Globular Clusters

Week of October 18, 1999

So far in our hop from cluster to cluster we've seen associations, then constellations, and then our first truly bound group of open clusters.

The next step up is a big one. Some open clusters have tens of thousands of stars, but a decent globular cluster has over a million. Named because they are roughly spherical in shape, globular clusters are dense balls of stars. They may have as many as half of their millions of stars packed in a space only a few light years across. For comparison, the nearest star to the Sun is over 4 light years away. Compared to the core of a globular, space near the Sun is a high grade vacuum. Well, it really is anyway, but you get the picture.

Our Milky Way galaxy sports about 100 globulars that we have detected. There may be many more, but they may be behind the Galaxy where it's very hard to see them. The nearest is pretty far away; about 8000 light years. The farthest is more than 10 times that far, and in total they form a spherical halo around our Galaxy. As a matter of fact, they were used to show that the Sun is not the center of the Universe! Before it was known that the Milky Way was just a galaxy, and not the whole Universe, people wondered about our position in it. Most thought the Sun was at the center, carrying on a tradition of geo (or in this case, helio) centrism started as far back as Aristotle. However, The famous astronomer Harlow Shapley showed that the globular clusters were actually in a halo around the Milky Way, and that when you plot the positions of the clusters you see that their positions are not centered on the Sun. Ironically, years later, Shapley would debate another astronomer, claiming that the Milky Way was indeed the whole Universe, and not just on piece of it. He was wrong... but that's next week's Snack.

The picture above is again from Bill Arnett's wonderful website The Messier Objects. There are lots of great images there. For more about Messier objects, you can also read my own Bitesize Snack about them.