Globular star cluster. Artwork of a dense ball of stars forming a structure known as a globular star cluster. A globular star cluster can contain over a million stars within a sphere only a few hundred light years across. Globular star clusters formed early in the history of the universe, and their stars are thought to be among the oldest known. Many globular star clusters are far older than the galaxies they now orbit. The Milky Way galaxy has a halo of orbiting globular star clusters.