Brown dwarf stars
What a brown dwarf might look like up close
The very beginning of a star forming is a brown dwarf: a star that hasn't gotten enough mass together yet to begin nuclear fusion. They are the smallest and coolest kind of star. Brown dwarfs aren't really brown, but they are a very dull red color. They hardly shine at all. Their surface temperature is usually only about 1000 degrees Kelvin, or about one-sixth the surface temperature of our sun. They are so small that the biggest brown dwarf stars are only about 84 times the mass of the planet Jupiter, and only about 8 percent the mass of our Sun.
The smallest brown dwarf stars are even smaller than that; in fact, it can be hard to tell the difference between the smallest brown dwarf stars and the biggest planets. Most people think of the smallest brown dwarf stars as being about thirteen times as big as Jupiter. But brown dwarfs don't grow out of planets; they form out of the gas clouds between the stars, in space.
Even though brown dwarf stars are too small to start regular hydrogen fusion, they do still give off a lot of heat, partly because of gravity pulling them together and making them smaller, and partly because of a little bit of fusion going on among heavy atoms inside the brown dwarfs.
There aren't very many brown dwarf stars, because most of them soon do gather enough mass to begin nuclear fusion, and then they become ordinary main-sequence stars like our Sun. If they don't get enough mass together to become regular stars, then they only last about ten million years, and then they run out of the heavy atoms they have been using for fuel, and the brown dwarfs fade to black.