M13
Messier 13 or M13, also designated NGC 6205 and sometimes called the Great Globular Cluster in Hercules or the Hercules Globular Cluster, is a globular cluster of several hundred thousand stars in the constellation of Hercules. M13 was discovered by Edmond Halley in 1714, and cataloged by Charles Messier on June 1, 1764 into his list of objects not to mistake for comets; Messier's list, including Messier 13, eventually became known as the Messier Catalogue.
About a third of the way from Vega to Arcturus, four bright stars in the constellation of Hercules form the Keystone asterism. M13 can be seen partway between Zeta Herculis and Eta Herculis. Although only telescopes with great light-gathering capability fully resolve the stars of the Cluster, M13 can be visible to the naked eye depending on circumstances. With a low-power telescope, Messier 13 looks like a comet or fuzzy patch. The cluster is visible throughout the year from latitudes greater than 36 degrees north, with the longest visibility during northern hemisphere spring and summer.
It is located at right ascension 16h 41.7m, declination +36° 28'. With an apparent magnitude of 5.8, it is barely visible with the naked eye on clear nights. Its diameter is about 23 arc minutes and it is readily viewable in small telescopes. Nearby is NGC 6207, a 12th magnitude edge-on galaxy that lies 28 arc minutes directly northeast. A small galaxy, IC 4617, lies halfway between NGC 6207 and M13, north-northeast of the large globular cluster's center.
Traditional binoculars make the Hercules Globular Cluster look similar to a round patch of light. At least four inches of telescope aperture will allow observing the stars that constitute M13 as small pinpoints of light. However, only larger telescopes allow resolving stars further into the center of the cluster.
