Globular Cluster

Globular Clusters  are very dense, spherical collections of stars which orbit the a galaxy as a satellite. They are found in the galactic halo, and galaxies can host from hundreds to thousands of them. The Milky Way hosts 150 to 158 confirmed globular clusters, while Andromeda may host as many as 500 clusters. Giant elliptical galaxies at the centres of galaxy clusters are by far the most abundant source of globular clusteres: M87 hosts about 13,000 globular clusters.

Globular clusters are far denser than star clusters in the galaxy itself, and may have around 0.04 stars per cubic parsec in the outer envelope of the cluster, to as many as 100 to 1000 stars per parsec in the dense, globular core. Globular clusters consist primarily of old, low-metallicity population II stars. They range in size from a radius of about 10 parsecs to 50 parsecs.

Both the high stellar density and the low metallicity of globular clusters are factors which greatly inhibit the formation of planets and life.

Examples of globular clusters include M80 and Palomar 12.

Worldbuilding in Practice
"RGS 23 is a small globular cluster orbiting the Ripple Galaxy. It itself is rather unremarkable: it is about 80 light years across and contains about 1 million stars. More importantly to space exploration, RGS 23 is close enough to the elliptical Inti that, once colonies are firmly rooted there, it can act as a stepping stone between Inti and the Ripple Galaxy and possibly even the nearby elliptical radio galaxy, TGC 43."