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100 billion failed stars in Milky Way?

A recent survey revealed 1 brown dwarf for every 2 stars in the star cluster RCW 38 and other clusters. “There are a huge number of brown dwarfs out there,” astronomers said.

View larger. | False-color near-infrared image of the core of young massive cluster RCW 38, located 5,500 light-years from our sun. The field of view is about 1.5 light-years across. The insets show the faintest and least massive cluster candidate brown dwarfs, weighing only a few tens of Jupiter masses. Image via astronomers Koraljka Muzic/ Aleks Scholz/ Rainer Schoedel/ Vincent Geers/ Ray Jayawardhana/ Joana Ascenso/ Lucas Cieza.

At last week’s National Astronomy Meeting in the UK, a seven-member team of astronomers from around the world said our Milky Way galaxy could harbor some 100 billion brown dwarfs. Often described as failed stars, brown dwarfs are hybrid objects, too massive to be called planets, but not massive enough to spark thermonuclear fusion reactions in their cores and thus shine as stars do. The new result comes from a survey made using the Very Large Telescope (VLT) in Chile. The team looked carefully at five star-forming regions in our Milky Way, plus the relatively distant star cluster RCW 38.

From their survey, the team estimates that our Milky Way galaxy has at least 25 billion to as many as 100 billion brown dwarfs. Their statement said:

Even this could be a significant underestimation, as there are many lower mass and fainter brown dwarfs, present everywhere in star clusters.

The researchers – led by astronomer Ray Jayawardhana at York University in Toronto, Canada – found that there’s approximately one brown dwarf for every two stars in the star cluster RCW 38, similar to the ratio they had found previously in nearer, less dense star clusters. Team member Aleks Scholz at the University of St. Andrews in the UK said:

We’ve found a lot of brown dwarfs in these clusters. 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.

Jayawardhana and his collaborators have been searching for young brown dwarfs since 2006. Overall, astronomers have identified thousands of these objects since their discovery in 1995. So far, most known brown dwarfs lie within 1,500 light-years of our sun, relatively nearby astronomically speaking, simply because brown dwarfs are intrinsically faint and difficult to observe at greater distances. That is why the observation of the star cluster RCW 38 – 5,500 light-years away – is significant. The astronomers statement explained:

The fact that they have found just as many brown dwarfs in RCW 38 suggests that the environment where the stars form, whether stars are more or less massive, tightly packed or less crowded, has only a small effect on how brown dwarfs form.

The team used an adaptive-optics camera at the VLT to capture images that were as sharp as possible, given the RCW 38’s great distance from Earth and the proximity of the faint brown dwarfs to bright and massive stars.

General size comparison between a low mass star, a brown dwarf, and the planet Jupiter. In this image the brown dwarf is shown to be about 15% larger than Jupiter. Image via Wikimedia Commons.

Bottom line: Astronomers surveyed star-forming regions and the distant star cluster RCW 38, seeking brown dwarfs. They found approximately one brown dwarf for every two stars in RCW 38 and believe there may be some 25 billion to 100 billion brown dwarfs in the Milky Way.

Deborah Byrd