• Astronomers discovered a cluster of thousands of young stars appearing in the most ancient reaches of the Milky Way.
• The new stars seem to come from two nearby galaxies, the Large and Small Magellanic Clouds, which are on track to crash into our galaxy in 2 or 3 billion years.
• The discovery indicates that this galactic collision could happen sooner than scientists expected.
A future galactic collision is already causing new stars to form at the edge of the Milky Way, astronomers say.
The farthest reaches of our galaxy contain its oldest stars. So astronomers were surprised when they recently found a cluster of thousands of young stars at the edge of the Milky Way.
"It's really, really far away," Adrian Price-Whelan, an astronomer at the Flatiron Institute who led the team behind the discovery, said in a press release. "It's further than any known young stars in the Milky Way, which are typically in the disk. So right away, I was like, 'Holy smokes, what is this?'"
The researchers presented their findings at the annual meeting of the American Astronomical Society on Wednesday.
They think the mysterious young stars come from two nearby galaxies called the Large and Small Magellanic Clouds, which are on track to crash into our galaxy in 2 or 3 billion years. But the new arrivals indicate that collision could happen in half the time scientists previously expected.
The stars came from a stream of alien gas
Price-Whelan's team identified the new stars using data from a European space telescope called Gaia, which launched in December 2013 with the ambitious goal of making a 3D map of the galaxy.
By removing known star clusters from the data, the researchers narrowed in on a group of stars that are about 117 million years old. That's quite young compared to the 13-billion-year-old ancients at the edges of the Milky Way.
The stars' similar ages and locations suggest that they all formed together.
To figure out where the group came from, the researchers analyzed light from the 27 brightest stars in the cluster. The frequencies of that light showed that the stars didn't contain much metal, even though metal-heavy material makes up the edges of the Milky Way.
However, the stars' contents did look similar to those of the nearby Magellanic Stream —a river of gas that stretches from the Large and Small Magellanic Clouds towards the Milky Way.
The researchers think the new star cluster began as a cloud of gas in the Magellanic Stream, which then compressed to form stars as the tug of the Milky Way pulled it in.
A galactic merger could come sooner than expected
Since it's easier to measure the distance of stars than clouds of gas, the discovery allowed the scientists to estimate the distance of the Magellanic Stream with unprecedented accuracy. They predicted that the stream is 90,000 light-years away from the Milky Way — half the distance scientists previously thought.
"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," David Nidever, a physicist at Montana State University who analyzed the stars' metal content, said in the release.
Studying these new stars could also help astronomers settle a debate about whether the Magellanic Clouds have ever passed through the Milky Way in the past.
But don't worry: The future collision won't be catastrophic, since galaxies are mostly empty space. That means they usually pass through each other with very few collisions between stars, if any.
In fact, Nidever said, the merger could be good for our galaxy.
"Right now, our galaxy is using up gas faster than it's being replenished," he said. "This extra gas coming in will help us replenish that reservoir and make sure that our galaxy continues to thrive and form new stars."