A trace of lithium proved brown dwarfs are failed stars
True stars destroy their lithium quickly, because the heat needed to fuse hydrogen is also enough to burn it. So when a spectrum taken by the Keck telescope in 1995 showed a faint object in the Pleiades still holding its original lithium, astronomers knew its core had never ignited. They had confirmed one of the first brown dwarfs.
Brown dwarfs sit between planets and stars, weighing roughly 13 to 80 times as much as Jupiter. That is too little to sustain hydrogen fusion, but enough to fuse deuterium, a heavier form of hydrogen that burns at lower temperatures, and the heaviest can also consume lithium. Without a steady fusion engine, they cool over time, sliding through ever cooler spectral classes labelled M, L, T and Y. Despite the name, most would look magenta, purple or nearly black to the eye.
Shiv Kumar predicted such objects in the 1960s, calling them black dwarfs, but that label already belonged to burnt-out white dwarfs. In 1975 Jill Tarter, then a doctoral student at Berkeley, suggested brown, meaning a colour between red and black. Finding them proved frustrating for decades, because they give off most of their light in the infrared, where early detectors were too crude.
The breakthrough came in 1994. A Spanish team led by Rafael Rebolo spotted Teide 1 in the Pleiades, estimated at about 55 Jupiter masses, and the lithium test confirmed it. Around the same time, astronomers at Caltech and Johns Hopkins found a companion to the star Gliese 229 whose spectrum showed methane, previously seen only in giant planets and Saturn's moon Titan and impossible in a genuine star. That discovery defined the cooler T class. An odd companion to a white dwarf, spotted in 1988 and baffling for years, is now recognised as the prototype of the L class.
Thousands are now known thanks to better infrared surveys. The nearest pair, Luhman 16, lies about 6.5 light years away, so only Alpha Centauri and Barnard's Star are closer neighbours of the Sun. Of the ideas proposed for how they form, turbulent collapse of gas clouds best matches observations, producing the full range of masses from small stars down to these would-be suns.
Source: Brown dwarf
