Flexi Says: Star clusters are small groups of stars. A star cluster is smaller than a galaxy. There are two main types, open clusters and globular clusters. Both types are held together by gravity. Open clusters are groups of up to a few thousand stars. They often contain glowing gas and dust and tend to be blue in color. The stars in an open cluster are young stars that all formed from the same nebula. The stars in an open cluster will likely disperse over time. Globular clusters are groups of tens to hundreds of thousands of stars. Gravity holds these stars tightly together. Globular clusters have a definite, spherical shape. They contain mostly old, reddish stars. Near the center of a globular cluster, the stars are closer together.