Brown Dwarf, a celestial object more massive than a planet but less massive than a star. Such an object is very dim and difficult to detect. The first reliable observations of brown dwarfs were made in the mid-1990's, long after the existence of such objects was first suspected. Some astronomers believe brown dwarfs may exist in large numbers, helping account for the so-called missing mass" of galaxiesmatter galaxies seem to contain that cannot be accounted for by observed celestial objects.
A brown dwarf probably forms from concentrated dust and gases in a way similar to that in which stars form. As the matter forming the brown dwarf becomes compressed by gravitational forces, enough heat is produced to make the surface glow a very faint orange or deep red. However, because of the brown dwarf's small mass, the core does not become hot enough to sustain nuclear fusion, the main source of a star's energy. A brown dwarf is therefore sometimes referred to as a failed star."