ic "dark matter." Indeed, we currently have a hard time directly seeing an old brown dwarf beyond 100 pc. After 20 years of searching and false starts , the first confirmed brown dwarfs were announced in 1995. This was due to a combination of increased sensitivity, better search strategies, and new m eans of distinguishing substellar from stellar objects. Since then, a great deal of progress has been made on the observational front. We are now in a position to say a substantial amount about actual brown dwarfs. We have a rough idea of how many of them occur as solitary objects and how many are f ound in binary systems. We have obtained the first glimpse of atmospheres i ntermediate in temperature between stars and planets, in which dust formati on is a crucial process. This has led to the proposal of the first new spec tral classes in several decades and the need for new diagnostics for classi fication and setting the temperature scale. The first hints on the substell ar mass function are in hand, although all current masses depend on models. It appears that numerically, brown dwarfs may well be almost as common as stars (though they appear not to contain a dynamically interesting amount o f mass).