M 55 - Globular Star Cluster

M 55 is a large Globular Star Cluster of around 100.000 stars. Only 20.000 LY away in the constellation Sagittarius, M 55 appears to earth-bound observers to be nearly 2/3 the size of the full Moon. Globular star clusters like M 55 roam the halo of our Milky Way Galaxy as gravitationally bound populations of stars known to be much older than stellar groups found in the galactic disk. Astronomers who make detailed studies of globular cluster stars can accurately measure the cluster ages and distances. Their results ultimately constrain the age of the Universe and provide a fundamental rung on the astronomical distance ladder.