100 billion brown dwarfs may populate our galaxy

The Milky Way may contain as many as 100 billion brown dwarfs, according to new research to be published in the Monthly Notices of the Royal Astronomical Society.

Brown dwarfs are failed stars that were not quite heavy enough to sustain the hydrogen fusion reactions that make real stars shine. They weigh about 10 to 100 times as much as Jupiter, which means their internal gravitational pressure is enough to fuse deuterium (a kind of hydrogen that contains an extra neutron in each atom) and sometimes also lithium. This means they glow only dimly. Most of the radiation they do give off is in the infrared spectrum and hence invisible to the human eye, though some would appear faintly purple or red. {%recommended 1586%}

All of this makes brown dwarfs very hard for astronomers to spot. While scientists have speculated since the 1960s that they might exist, the first definite sightings did not occur until 1995. 

The new research, by an international team of astronomers lead by Koraljka Muzic from the University of Lisbon and Aleks Scholz from the University of St Andrews, used the European Southern Observatory’s Very Large Telescope to make infrared observations of distant dense star clusters where many new stars were being formed. They counted as many brown dwarfs as they could, and came to the conclusion there were about half as many brown dwarfs as stars.

Earlier studies of brown dwarfs – which focused mainly on regions nearer to Earth where stars are less dense, simply because they are easier to see if they are closer – concluded the substellar objects were much less common.

The researchers estimate the minimum number of brown dwarfs in the Milky Way is at least 25 billion and as high as 100 billion – but they note even this upper figure may be an underestimate, given the probability of there being many more failed stars too faint to see at all.