The US is a nation of immigrants, by and large, and now it seems as if the Milky Way galaxy has its share of immigrants, too, in the form of globular star clusters. A new paper from Australian scientists at Swinburne University of Technology demonstrates that a significant chunk of our galaxy's star clusters were born elsewhere in the universe, and kind of migrated to the Milky Way. Or, as astronomer Duncan Forbes put it, they're "not natives, but aliens from other galaxies."
Now, astronomers had suspected this might be the case, but this is science: you need evidence. And the Hubble Space Telescope was there to help. Forbes and his colleagues examined Hubble data on Milky Way stay clusters — not an easy feat, since each of them contain at least 10,000 stars, and as many as several million stars in some cases. And they painstakingly noted the age and chemical properties of each cluster. Those properties make up a cluster's "signature" and can tell astronomers where it originated.
The answer: about one-quarter of our galaxy's globular star clusters — like Messier 80 (pictured above) — are foreign-born, and I doubt they have a green card. But before people start clamoring for NASA to institute an immigration department to keep these hordes of invaders in check, they didn't just come here randomly. The Milky Way lured them in and gobbled them up.
In fact, a side note to these new findings is that our galaxy is a pretty hungry beast, and may have swallowed up a lot more dwarf galaxies than astronomers previously believed. Those are smaller not-quite-big-enough-to-be-full-galaxies that have been absorbed and broken up to be assimilated into the Milky Way. The only traces left of the original dwarf galaxies are these globular star clusters. Only two such dwarf galaxies have been confirmed thus far, but Forbes says his research indicates there could be at least half a dozen more yet to be discovered.
So maybe it's not so much that the Milky Way is being invaded by waves of immigrants, and more like the Milky Way is the Borg, seeking to assimilate everything into its galactic self.