Globular and Open Clusters

Along with multiple star systems, there are also larger groups of stars called star clusters. Star clusters are divided into two main groups: globular clusters and open clusters. Globular clusters form in the halos of galaxies (areas outside the visible parts of galaxies), and consist of very old stars compared to the ones near the center. They may have hundreds of thousands of stars, all of which are gravitationally bound together, and orbiting the galaxy as a single satellite. Globular clusters are also very dense, and most of their mass usually is packed tightly in the center of the cluster.

Open clusters, however, are totally different. They don't form in all galaxies, and consist of very young stars. They are formed by a giant molecular cloud (see the formation of the solar system) that is way too big for one star. The upper limit of members in these clusters is only a few thousand. Unlike globular clusters, open clusters are very loosely gravitationally bound. They orbit near the center of a galaxy, and any interactions between them and other objects may scramble them. But even if they aren't disturbed, they always break up after a few hundred million years.