Gaia tracks sibling stars in Milky Way

Stars are born in huge clouds of gas and dust in space. As we look out into our Milky Way galaxy, we see some stars that are still hanging out in their original star families; we say these stars reside in open star clusters. Our sun was surely born in such a cloud, whose stars have now dispersed into the Milky Way at large, moving with the general stream of stars around the galaxy’s center. The search for the sun’s lost siblings is ongoing. But what if we could recognize not just our sun’s siblings but sibling stars across a wide expanse of the Milky Way? In fact, we can. The amazing Gaia spacecraft of the European Space Agency (ESA) has given us that ability. And, contrary to what astronomers believed – rather than leaving home young, as expected – the star siblings revealed by Gaia have been found to stick together in long-lasting star groups. Astronomers are currently referring to these groups of star siblings as “strings.”

An August 28, 2019, statement from ESA explained why information about star families or strings has been so long in coming:

Exploring the distribution and past history of the starry residents of our galaxy is especially challenging as it requires astronomers to determine the ages of stars. This is not at all trivial, as ‘average’ stars of a similar mass but different ages look very much alike.

To figure out when a star formed, astronomers must instead look at populations of stars thought to have formed at the same time – but knowing which stars are siblings poses a further challenge, since stars do not necessarily hang out long in the stellar cradles where they formed.

Circle with many dots and lots of thick and thin wiggly colored lines going different directions.
A face-on view of star families – sibling stars born from a single cloud of gas – in our Milky Way galaxy, within 3,000 light-years of our sun (center of image). The Milky Way is 100,000 light-years wide. Stars in clusters today appear as dots. Co-moving groups – stars born together and still moving together in space – appear as thick lines. The diagram is based on data from the astoundingly useful second data release of the European Space Agency’s Gaia mission. Image via M. Kounkel & K. Covey (2019).

Marina Kounkel of Western Washington University is lead author of the new peer-reviewed study, which was published August 23, 2019, in The Astronomical Journal. She explained:

To identify which stars formed together, we look for stars moving similarly, as all of the stars that formed within the same cloud or cluster would move in a similar way.

We knew of a few such ‘co-moving’ star groups near the solar system, but Gaia enabled us to explore the Milky Way in great detail out to far greater distances, revealing many more of these groups.

Kounkel used data from Gaia’s second data release in April 2018 to trace the structure and star formation activity of an area of space surrounding our solar system, and to explore how this changed over time. This data release lists the motions and positions of over a billion stars with a precision made possible only since Gaia’s launch in 2013. Gaia has been using the unglamorous-sounding tool of astrometry to do something quite amazing. The satellite is charting a three-dimensional map of our galaxy, pinpointing the locations, motions, and dynamics of Milky Way stars, along with additional information about many of these stars. I can’t emphasize enough how Gaia is making possible a view of the Milky Way we never had before. As regards this study, ESA said:

The analysis of the Gaia data, relying on a machine-learning algorithm, uncovered nearly 2,000 previously unidentified clusters and co-moving groups of stars up to about 3,000 light years from us – roughly 750 times the distance to Proxima Centauri, the nearest star to the sun.

The study also determined the ages for hundreds of thousands of stars, making it possible to track stellar ‘families’ and uncover their surprising arrangements.

Dark blue oval with splotchy pink stripe through the center.
Stellar families in Gaia’s sky. Image via ESA/Gaia/DPAC; Data: M. Kounkel & K. Covey (2019).

Kounkel added:

Around half of these stars are found in long, string-like configurations that mirror features present within their giant birth clouds.

We generally thought young stars would leave their birth sites just a few million years after they form, completely losing ties with their original family – but it seems that stars can stay close to their siblings for as long as a few billion years.

And here’s another interesting fact about these star strings, as revealed by Gaia:

The strings also appear to be oriented in particular ways with respect to our galaxy’s spiral arms – something that depends upon the ages of the stars within a string. This is especially evident for the youngest strings, comprising stars younger than 100 million years, which tend to be oriented at right angles to the spiral arm nearest to our solar system.

The astronomers suspect that the older strings of stars must have been perpendicular to the spiral arms that existed when these stars formed, which have now been reshuffled over the past billion years.

Kevin Covey, also of Western Washington University, is a study co-author. He said:

The proximity and orientation of the youngest strings to the Milky Way’s present-day spiral arms shows that older strings are an important ‘fossil record’ of our galaxy’s spiral structure.

The nature of spiral arms is still debated, with the verdict on them being stable or dynamic structures not settled yet. Studying these older strings will help us understand if the arms are mostly static, or if they move or dissipate and re-form over the course of a few hundred million years – roughly the time it takes for the sun to orbit around the galactic center a couple of times.

Further Gaia releases, including more and increasingly precise data, are planned for the coming decade, providing astronomers with the information they need to unfold the star-formation history of our galaxy. Timo Prusti, Gaia project scientist at ESA, said:

Gaia is a truly ground-breaking mission that is revealing the history of the Milky Way – and its constituent stars – like never before.

If you’ve been following Gaia, you’ll agree!

Elongated blob of colored strings and dots.
An edge-on view of stellar groups and strings in our Milky Way galaxy. Image via M. Kounkel & K. Covey (2019).

Bottom line: A new analysis of data from Gaia’s second data release has revealed that star siblings – stars born from the same cloud of gas and dust in space – stick together in long-lasting star groups moving around the center of the Milky Way. Astronomers are referring to these groups as “strings.”

Source: Untangling the Galaxy. I. Local Structure and Star Formation History of the Milky Way


August 29, 2019

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