Globular Star Clusters

Globular star clusters are enormously dense star clusters. They typically contain more than 100,000 stars. Some contain more than a million stars. Unlike small galactic star clusters, globular star clusters have enough mass to keep all member stars gravitationally bound to the cluster. Globular star clusters tend to be spherical in shape. They do not typically orbit the plane or our galaxy but are in randomly inclined orbits through the halo of the Milky Way. The Milky Way may contain 150–200 globular star clusters. Almost all globular star clusters contain metal-poor Population II stars. Globular star clusters are very old compared to the galactic star clusters found in the disk of the Milky Way. American astronomers Harlow Shapley and Helen Sawyer developed a classification scheme for globular clusters. The Shapley-Sawyer classifications are numbered with Roman numerals I to XII based on the concentration of stars in the cores of the clusters. Level I clusters have the highest concentration of stars in their cores. Level XII clusters have virtually no concentrated cores.