A river of stars discovered in the cosmic neighborhood

The starts that form the cosmic river in red with a Gaia map in the background (Image Meingast et al / Gaia DR2 skymap)
The starts that form the cosmic river in red with a Gaia map in the background (Image Meingast et al / Gaia DR2 skymap)

An article published in the journal “Astronomy & Astrophysics” reports the discovery of what was called a river of stars, a stream of stars a little over 300 light years away from us that occupies most of the southern sky. Astronomers of the University of Vienna used information collected by ESA’s Gaia space probe and published in the so-called Data Release 2 (DR2) to discover at least 4,000 stars that have been moving together in space since their formation, which was about a billion years ago.

The stars of the Milky Way are generally formed in groups that can also be very numerous within gas clouds but these star clusters tend to fall apart due to the many gravitational influences existing within the galaxy. For this reason, there are many newborn clusters still inside molecular clouds, few clusters of middle and old age in the galactic disk, and even less massive and old clusters in the galactic halo.

Dr. Stefan Meingast, the first author of this research, pointed out that even in the solar neighborhood there are some clusters that are massive enough to remain bound for several hundred million years. The consequence is that, in principle, even remains of clusters that form a stream should be part of the galactic disk. To find their traces, together with his collaborators he used the precision of the measurements carried out by the Gaia space probe to measure the 3D motions of stars in space.

Dr. João Alves, the second author of the article, compared the research to that of the classic needle in the haystack and explained that astronomers have been watching the new stream for a long time time as it covers most of the night sky but only now, thanks to the work carried out by Gaia, they became aware of its nature and that it’s really close in astronomical terms. Proximity is a big advantage because those stars are not too dim or blurry for further detailed exploration.

The Gaia space probe is conducting an extraordinary work collecting a lot of information on a huge amount of cosmic objects but its sensitivity is limited so it was possible to select only 200 of them that revealed a group of stars that showed precisely the expected characteristics in a cluster of stars born together but in the process of separation due to the gravitational field of the Milky Way. By extrapolating that data, the researchers estimated that at least 4,000 stars are included in that cosmic river.

Verena Fürnkranz, a student at the University of Vienna and third author of the article, added that when she and the other researchers investigated that particular group of stars in greater detail they understood that they had found what they were looking for: a structure with the appearance of a stream that extends for at least 1,300 light years across the sky.

This river of stars will be dispersed over time like the other clusters but it’s a process that could go on for a few hundred million years. In the meantime, it will be useful for example to assess the gravitational effects of the Milky Way in that area. The information published in Gaia’s Data Release 2 on April 25, 2018 are producing many studies, in this case offering a new vision of stars that were really before our eyes. The researchers hope to find more structures of this kind but it’s just one of many types of astronomical research that are taking advantage of Gaia’s work.

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