Stellar clusters

So it turns out this week is a little crazy for me, so here's another short post on stellar clusters.

In general, a stellar cluster is a group of stars that are close together. There are two kinds - open and globular clusters, and while they may look similar, they're actually completely different. Open clusters are groups of young stars like the Pleiades. They formed from the same cloud of the interstellar medium, and since they're relatively young, they're also pretty high in metals (remember, for astronomers, any atom heavier than helium is a metal). It turns out, though, that because of the way fragmentation and the resulting star formation work, open clusters are only clusters for a short period of time. They're not gravitationally bound, so after a few million years, they spread out, and there's no more evidence that a cluster even existed. Chances are good that the sun started out as a member of an open cluster, the stars of which spread out into their current configuration.
Globular clusters, on the other hand, seem to have formed around the same time as galaxies, so the stars in them are very old (on the order of billions of years) and therefore very low in metals. They're gravitationally bound, as well, which is why we still see them as clusters today. And where open clusters consist of ten to ten thousand stars, globular clusters have more like a million stars in them.
The handy thing about both kinds of clusters is that the stars in them formed around the same time, which allows researchers to get very accurate information on their composition and age.