An object that, because of its low mass (less than 0.08 solar masses), never becomes hot enough to begin hydrogen fusion in its core although it can produce some nuclear energy for a few million years from the fusion of deuterium, a heavy isotope of hydrogen, which requires lower temperatures than normal hydrogen fusion; hence it is considered to be not a star, but a substellar object. Brown dwarfs have surface temperatures of about 2000 K and cooler. They thus have a very low luminosity and are difficult to detect. Brown dwarfs may be at least as numerous as visible stars but because of their low masses are not thought to be a significant component of the galactic dark matter. The first brown dwarf to be identified with certainty was a companion to the nearby red dwarf Gliese 229, photographed by the Hubble Space Telescope in 1995. Brown dwarfs are now assigned the spectral types L, T, and Y (see L Dwarf; T Dwarf; Y Dwarf). An object below about 0.01 solar masses (about 10 Jupiter masses) is regarded as a planet.