For those of you worried about the nearby Andromeda galaxy colliding with our own Milky Way in the distant future, here’s a little perspective: Turns out our spiral galaxy is still reeling from a hard knock it suffered as recently as 100 million years ago -- but it’s likely to recover just as quickly, say researchers at the Fermi National Accelerator Laboratory in Batavia, Ill.

Fermilab scientists looked at about 300,000 stars cataloged by the Sloan Digital Sky Survey – which has mapped a full 35% of the sky using an optical telescope in New Mexico – and noticed something strange: Some nearby stars north and south of the Milky Way’s plane were out of sync relative to one another.

As these stars spin around the Milky Way’s flattened disc at about 220 kilometers per second, they also bob up and down at roughly 20 to 30 kilometers per second. Since they’re doing about the same thing, the distribution and motion of these stars along the north-south divide should be fairly symmetric.

But to the scientists’ surprise, neither the stars’ positions nor their motions were as regular as they expected. At first, they thought it might have to do with interference from all the interstellar dust hanging out between the stars, or something to do with how they selected which stars to study.


Eventually, they came up with an explanation: The Milky Way had suffered a hit about 100 million years ago, and the galaxy is still “ringing” like a bell from the force of it -- which would explain why so many stars were out of place. It could have been a smaller galaxy, or a large structure made out of dark matter – mysterious, invisible stuff that doesn’t interact much with regular matter, but whose gravitational influence can have a powerful effect -- especially since there’s more than four times as much dark matter in the universe as there is regular matter.

Though they’re not sure what caused this galactic “ringing,” the researchers say it should die down in another 100 million years, allowing the stars to settle back into equilibrium -- so long as another galaxy or giant dark-matter clump doesn’t hit.

But if we’ve survived a hit before, we can survive it again. Perhaps this will assuage any folks worried about the Milky Way-Andromeda mashup set to take place in 4 billion years. After all, even though those beautiful images of the galaxy look thick with stars, there’s actually a ton of space between each one. So even though the entire solar system may be flung farther out in the galaxy, it’s unlikely we earthlings would really feel a thing.

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