Star Clusters
Star clusters are groups of stars that are held together by gravity. They can be classified into two main types: open clusters, which contain hundreds to thousands of stars and are found in the disk of a galaxy, and globular clusters, which contain thousands to millions of stars and are found in the halo of a galaxy. Star clusters are important for studying stellar evolution, as the stars within a cluster are all roughly the same age and composition, allowing for comparisons to be made between them.