Bad Astronomy

Alien clusters invade our galaxy!

Is this the face of an alien?

According to a new study, the answer is probably yes.

That’s Palomar 5, a globular cluster very roughly 75,000 light years away. Globulars are ball-shaped collections of hundreds of thousands of stars, and surround many large galaxies, kinda like bees swarming around a hive. There are at least 150 orbiting our own galaxy, the Milky Way.

The question is, how many of these formed here, along with our galaxy 12 billion years ago, and how many formed around other galaxies and were subsequently subsumed into us?

The Milky Way is a giant galaxy, and we know it got that way by eating – astronomers call it cannibalizing, because we’re zombie fans – smaller galaxies. We see the remnants of some of those meals as streams of stars that got torn out of the galaxies as they got digested, and sometimes we see the residual core of stars from the galaxy itself still somewhat intact – indigestible bit of chewing gum, you might say.

So the astronomers in the new study asked themselves: just how many of the Milky Way’s globular clusters formed along with the original Milky Way, and how many came from other galaxies that were eaten? The answer they found was surprisingly high: it may be as many as 1/4 of them!

They examined what’s called the Age-Metallicity Relation in the clusters, a way of figuring out the age of a cluster by looking at the relative numbers of heavy elements in it (astronomers call any element heavier than helium a metal). What they found is that most of the globular clusters in and around the Milky Way are about the same age as the galaxy itself: 12 billion years, give or take. However, quite a few are much younger, by several billion years.

Some we already know about, and are associated with known cannibal events (the Sagittarius dwarf galaxy upon which we’re currently dining, for one). But still, something like 30 - 50 clusters still remain that are too young to have been native to the Milky Way – and Palomar 5 (in the picture above) is among them.

This implies something like 6 - 8 galaxies have been eaten by our galaxy to make it what it is today. That’s pretty neat. I’ve often wondered just how many galaxies were sacrificed to make the Milky Way one of the biggest galaxies in the Universe – and it really is; while there are plenty that are bigger, and some that are a lot bigger, we’re still in the upper echelons of the cosmos if you rate galaxies by sheer size and mass. Now it looks like we had to eat a half dozen less fortunate galaxies to get where we are today.

And we’re not done yet. In a billion years, maybe two, it’s likely that the Milky Way and the massive Andromeda galaxy will collide – perhaps not directly at first, but over hundreds of millions of years they’ll merge into one even more gianter galaxy, potentially igniting a burst of star formation and tossing around stars like bugs in the wind. We may wind up consuming the score of dwarf galaxies in our own Local Group as well.

What I find interesting is that the Sun will still be around then; it won’t go red giant on us for another few billion years after the galactic merger. We may very well get a pretty good view of the coming cosmic collision. Well, maybe not us in particular, but whoever’s still around in a billion years. What a view they’ll have!

Image credit: Sloan Digital Sky Survey