New classes introduced following the discovery of brown dwarfs are L and T, only loosely defined at present. Cool dwarf stars and the younger, warmer brown dwarfs have similar appearances and share portions of the M and L classifications. They may be stars or brown dwarfs, depending on their mass. The coolest, least massive stars fall into the warmer half of this temperature range. Their spectra show strong absorption by methane and water.
Brown dwarfs are intriguing objects, intermediate between stars and planets.
Stars can shine constantly for billions of years because they generate nuclear energy from the fusion of hydrogen into helium. But brown dwarfs cannot sustain nuclear power production. After a modest initial flush, they cool off and become progressively fainter. Young brown dwarfs are now known to exist in the hundreds in the Sun's neighbourhood. Over most of this range their appearances are similar to cool stars of the same temperature.