Hercules cluster, giant globular star cluster in the northern constellation Hercules; cataloged as M13 or NGC 6205. The cluster is just visible to the naked eye and is the best example of a globular cluster visible in the Northern Hemisphere. Its angular diameter is about 2/3 that of the full moon, and its linear diameter is 100 light-years. The entire cluster contains perhaps 100,000 or more stars. In its central portion, the stars are so close together that they cannot be resolved, despite the cluster's relatively near distance of 35,000 light-years; the separation between stars is estimated to be only a few astronomical units as compared to the normal interstellar separation of a few light-years.