WHAT IS A BROWN DWARF?
A brown dwarf is a substellar object that forms through gravitational collapse like a star but does not have enough mass to sustain long-term hydrogen fusion in its core. They occupy the unusual region between the largest planets and the smallest true stars. Because they cannot maintain ordinary hydrogen fusion, brown dwarfs gradually cool and become fainter over time.
WHY CAN'T IT BECOME A STAR?
For a collapsing object to become a normal hydrogen-burning star, its core must reach sufficient temperature and pressure for sustained hydrogen fusion. Brown dwarfs do not reach the required conditions. Some of the most massive brown dwarfs can briefly fuse deuterium, but this is not the sustained hydrogen fusion that powers ordinary stars.
BROWN DWARFS VS PLANETS
Brown dwarfs can have masses similar to giant planets, but their formation is an important distinction. Many brown dwarfs are thought to form from the gravitational collapse of gas clouds, similar to stars. Planets generally form inside disks around young stars. The exact boundary between massive planets and brown dwarfs can therefore be complicated.
STRANGE ATMOSPHERES
Brown dwarfs have cool, complex atmospheres containing clouds and unusual chemical compounds. Depending on their temperature, they can contain clouds made from materials such as silicates and iron droplets. Their atmospheres also produce distinctive spectral signatures, allowing astronomers to study these objects even when they are too faint to see easily in visible light.