Protostars with masses less than roughly 0.08 M ☉ (1.6 × 10 kg) never reach temperatures high enough for nuclear fusion of hydrogen to begin. These are known as brown dwarfs. The International Astronomical Union defines brown dwarfs as stars massive enough to fuse deuterium at some point in their lives (13 Jupiter masses (M J), 2.5 × 10 kg, or 0.0125 M ☉). Objects smaller than 13 M J are classified as sub-brown dwarfs (but if they orbit around another stellar object they are classified as planets). Both types, deuterium-burning and not, shine dimly and fade away slowly, cooling gradually over hundreds of millions of years.