Messier 23 — Open Cluster in Sagittarius

M23 is a large, rich open star cluster in the constellation Sagittarius, located about 2,150 light-years from Earth. It contains roughly 150 stars spread across about 15 light-years of space. At magnitude 5.5 with an apparent diameter of about 27 arcminutes (nearly the size of the full Moon), it is a bright and impressive cluster. M23 is estimated to be about 300 million years old. Its brightest stars are blue-white main sequence stars of spectral type B9, with several notable red and orange giants adding color contrast.

Located about 5 degrees northwest of M24 (the Small Sagittarius Star Cloud). At magnitude 5.5, it can be glimpsed with the naked eye under dark skies. Binoculars reveal a beautiful, large, rich cluster of dozens of stars. A telescope at 40-75x provides an outstanding view, resolving the cluster into a rich field of stars with arcs and chains. The cluster is large enough that too much magnification breaks up its visual impact. Best observed from July through September.

Discovered by Charles Messier on June 20, 1764. He described it as a cluster of small stars without any nebulosity. Admiral Smyth later described it as a 'splendid cluster' and noted the beautiful arcs and festoons of stars visible in moderate telescopes.