Star Clusters are a group of physically associated stars, presumed to share the same origin. There are two main types, open clusters and globular clusters.

Very young stars are often found in loose groupings called associations.

Globular clusters

Globular clusters are gravitationally bound concentrations of approximately tenthousand to one million stars. They consist primarily of very old stars and populate the halo or bulge of the Milky Way and other galaxies with a significant concentration toward the Galactic Center. These clusters have about the same mass as the smallest galaxies, and are among the oldest objects in galaxies. More recent estimates yield an age of 12 to 20 billion years. A typical globular cluster is a few hundred light-years across. Our galaxy has about 200 globular clusters.

Open clusters

Open clusters are loose aggregations of dozens or hundreds of young stars. They are generally not gravitationally bound and will disperse in a relatively short period of time, astronomically speaking. They are believed to originate from large cosmic gas/dust clouds in the Milky Way, and to continue to orbit the galaxy through the disk and are often associated with more diffuse nebulosity, as well. Also called "galactic clusters" because they are usually found in the plane of the galaxy. A typical open cluster is less than 50 light-years across.