What are star clusters made of? What forms a star cluster?
Star clusters are groups of stars which are close together in space, rather than just accidentally lined up one behind the other. There are several different kinds. Among the most spectacular are huge balls of many thousands of stars, called globular clusters, which formed early in the history of galaxies, when they were rich in star-forming gas clouds. Other much smaller clusters, called open clusters, are visible where groups of stars have formed nearby more recently and are now drifting apart.
