There are two classifications of star clusters: globular clusters and open clusters. You can see some examples of each with the naked eye in tonight’s night sky, but there are even more that can be seen with binoculars and low-power telescopes. There are also distinct differences between the two types.
Globular clusters are full of the wizened old stars of the galaxy. These stars have been around for 12 to 20 billion years (edited to add: as Ken pointed out, this would make these stars older than the universe which isn’t true. This estimate was come up with by analyzing the spectra of the cluster, and it should be limited by the age of the universe, so 12-14 billion years is a better number to keep in mind), and they clump tightly together in groups of about 10,000 to 1 million stars.
Globular Clusters are the oldest stars in our galaxy (called “Population II” by astronomers). These are also pretty much the only stars in our galaxy that you’ll find orbiting above or below the main “disk” of the Milky Way. If you imagine our galaxy as a sphere, almost everything (including us, black holes, and most other stars) is in a Frisbee-shaped disk filling up the middle of the sphere – leaving most of the sphere empty. Globular clusters can be anywhere inside that sphere. This is also where we suspect most of the dark matter in our galaxy to be.
Age: 12-20 14 billion years old
Number: 10,000 to 1,000,000 stars
Size: A sphere 10-200 light-years in diameter
Good Cluster to See Tonight: The Hercules Globular Cluster (M13), in Hercules – rising a little north of due East around 11 p.m. You’ll see the dimmest little fuzzy patch near Hercules’ leg. You may have to look away and use your peripheral vision to see it.
Open clusters are the uppity young sprouts. They’re usually less than 100 million years old, and group together in gangs of about 100 stars. The spectacular thing about open cluster stars is that they’re all from the same “litter,” the stars usually all formed together out of the same diffuse nebula, which you can sometimes see – as in the Pleiades.
Age: Less than 100 million years old
Number: Commonly 100-ish stars (less than a few thousand)
Size: A sphere 3-20 light-years in diameter
Good Clusters to See Tonight: The Pleiades and the Hyades, in Taurus – setting near West around 11 p.m. The Pleiades are a tiny group of what looks to be 6-8 stars. The Hyades are a medium-sized V of stars that makes the “face” of Taurus the bull.
Where’d I Get My Info?
PSC’s 2007 March-April Starmap