100 Billion Brown Dwarfs in the Milky Way?

A new study of a nearby cluster of newly formed stars reveals that brown dwarfs might rival stars in the Milky Way in number, with one brown dwarf for every two bona fide stars.

Artist's illustration of brown dwarf

Artist's illustration of a dimly glowing brown dwarf.
NASA / JPL-Caltech / R. Hurt (SSC)

Since their discovery in 1995, brown dwarfs, so-called “failed stars,” have fascinated astronomers. At less than 8% the mass of the Sun, brown dwarfs fail to ignite fusion in their core. Glowing initially with the heat of their formation, they slowly cool over billions of years like dim red embers. Due to their intrinsic faintness, most of the thousands of known brown dwarfs are relatively close to the Sun, within a few thousand light years. But there are brown dwarfs beyond the grasps of our telescopes — we just don’t know exactly how many.

Now, an international team of astronomers, led by Koraljka Mužíc (University of Diego Portales, Chile, and University of Lisbon, Portugal), has estimated that the Milky Way contains many more brown dwarfs than we once thought. The article is available on the astrophysics arXiv.

Star-forming region RCW 38

A near-infrared false-color image of the star-forming region RCW 38 taken with the European Southern Observatory's Very Large Telescope in Chile.
ESO / K. Muzic (Univ. of Lisbon) / A. Scholz (Univ. of St Andrews) / R. Schoedel (Institute of Astrophysics in Andalucía) / V. Geers (UKATC) / R. Jayawardhana (York University) / J. Ascenso (Univ. of Porto & Univ. of Lisbon) & Lucas Cieza (Univ. Diego Portales)

Mužíc and collaborators trained the European Southern Observatory's Very Large Telescope (VLT) on the RCW 38 star-forming cluster 5,500 light-years from the Sun. Most of the Milky Way’s stars began in similar clusters over billions of years. Such clusters are still valued for studying how stars form and how environment — the physical properties of the gas and dust in the natal cloud — can influence the distribution of the stars that form in the cluster. Even small cluster-to-cluster variations can have a big impact on the final stellar population of the Galaxy.

RCW 38 is massive, as star clusters go, and vigorously forming new stars. Mužíc's team imaged deep into the cluster, identifying bright massive stars and dim brown dwarfs within the same data. Using the VLT, the team was able to capture brown dwarfs with masses down to about 2% that of the Sun.

The team found more brown dwarfs that expected from previous cluster studies. That suggests that brown dwarfs may be more common than previously thought — in this cluster, there’s approximately 1 brown dwarf for every two bona fide stars. When applied to the rest of the galaxy (which contains some 200 billion stars), that adds up to about 100 billion brown dwarfs, dimly gliding along in the the Milky Way.

Brown dwarfs in star-forming region RCW 38

The central part of star-forming region RCW 38 contains brown dwarfs by the dozen. A few examples are shown in the insets.
ESO / K. Muzic (Univ. of Lisbon) / A. Scholz (Univ. of St Andrews) / R. Schoedel (Institute of Astrophysics in Andalucía) / V. Geers (UKATC) / R. Jayawardhana (York University) / J. Ascenso (Univ. of Porto & Univ. of Lisbon) & Lucas Cieza (Univ. Diego Portales)

This result will be directly tested on two fronts. First, studies will continue to refine the brown dwarf populations within other star clusters, such as the Orion Nebula Cluster. Next, large upcoming surveys such as WFIRST and LSST will image the galactic field to faint brightnesses, enabling astronomers to extend the local brown dwarf census. These faint almost-stars may prove to be just as ubiquitous as their stellar siblings.

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