Billion-Year-Old River of Stars Seen Flowing Near the Sun

A river of stars in our galactic neighborhood has recently been spotted by astronomers using the Gaia satellite. This family of stars travel together, with their edge currently just 330 light years distant from the Sun — relatively close on a galactic scale.

This grouping of more than 4,000 stars covers most of the night sky as seen from Earth. Roughly one billion years old, and traveling around the Milky Way once every 250 million years, they have orbited around the edge of the Milky Way Galaxy four times since their formation. Although these stars have been seen before by astronomers, this unusual formation was largely unexpected.

“Most star clusters in the Galactic disk disperse rapidly after their birth as they do not contain enough stars to create a deep gravitational potential well, or in other words, they do not have enough glue to keep them together. Even in the immediate solar neighborhood, there are, however, a few clusters with sufficient stellar mass to remain bound for several hundred million years. So, in principle, similar, large, stream-like remnants of clusters or associations should also be part of the Milky Way disk.“ said Stefan Meingast, lead author of the paper published in Astronomy & Astrophysics.

A river of stars in the Milky Way galaxy.
A river of stars mapped against the Milky Way, as seen by Gaia. Image credit: Gaia DR2 skymap

The Milky Way Galaxy is surrounded by local groups of stars, but these are either loosely-bound together in open clusters, or found in the spherical form of globular clusters.

Andromeda Galaxy
The Andromeda Galaxy (also called M31) is the largest close neighbor of The Milky Way, and looks much like our galaxy would, seen from the outside. Image credit: NASA/MSFC/Meteoroid Environment Office/Bill Cooke

The stream of stars was discovered by researchers using the Gaia telescope, operated by the European Space Agency (ESA). The observatory identified just 200 stars in the group, but extrapolation of the finding reveals there are likely more than 4,000 stellar bodies within the stream. This grouping of stars extends for several hundred light years in length. They stars likely formed together, but the group is being slowly torn apart by gravity from our home galaxy.

Estimates of the number of stars in the Milky Way suggest there are between 200 and 400 billion stellar bodies in galaxy. We now know that most stars are accompanied by planets, meaning there are likely trillions of alien worlds within our family of stars.

The Gaia spacecraft was designed to study the positions and velocities of one billion stars of the Milky Way, in an effort to better understand the structure and nature of our galaxy.

The Milky Way as seen from Earth
This is how we are used to seeing the Milky Way galaxy — from Earth. Image credit: bromatofiel/Flickr

“The detailed knowledge of our Galaxy obtained from the study of the Gaia data, will provide a firm base for the analysis of other galaxies for which this level of accuracy cannot be achieved through direct observations like in our own Galaxy, where we have a close-up and inside view,” the ESA reports.

Until 1929, astronomers believed that the Milky Way was the only galaxy in the Universe. Now, we know the visible Universe contains between several hundred billion to two trillion other families of stars. Examination of streams like this one can help astronomers learn more about the nature of stars, and how they interact with galaxies like the Milky Way.


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