What is a brown dwarf?

A brown dwarf is an object between roughly 13 and 80 Jupiter masses — too massive to be a planet, but not massive enough to sustain hydrogen fusion in its core. It glows faintly from leftover heat and cools steadily forever.

Why is a brown dwarf called a failed star?

Because it forms the same way a star does, by gravitational collapse of a gas cloud, but never reaches the roughly 3 million kelvin core temperature required to fuse hydrogen. It can burn deuterium briefly, then fades.

How hot is a brown dwarf?

They span roughly 250 K to 2,500 K depending on mass and age. The coldest known examples, class Y dwarfs, are around room temperature or below — the coldest, WISE 0855-0714, is near the freezing point of water.

Are brown dwarfs common?

Yes, though they are hard to find because they are so faint in visible light. Infrared surveys such as WISE suggest they may be nearly as numerous as stars, with several known within 10 light-years of the Sun.

Brown Dwarfとは何か？

A brown dwarf is a substellar object: heavier than any planet, lighter than any star. Between roughly 13 and 80 Jupiter masses, it can fuse deuterium for a while but never ignites ordinary hydrogen fusion — so instead of settling onto the main sequence, it simply cools and fades for the rest of time.

The lower boundary is the deuterium-burning limit near 13 Jupiter masses; below that an object is a planet by the usual convention. The upper boundary is the hydrogen-burning limit at about 80 Jupiter masses, or 0.08 solar masses, where core temperature finally reaches the roughly 3 million kelvin needed for sustained proton fusion. In between sits a class of object that was purely hypothetical until the first confirmed detection in 1995.

Remarkably, brown dwarfs are all about the same size — roughly the radius of Jupiter — across that entire mass range. Electron degeneracy pressure props them up, and it barely cares how much mass you add. A 70-Jupiter-mass brown dwarf is not much larger than a 15-Jupiter-mass one, just far denser.

Their atmospheres are the closest thing in nature to a planet's weather on a stellar body: banded, turbulent, and full of clouds of iron, silicate dust, and at the coldest end water ice. The Y-class dwarf WISE 0855-0714 has an effective temperature near 250 K, colder than the Arctic. Luhman 16, a binary pair only 6.5 light-years away, has been mapped well enough to show its clouds rotating in and out of view.