Astronomers from Yale University have helped identify a tiny star system that orbits around our Milky Way Galaxy. According to their research, this finding points to the existence of a new class of very weak satellite star systems that orbit around large galaxies.

An international team led by researchers from Yale and the University of Victoria in Canada recently announced the discovery of Ursa Major III/UNIONS 1 (UMa3/U1), the faintest and least massive star system ever found. This newly discovered system is only 20 light years wide and contains around 60 “mature” stars, which in this case means over 10 billion years old. Its mass is 15 times smaller than the faintest dwarf galaxy.

The researchers studied UMa3/U1 in detail using instruments like the Deep Imaging Multi-Object Spectrograph (DEIMOS) at the W.M. Keck Observatory in Hawaii, the Northern Ultraviolet, Optical, Infrared Survey (UNIONS) on the Canada-France-Hawaii Telescope, and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) at the University of Hawaii’s Institute for Astronomy.

The Milky Way
The Milky Way. Credit: NASA / Wikimedia Commons

There are so few stars in UMa3/U1 that one might question if it’s just a chance grouping of similar stars. However, measurements from DEIMOS clearly show all the stars moving through space at very similar velocities and appearing to share similar chemical compositions. This indicates they are physically associated with each other.

For now, the new star system has two names since astronomers can’t confirm if it’s a dwarf galaxy or star cluster. At the center of this question is the role of dark matter, invisible material thought to form the “scaffolding” of the universe. Future observations with Keck may help clarify if UMa3/U1 is a dark matter dominated galaxy.

The tiny object’s long-term survival is very surprising, as tidal forces from our Galaxy’s disk should have torn it apart by now. Its intact nature leads to two possibilities – it contains large amounts of dark matter for stability, or it’s a star cluster observed just before its imminent demise. Either way, more may be discovered about these extremely weak star systems that have so far eluded detection.


Yale University | Simon E.T. Smith, William Cerny, et al., The Discovery of the Faintest Known Milky Way Satellite Using UNIONS. The Astrophysical Journal, vol.961, no.1, DOI 10.3847/1538-4357/ad0d9f

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