Brown dwarfs started out the same as ordinary stars, collapsing from giant nebulas of dust and gas. Most brown dwarfs are not quite massive enough to sustain a nuclear fusion reaction at their cores. Brown dwarfs, therefore, are transitional objects, standing between stars and giant gas planets. The mass of a brown dwarf can range from 13 to 90 times the mass of the planet Jupiter, or up to about a tenth the mass of the sun.
Brown dwarfs are classified according to the spectrum of the light detected from them. M-class stars are the coolest stars that can burn successfully, and are the most common stars in the universe (about 75 percent of the stars in the sun’s neighborhood). Many M-stars are red dwarfs, but some are brown dwarfs.
L-class brown dwarfs are cooler than M stars. Some L stars can support hydrogen fusion, but most do not. T-class brown dwarfs have surface temperatures in the range of 800 to 1,880 degrees Fahrenheit (430 to 1,030 degrees Celsius). T dwarfs are largely composed of methane.
Y-class brown dwarfs are the coolest of the dwarfs. Their temperature may be as low as that of a household oven, or even the temperature of a human body.
Some brown dwarfs are orbited by planets, or sometimes discs of matter similar to those in developing solar systems (artist’s rendering, inset). Astronomers have found microscopic particles of the mineral olivine orbiting in the dust disc. Olivine crystals are thought to be the initial stage of the process that builds rocky planets such as Earth.
In 2014, astronomers were able to map features in the atmosphere of T-class brown dwarf Luhman-16B. The “clouds” are believed to be droplets of molten iron and minerals, floating in an atmosphere of hydrogen at 2,000 degrees Fahrenheit (1,100 degrees Celsius).