BROWN DWARFS

Shining only very dimly, these low-mass objects are sometimes classed as failed stars

Brown dwarfs are the smallest stars, with masses in the range of 15 to 75 times the mass of the planet Jupiter. Like an ordinary star, a brown dwarf starts life by collapsing under its own gravity from a cloud of interstellar gas, but it doesn’t have enough mass for the core temperature to rise to the point where hydrogen undergoes nuclear fusion. In other words, it never reaches the main sequence phase of stellar evolution. So why is a brown dwarf a star and not simply a very large planet? The reason is that while it’s insufficient for ordinary hydrogen fusion, the core temperature is high enough for another kind of fusion involving a scarcer isotope called deuterium. This means the brown dwarf shines, albeit very dimly, with its own light – something a planet can’t do. Even after all the deuterium is used up, the brown dwarf’s retained heat means that it still radiates more energy than a planet. As it slowly cools down, this radiation declines from reddish light similar to a more conventional hydrogen-burning red dwarf star to very dim infrared light that is only barely perceptible, even with a powerful telescope.