Celestial Speeders

The Milky Way's stars move in predictably lazy ways--but 10 of them are breaking all the rules

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Roger Ressmeyer / Corbis

It's hard to see the few stars that break the march of the night sky, but they're there, racing away at 1 million m.p.h.

In a cosmos filled with peripatetic objects like comets, stars appear to be reliable anchor points. But even in the otherwise orderly Milky Way, at least 10 stars have jumped the rails, blasting along at more than a million miles (1.6 million km) per hour. Mysterious as all of them seem, there's one that's a true puzzle.

It was in 2005 that the first hypervelocity star was found, but astronomers have believed since the 1980s that these stars might exist. A pair of gravitationally linked stars could wander too close to the black hole that lurks in the galactic core, causing one to be sucked in and the other to be violently flung off into space.

Nine of the 10 known hypervelocity stars were probably set in motion this way, but two postdoctoral students at the Carnegie Institution of Washington suggest that the 10th, known as HE 0437-5439, came not from our galaxy but from the nearby Large Magellanic Cloud (LMC). Mercedes Lopez-Morales and Alcestes Bonanos analyzed the position and speed of HE 0437-5439 and calculated that if it came from the Milky Way's core, it must have been traveling for 100 million years. Yet its color and mass put its age at 35 million years. The alternative is that it came from the LMC, which means it didn't have to travel so far. The scientists studied the composition of the star and found that it matched that of LMC stars--a sign that it's indeed an immigrant. The Milky Way's hypervelocity stars are probably outward bound too. There's little risk that we could get whacked by one as it leaves, though. Stars are big, but space is much, much bigger.