Messier 55 — Globular Cluster in Sagittarius

M55 (NGC 6809) is a large, loose globular cluster in the constellation Sagittarius, located approximately 17,600 light-years from Earth. It shines at magnitude 7.0 and spans about 19 arcminutes — roughly two-thirds the apparent diameter of the full Moon. Its true diameter is about 96 light-years. M55 is notable for being one of the least concentrated globular clusters, classified as concentration class XI on the Shapley-Sawyer scale (where XII is the loosest). This sparse structure means it lacks a strongly condensed core, giving it an almost open-cluster-like appearance. The cluster contains roughly 100,000 stars and is estimated to be about 12.3 billion years old. It has relatively low metallicity, consistent with its great age.

Located about 7.5 degrees east-southeast of the star Zeta Sagittarii, in a relatively sparse star field south of the main Sagittarius Teapot. M55 can be a challenging target from mid-northern latitudes due to its southerly declination (-31 degrees); it barely clears the horizon from locations above 40 degrees north latitude. When accessible, binoculars show a large, faint, round glow. A 4-inch telescope at 50-80x begins to resolve the cluster into a granular haze of faint stars. An 8-inch telescope at 100-150x transforms M55 into a stunning field of resolved stars scattered loosely across the eyepiece — one of the most satisfying views of any globular. Best observed from July through September when Sagittarius is highest.

Discovered by Nicolas Louis de Lacaille in 1752 during his survey of southern skies from the Cape of Good Hope. Charles Messier added it to his catalog in 1778 after some difficulty locating it from his Parisian latitude. The cluster's low concentration and southern declination made it one of the more challenging Messier objects for European observers.