Stars on the outskirts of the Milky Way

Astronomers have found a flock of young stars where they didn’t expect to: on the outskirts of the Milky Way among the galaxy’s oldest stars. 

If that wasn’t surprising enough, spectral analysis suggests they have an extragalactic origin. 

As Adrian Price-Whelan told this week’s meeting of the American Astronomical Society in Hawaii, the stars seemingly formed not from material from the Milky Way, but from the two nearby dwarf galaxies known as the Magellanic Clouds. 

This, he says, suggests that a stream of gas extending from the galaxies is about half as far from crashing into the Milky Way as previously thought.

Price-Whelan, from the Flatiron Institute in New York, led the team which discovered the cluster – now known as Price-Whelan 1. He is also the lead author of a paper in The Astrophysical Journal.

He started the ball rolling by searching the Gaia spacecraft’s dataset for very blue stars, which are rare in the Universe, and identifying clumps of stars moving alongside them. After cross-matching with and removing known clusters, one remained.

Price-Whelan 1 is relatively young at 117 million years, and inhabits a region near a river of gas, dubbed the Magellanic Stream, that forms the outermost edge of the Large and Small Magellanic Clouds and reaches towards the Milky Way. 

The researchers propose that it formed as gas from the Magellanic Stream passed through the gases surrounding the Milky Way. This created a drag force that compressed the Magellanic Stream gas. 

This drag, along with tidal forces from the Milky Way’s gravitational tug, condensed the gas enough to trigger star formation. Over time, the stars zoomed ahead of the surrounding gas and joined the Milky Way.

Because it tricky to gauge the distance of gas from Earth, until now astronomers haven’t been sure how far the Magellanic Stream is from reaching the Milky Way. 

Now that they have stars to work with, Price-Whelan and colleagues suggest it’s about 90,000 light years– roughly half the distance previously predicted.

“If the Magellanic Stream is closer, especially the leading arm closest to our galaxy, then it’s likely to be incorporated into the Milky Way sooner than the current model predicts,” says co-author David Nidever, from Montana State University, US. 

“Eventually, that gas will turn into new stars in the Milky Way’s disk. Right now, our galaxy is using up gas faster than its being replenished. This extra gas coming in will help us replenish that reservoir and make sure that our galaxy continues to thrive and form new stars.”