Unknown Force Kicks Stars Out of Milky Way. Really.

If you could see it from the outside, the Milky Way would look like a majestic spiral of stars and glowing gas, rotating on its axis once every 200 million years or so like a gigantic, pinwheel 100,000 light-years across. Up close, though, you’d see that each star is jostling along on its own, moving through an entirely independent orbit. It’s more like an unruly crowd, going in the same approximate direction at the same approximate speed, than a military formation marching in lockstep.

At least a few stars, however, are headed somewhere else entirely, and in a big hurry. They’re known as hypervelocity stars, and they’re going so fast that they’re on their way out of the Milky Way altogether. While the astronomers think they have an explanation for the 18 hypervelocity stars discovered since 2005, a new group of 20, just announced at a meeting of the American Astronomical Society outside of Washington, DC, seems to make no sense at all.

“What’s going on?” asks Vanderbilt University grad student Lauren Palladino, lead author of a paper in the Astrophysical Journal describing these cosmic speedsters. “We don’t know.”

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What astronomers do know is that virtually all of the hypervelocity stars seen until now share three characteristics: First, they’re very massive and very bright. Second, they’re moving at a minimum of 1.5 million mph (2.4 million k/h). Escape velocity for the Milky Way—or the speed required to break the galaxy’s gravitational grip—is about 1 million mph (1.6 million k/h); by comparison, escape velocity for Earth is a paltry 25,000 mph (40,000 k/h). Finally, all of the hypervelocity stars previously observed have been shooting out of the center of the galaxy.

The explanation for all of this, theorists agree, begins with a pair of binary stars. The fact that the two stars are orbiting each other gives them a lot of angular momentum. Every so often, a binary approaches too close to the giant black hole at the core of the Milky Way. One of the stars is swallowed up; the other retains most of the momentum of the previous two-star system and that, combined with the black hole’s gravity, slingshots it outward at high speed.

But there’s no reason only massive stars should be affected, so Palladino and several colleagues set out to look for smaller, Sun-like stars racing into extragalactic oblivion as well. They relied on the powerful Sloan Digital Sky Survey, which has been taking precise, detailed images of the night sky since the late 1990’s, and compared them with earlier sky images, to look for small, unusually fast-moving stars.

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A handful of these rogue stars did turn up in the search, but when the astronomers traced their trajectories backward, says Palladino, “they were coming from random directions. None of them started out in the galactic center.” She and her colleagues considered the possibility that at least some of the stars might have been ejected long ago from the Andromeda galaxy; maybe they were just passing through our neighborhood today. But no, that wasn’t the case either, since the same backward trajectory tracking did not put their point of origin in the Andromeda neighborhood.

Theorists have offered a few ideas about what might have jolted these stars out of their calm orbital amble and sent them shooting toward the depths of intergalactic space. Maybe they were spun out by a gravitational slingshot effect inside dense clusters of stars, or maybe they were whipped to high speed by being too close to an exploding supernova.

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But the investigators concede that these explanations aren’t very persuasive and that the answer, for now, remains elusive. “It’s very hard to kick a star out of the galaxy,” said Kelly Holley-Bockelmann, Palladino’s advisor and a co-author on the paper, in a press release.

Whatever the explanation for the rogue stars, for any intelligent beings living on planets that might be orbiting one of them, nothing much would appear to be amiss for eons. It will take many millions of years before the stars approach the outskirts of the Milky Way. After that, the skies would go gradually dark—aside from whatever other planets and moons were part of the stars’ retinue, and the vanishingly faint smudges of light that marked distant galaxies. In theory, the speeding stars might reach one of those faint smudges, but that would take tens of billions of years—and unless you sit in the middle of a rich cluster of galaxies, the distances are so great and the galaxies spread so thin that the odds are very much against it. Instead, the castaway star, with its castaway planets, would simply wander the cosmos forever.

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