Globular Clusters: A minor mystery?

“I may be an old lion, but if someone puts his hand in my mouth, I can still bite it off.” –Wilhelm Steinitz

When you look at a typical galaxy, you usually find a disk, a bulge, and a few dots diffusely strewn about the exterior.

Like an old lion, these dots have been around for a very long time: often for longer than the galactic disk itself! Just what are these things? Well, we can learn a little more if we look in the X-ray (with Chandra) and the infrared (with Spitzer), in addition to a visible light image (from Hubble). Let’s take a look at a composite:

Just what are these dots that are bright in X-rays and visible light, but dim in the infrared? Say hello to Globular Clusters. A typical galaxy, like our Milky Way, has a couple of hundred globular clusters in it. Just what is a globular cluster? Let’s go in for a closer inspection.

This is globular cluster M72, as taken by Hubble back in 1999. M72 is a pretty typical globular cluster; average size, brightness, etc. (Perhaps it’s even a little bit younger than most.) But what does that mean? Let’s recall what’s going on in our own neighborhood.

Each ring on this diagram is about 13 light-years in radius. By contrast, the globular cluster M72 is about 21 light years in radius.


In other words, if we drew a sphere around the Sun of the same radius as M72, we’d get under 100 stars. How many stars do you suspect are in a globular cluster like M72?

About a million. Don’t believe me from that image above? Click on it. And what you’ll see is that there really are about a million stars in there! In fact, here’s a little snippet of the middle of this image.

This “snippet” is less than 1% the size of the image above it! Yes, there are really over a million stars in this globular cluster. And while there used to be tens of thousands of them around our galaxy, there are only about 200 left, destroyed by gravity and repeated collisions with our galactic disk.

But that’s not the mystery. There are two, and I’ll have the resolutions for you next week, but I want to give you the weekend to think about it.

Mystery #1: There’s nothing smaller! Yes, we have dwarf galaxies with only a few tens of millions of stars in them, but a small globular cluster may only have a few hundred thousand stars (like M4, our closest globular cluster, above). But while we can make things bigger and bigger, there’s nothing in the Universe that’s a smaller structure (containing stars) than a globular cluster! Why couldn’t there be collections of a few, or a few hundred, or a few thousand stars just on their own in this Universe? A few hundred thousand stars seems to be the magic number, and we’ve discovered many thousands of globular clusters with these properties.

Mystery #2: Why aren’t they ever found on their own? People have looked, and globular clusters only exist bound to other, larger structures; they’re never on their own!

Interesting, isn’t it? These super-dense balls with hundreds of thousands (or even millions) of stars in them, orbiting around galaxies everywhere, but never smaller than a few hundred thousand stars and never on their own. Weird! So think about it, tell me your guesses, and I’ll be back with the answer next week!