A star cluster is made up of stars that are all at about the same distance from us. These stars were also born around the same time from the same gas cloud. There are two major types of star clusters: open clusters and globular clusters. Open Clusters Open clusters are made up of a few hundred to a few thousand stars which are loosely arranged so that the cluster has no particular shape. In an open cluster, most of the individual stars are easy to see. Open clusters are made up of relatively young stars (usually younger than the Sun) and are found in the disk (flat part) of our galaxy (you will learn more about the galaxy in the next lab; we are also in the disk). There are thousands of open clusters in our galaxy, but we are only able to see a small fraction of them because their light is dimmed by intervening dust in the plane of our galaxy.

Globular Clusters Globular clusters, as the name suggests, look more like globes with a dense concentration of stars at their centers. They contain hundreds of thousands of stars! It is very difficult to make out individual stars in the center because they blend together. Globular clusters contain relatively old stars (usually older than the Sun). There are only about 150 known globular clusters in our galaxy. Globular clusters are in what is called the Galactic halo, which surrounds the disk, and are quite far from us, as much as 50,000 (5 x 104) pc away or more !