brown dwarf

A brown dwarf is a failed star: an object that formed like a star, from a collapsing clump of gas, but ended up too light to ever ignite ordinary hydrogen fusion in its core. It sits in the no-man's-land between the smallest true stars and the largest planets, too big to be called a planet, too small to shine as a star. Despite the name, brown dwarfs are not brown; they would glow a dim dull red or, when cool, almost invisibly dark.

The dividing line is about 8 percent of the Sun's mass, roughly 80 times the mass of Jupiter. Below this, the core never gets hot enough, about ten million degrees, to fuse ordinary hydrogen in a self-sustaining way, so the object can never settle onto the main sequence as a true star. A brown dwarf does briefly fuse deuterium (a heavy form of hydrogen) and, if heavy enough, lithium, but that flickers out, and thereafter it has no internal furnace at all. It simply glows from leftover heat of formation and slowly fades and cools over billions of years, drifting from a faint red to ever cooler, darker, infrared-only embers. Their atmospheres are cool enough for clouds and even rain of exotic minerals.

Brown dwarfs matter because they mark the very bottom rung of star formation and tell us where the star-making process finally fails. They are the lowest-mass end of the initial mass function, and their abundance is a direct test of how low fragmentation can go. Because they are so faint, they are best hunted in the infrared; whole new spectral classes (L, T, and the very coldest, Y) were created to classify them. The boundary with planets is partly a matter of definition and origin, which is one reason brown dwarfs blur the once-tidy line between stars and planets.