Astronomers are puzzling over how some stars—so-called hypervelocity stars—can surpass the Milky Way’s speed limits, moving fast enough to escape our galaxy entirely. The first such galactic outcast was discovered in 2005, whizzing along at about three million kilometers per hour—fast enough to fly from New York City to Los Angeles in less than five seconds. Its path, like that of many other hypervelocity stars that were subsequently found, traced back to the vicinity of a four-million-solar-mass black hole lurking at the galactic center—a supermassive monster with more than enough heft to hurl a star at such high velocity. Now, however, new results from the European Space Agency’s Milky Way–mapping Gaia spacecraft and other projects are hinting that another culprit—a never before seen class of black holes—may be responsible for some of our galaxy’s emigrants.

One hypervelocity star, ignominiously known as PG 1610+062, is the catalyst for much of the research. Initially observed with little fanfare in 1986, PG 1610 is not actually moving quite fast enough to leave our galaxy. In 2015 astronomers routinely studying the star realized it had been misclassified and actually shone brighter than expected; PG 1610, it turns out, is farther away than previously appreciated. Combined with Gaia’s most recent observations, the new distance showed the star to be on a path that never took it to the galactic center, where an encounter with the supermassive black hole located there could have imbued it with greater speed.

The most likely culprit for accelerating PG 1610, some researchers say, is an intermediate-mass black hole (alternatively, others suggest an extremely massive star could possibly do the job). Intermediate-mass black holes remain theoretical and have never been directly detected. But PG 1610 and other stars like it could help change that. “Runaway stars could be a good way to find those intermediate-mass black holes,” says Andreas Irrgang of Germany’s Friedrich-Alexander University of Erlangen–Nuremberg, who is the first author on a new preprint paper reporting the results that will be published in Astronomy & Astrophysics.

Runaway Star

The canonical theory for the origins of hypervelocity stars holds that they began their lives in a tight orbit around a companion star near the galactic center. Drifting too close to the supermassive black hole, the companion star was stripped away and captured, unleashing the other star to fly away with astonishing swiftness.

That consensus view began to crumble last year, when the Gaia team released new data tracking the motions of nearly 1.7 million stars across the sky. Using those data, astronomers traced the trajectories of many of the previously discovered fast-moving stars, finding that, like PG 1610, nearly a third of them were embedded in the Milky Way’s starry disk, never having approached the galactic center. For such stars headed out of the Milky Way, something besides a supermassive black hole must be providing the necessary boost.

In the case of PG 1610, Irrgang and his colleagues suspect that an intermediate-mass black hole may be responsible, although an ultramassive star could also fit the bill. Both objects are thought to exist in the dense hearts of very young and massive star clusters. A pair of close stars encountering either object would react much the same way as stars that come near to a supermassive black hole—with one being captured into orbit, and the other being cast away at high speed. Whether or not the latter escapes the galaxy depends on which direction it is thrown.

“It’s basically like throwing a ball off a moving train,” says hypervelocity star hunter Monica Valluri of the University of Michigan. If the star is ejected in the direction of the galaxy’s spin, it gets a boost in speed. If it is fighting galactic spin, however, it will slow down and not escape the Milky Way. In other words, PG 1610 remaining in our galaxy was purely a matter of chance; other hypervelocity stars of its ilk in the Milky Way’s disk could just as easily find themselves outward-bound, slipping beyond our galaxy’s gravitational clutches.

At only 56,000 light-years away, PG 1610 is fairly close for a hypervelocity star. Its proximity allowed researchers to robustly study its chemical makeup—a feat that, to date, has only been achieved for one other hypervelocity star. “Because it is more nearby, we could really pinpoint the origin of the star in the galactic disk,” Irrgang says.

Live Fast, Die Young

Extremely massive stars, weighing in anywhere from 50 to a few hundred solar masses, are thought to occasionally form via stellar collisions inside of the youngest, largest star clusters. Astronomers have spotted stars as big as an estimated 300 solar masses in these hot, dense regions of the Milky Way—and some theorists believe such stars could potentially grow as large as 1,000 solar masses.

That bulk comes with a price: Such massive stars live only a few million years, an eyeblink as compared with our sun’s expected nine-billion-year lifetime. Consequently, if ultramassive stars are the source of hypervelocity stellar ejections from the Milky Way’s disk, they must do that work quickly, before their untimely death.

According to Irrgang’s observations, PG 1610 is only about 80 million years old and was ejected about 40 million years ago. Because stars in a cluster tend to be born around the same times, they should have roughly the same age, which would suggest a very long-lived ultramassive star ejected PG 1610—a scenario Irrgang deems “unlikely.”

Vasilii Gvaramadze, a researcher at Moscow State University, who models hypervelocity stars and was not part of the new study, is not ready to give up on ultramassive stars as a source. “It’s very difficult to estimate the age and flying time of stars,” he says. Estimates of both depend on different models, and a small error on either could have a significant effect in the final tally.

Mapping Midsize Black Holes

Once ultramassive stars explode in violent supernovae, theories suggest they leave behind an intermediate-mass black hole. Smaller than the monster in the Milky Way’s heart, these midrange beasts should weigh between 100 and a few thousand solar masses and would have plenty of time to cast stars from their birth cluster.

The problem is that no intermediate-mass black holes have ever been conclusively seen—evidence for their existence is compelling but circumstantial. “Already, this claim is a little bit controversial,” says Alessia Gualandris of the University of Surrey in England, who was not involved in the new study. “You’re trying to explain a process with something never directly detected.” Despite these reservations, Gualandris, who has modeled how hypervelocity stars interact with massive objects, suspects that intermediate-mass black holes are more likely to be the culprit than rare and fleeting ultramassive stars.

One reason for her suspicion emerged last year in additional Gaia results that showed one of the Milky Way’s hypervelocity stars was flung here from the Large Magellanic Cloud (LMC), a dwarf galaxy approximately 160,000 light-years from our own. Most dwarf galaxies are thought to lack sufficient material to form supermassive black holes—making intermediate-mass black holes the best explanation for any hypervelocity stars launched from the LMC, Gualandris says.

If intermediate-mass black holes are scattered throughout young, dense stellar clusters in the Milky Way, hypervelocity stars could help to find them. Irrgang and his colleagues have traced PG 1610 back to the Carina-Sagittarius spiral arm of the Milky Way, giving black hole hunters a place to look for the hard-to-spot objects.

“It still is not proof that [intermediate-mass black holes] exist, but it makes it important to try to understand how they formed,” Valluri says. “It will impact our understanding of how these stars evolve and how the most massive stars evolve into black holes.”