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Astronomers have known about celestial objects called globular clusters for centuries. They’re collections of maybe 100,000 stars, tightly bunched into a roughly spherical configuration, which orbit the central part of the Milky Way and other galaxies in a relatively lazy fashion, like a swarm of laid-back bees.
But observers have spotted one such cluster that’s behaving extremely strangely. This one is located in the vicinity of M87, a gigantic, blob-shaped galaxy—at least ten times as massive as the 300 billion star Milky Way—which lies about 53 million light years from Earth. The cluster doesn’t seem to be orbiting, and it’s anything but lazy: instead, it’s speeding in our direction at a blistering 2 million m.p.h., (3.2 million k/h) as though it’s been flung from its home galaxy like a bullet—except much, much faster. “We’ve found speeding, runaway stars before,” says Harvard astronomer Nelson Caldwell, who co-discovered this bizarre object. “But this is the first time we’ve found a runaway star cluster. We’re not really sure what’s going on.”
The best explanation, Caldwell believes, is that the globular cluster wandered too close to a pair of giant black holes thought to lie in the core of M87. The complex gravitational interaction between the cluster and the black holes could have provided the gunshot force to fling it into space. “It’s plausible,” says Caldwell, “but not entirely satisfying.”
One problem, he explains, is that the existence of both black holes in M87 is by no means proven, and a single one would not provide the necessary gravitational kick to account for what the astronomers have observed. What’s more, the globular cluster would have had to have approached to within just a few light-years of the black holes to pick up such enormous velocity. If it came that close, however, the enormous gravity of the holes should have stripped away a lot its stars, yet the cluster appears to be zipping through space reasonably intact.
Read more:
Galactic Cannonball Heading Our Way