Young stars have been found in an old part of our galaxy

A star cluster in the Milky Way’s halo may come from gas torn from two other galaxies

Milky way star cluster

A cluster of young stars (marked with blue stars) in the outer reaches of the Milky Way may have formed from gas torn off two smaller galaxies falling toward our own galaxy.

A. Price-Whelan

HONOLULU – A cluster of young stars in the Milky Way is hanging out where it seemingly shouldn’t exist.

Our galaxy is enveloped in an extensive halo of old stars and hot gas — gas which can’t cool down enough to clump together and form new stars. And yet, a flock of relatively new stars is hurtling through the halo, researchers reported January 7 during a news conference at a meeting of the American Astronomical Society.

The star cluster is about 120 million years old and sits about 94,000 light-years away from Earth. Astronomers found it by sifting through data from the European Space Agency’s Gaia satellite for young stars clumped together and moving in the same direction across the sky.

The cluster “didn’t have time to form somewhere else, so it was probably born near where we see it,” said Adrian Price-Whelan, an astrophysicist at the Flatiron Institute in New York City. “But how did it form there, where there’s very little cold gas that you need in order to form a new generation of stars?”

A clue, he said, lies with the Magellanic Clouds, two satellite galaxies of the Milky Way. The cluster appears to be speeding ahead of a stream of gas being torn from those galaxies by the Milky Way’s gravity, suggesting that perhaps new stars are popping out of shredded remains of these satellites.

If the cluster and the stream are connected, they reveal unknown conditions in the halo, said David Nidever of Montana State University in Bozeman at the same news conference. The stars appear to plow ahead, while gas in the halo drags on the stream, slowing it down. Taking into account the star cluster’s age and its 17,000-light-year distance from the leading edge of the stream, Nidever said that gas in the halo may be 10 times as dense as previously thought.

Newfound star cluster in the Milky Way animation
A newfound star cluster (blue dots) sits far above the spiral disk of the Milky Way (white dots) and likely formed out of material from the Magellanic Clouds (purple dots), two satellites of our galaxy.A. Price-Whelan, Simulation by J. Hunt

Christopher Crockett is an Associate News Editor. He was formerly the astronomy writer from 2014 to 2017, and he has a Ph.D. in astronomy from the University of California, Los Angeles.

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