Brown Dwarf

space · strange objects · physics

Not quite a star. Brown dwarfs are too small to start the nuclear fusion of regular hydrogen that makes stars shine brightly. They have between 13 and 80 times the mass of Jupiter, which puts them in a unique in-between zone in the universe.

They still glow. Even though brown dwarfs cannot fuse hydrogen like real stars, they are hot enough inside to fuse deuterium (a special form of hydrogen with an extra neutron). This gives them a faint glow and lets them emit heat into space.

Massive planets. Brown dwarfs are much bigger and heavier than any planet in our solar system, yet they are smaller and dimmer than the smallest stars. They sit in the mysterious gap between the largest gas giants and the tiniest stars.

Fusion in the core. The biggest brown dwarfs can even fuse lithium, another light element, in their cores because the pressure and heat are incredibly high there. This fusion gives them a bit more energy than smaller brown dwarfs.

Hard to spot. Brown dwarfs are so dim and cool compared to stars that they are difficult to find in the night sky, which is why astronomers sometimes call them 'failed stars.'