Science & Technology

A River of Stars: astronomers discover a nearby stellar stream

Night sky centered on the south Galactic pole in a so-called stereographic projection. In this special projection, the Milky Way curves around the entire image in an arc. The stars in the stream are displayed in red and cover almost the entire southern Galactic hemisphere, thereby crossing many well-known constellations. Background image: Gaia DR2 skyma
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Using the Gaia satellite astronomers from the University of Vienna have discovered a nearby river of stars – or stellar stream – containing 4000 stars, covering most of the southern sky. The discovery could have implications for the measurement of the 
Milky Way’s mass and our understanding of star formation. 

The paper – published in the journal Astronomy & Astrophysics – reveals that the stars in the stellar stream -which measures 400 parsecs in length and 50 parsecs in depth – have been moving in tandem through for roughly 1 billion years since they were formed.

Due to its proximity to Earth – only 100 parsecs from the Sun – this stream is a perfect workbench on which to test the disruption of clusters, measure the gravitational field of the Milky Way, and learn about coeval extrasolar planet populations with upcoming planet-finding missions. For their search, the authors used data from the ESA Gaia satellite.

Even though the Milky Way contains many clusters of stars – of varying ages and sizes – this stellar stream is fairly unusual as it hasn’t yet been pulled apart by tidal forces and other gravitational influences.

Stefan Meingast, lead author of the paper explains: “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 neighbourhood, 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.”

Artist’s impression of the Gaia satellite which researchers used to observe a nearby stellar stream (ESA)

The precision of the Gaia measurements allowed the authors to measure the 3D motion of stars in space. When carefully looking at the distribution of nearby stars moving together, one particular group of stars, as yet unknown and unstudied, immediately caught their attention. It was a group of stars that showed precisely the expected characteristics of a cluster of stars born together but being pulled apart by the gravitational field of the Milky Way.

João Alves, the second author of the paper, says: “Identifying nearby disk streams is like looking for the proverbial needle in a haystack. Astronomers have been looking at, and through, this new stream for a long time, as it covers most of the night sky, but only now realize it is there, and it is huge and shockingly close to the Sun.

“Finding things close to home is very useful, it means they are not too faint nor too blurred for further detailed exploration, as astronomers dream.”

The sensitivity limitations of the Gaia observations meant that the researchers’ selection only contained about 200 sources. An extrapolation beyond these limits suggests the stream should have at least 4000 stars, thereby making the structure more massive than most know clusters in the immediate solar neighbourhood.

The authors also determined the stream’s age to be around one billion years. As such, it already has completed four full orbits around the Galaxy, enough time to develop the stream-like structure as a consequence of gravitational interaction with the Milky Way disc.

This newly discovered nearby system can be used as a valuable gravity probe to measure the mass of the Galaxy. With follow-up work, this stream can tell us how galaxies get their stars, test the gravitational field of the Milky Way, and, because of its proximity, become a wonderful target for planet-finding missions. The authors hope to unravel even more such structures in the future with the help of the rich Gaia database.

Masters student at the University of Vienna and co-author of the paper, explains the significance of its results: “As soon as we investigated this particular group of stars in more detail, we knew that we had found what we were looking for: A coeval, stream-like structure, stretching for hundreds of parsecs across a third of the entire sky.

She adds: “It was so thrilling to be part of a new discovery” 

Original research: https://www.aanda.org/articles/aa/abs/2019/02/aa34950-18/aa34950-18.html

Featured image: Night sky centered on the south Galactic pole in a so-called stereographic projection. In this special projection, the Milky Way curves around the entire image in an arc. The stars in the stream are displayed in red and cover almost the entire southern Galactic hemisphere, thereby crossing many well-known constellations. Background image: Gaia DR2 skyma (Astronomy and Astrophysics)

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