An aborted star, one that is too small for nuclear reactions to be sustained in its core. They must be very common, but are very difficult to detect because of their low brightness. Brown dwarfs are not brown; at the beginning of their lives they radiate weakly in the red and subsequently dim. A brown dwarf is bigger than a planet, varying from 10 to 80 times the mass of Jupiter (i.e. 0.01 to 0.08 solar masses). There are two kinds of brown dwarf - the hottest (1500 K to 2500 K) are of type L; the coldest (200 K to 1500 K) are of type T. The hottest brown dwarfs are sometimes classified as very cold type M red dwarfs.
Comparative sizes of the Sun, the Earth and a brown dwarf. Credits: Gemini Observatory, artist's impression by Jon Lomberg