The Milky Way could contain at least 100 billion brown dwarfs

Over 100 billion – that is the number of brown dwarfs that could inhabit our galaxy. The huge figure was determined by a recent study led by Koraljka Muzic from the University of Lisbon and Aleks Scholz from the University of St. Andrews.

Brown dwarfs are colloquially known as ‘failed stars’ because they are not heavy enough for hydrogen fusion: the primary process that makes stars glow. Due to the lack of hydrogen fusion, brown dwarfs cannot shine brightly like their ‘brothers and sisters’, instead appearing magenta or purple. They are considered as by-products of processes that lead to the formation of other stars and planets.

When astronomers began their search for them, they found that most of the detectable brown dwarfs are found close to the Sun within distances of 1,500 light years. One light year is approximately 63,325 times the distance from the Earth to the Sun. Only stars in close proximity to the Sun are detectable, since they have a very faint appearance. All of the others are assumed to be located in nearby star-forming regions.

After the initial discovery of brown dwarfs in 1995, the international team of astronomers led by Muzic and Scholz started a new search for brown dwarfs in 2006. They investigated the regions around two star clusters – NGC 1333, in the constellation of Perseus, and RCW 38, in the constellation of Vela, which has more massive stars than NGC 1333.

Their results showed that regardless of the attributes that the star clusters have, such as how massive they are, or how packed together they are, the number of brown dwarfs always turns out to be roughly half of the total number of stars.

“We’ve found a lot of brown dwarfs in these clusters,” says Scholz to a journalist of the Royal Astronomical Society, “And whatever the cluster type, the brown dwarfs are really common. Brown dwarfs form alongside stars in clusters, so our work suggests there are a huge number of brown dwarfs out there.”

The group concluded that the number of brown dwarfs in the Milky Way is at least 25 billion, but could be up to 100 billion. They also suggested that this number could be massively underestimated, given the fact that a large population will be too faint to be detected. However, further research is necessary to verify such a claim and until then, we will keep counting.

Si Wu is a Physics alumnus from Imperial College London

Banner image: nebula, Vadim Sadovski