Astronomers have long suspected that some stars in the Milky Way can be accelerated to such a speed that they escape its gravitational pull. To find these hypervelocity stars, researchers used the European Space Agency’s Gaia telescope, which is mapping the velocity of over 7 million stars in the Milky Way.

As reported in the Monthly Notices of the Royal Astronomical Society, the team was able to find roughly 20 hypervelocity candidates. Seven stars were traced back to the galactic center but 13, including the fastest two, are actually coming from outside the Milky Way.

“Rather than flying away from the Galactic centre, most of the high-velocity stars we spotted seem to be racing towards it,” co-author Tommaso Marchetti, from Leiden University in the Netherlands, said in a statement. “These could be stars from another galaxy, zooming right through the Milky Way.”

These intergalactic stars might have originated from one of the nearby galactic companions of the Milky Way, such as the Large Magellanic Cloud, or maybe even somewhere further away. The stars are much closer to us than these satellite galaxies so we can study them in better detail, unlocking clues about the environments in which they were born. And the fact that they escaped their galaxies tells us a lot.

“Stars can be accelerated to high velocities when they interact with a supermassive black hole,” added co-author Professor Elena Maria Rossi, also at Leiden University. “So the presence of these stars might be a sign of such black holes in nearby galaxies. But the stars may also have once been part of a binary system, flung towards the Milky Way when their companion star exploded as a supernova. Either way, studying them could tell us more about these kinds of processes in nearby galaxies.”

The two fastest stars move at about 700 kilometers (430 miles) per second, almost three times faster than the average star in the Milky Way. This research wouldn’t have been possible without Gaia. The telescope has created the most detailed map of the Milky Way ever made, showing the positions and colors of about 2 billion stars. It is also studying the velocity of stars, which is a very laborious process.

“We eventually expect full 3D velocity measurements for up to 150 million stars,” explained co-author Anthony Brown, chair of the Gaia Data Processing and Analysis Consortium Executive. “This will help find hundreds or thousands of hypervelocity stars, understand their origin in much more detail, and use them to investigate the Galactic centre environment as well as the history of our Galaxy."

Gaia has truly earned its reputation as one of the most crucial tools of astronomy, both now and in decades to come.