M5, Globular Cluster in Serpens

Image by Jim Dixon
Globular cluster M5 was first seen by Gottfried Kirch and his wife Maria Margarethe on May 5, 1702, when they observed a comet, and described as a "nebulous star". Charles Messier found it independently on May 23, 1764, and described it as a round nebula which "doesn't contain any stars". William Herschel was the first to resolve this cluster into stars; he counted 200 of them with his 40-foot [FL] reflector in 1791, "although the middle is so compressed that it is impossible to distinguish the components."

M5 shows a distinct ellipticity, elongated at position angle 50 degrees (position angles give the orientation of a feature in the celestial sphere; they are measured between North and the direction considered, in counterclockwise sense); it is thought to be one of the oldest globular clusters, with a computed age of 13 billion years. Its diameter is about 165 light years, making it one of the larger globular clusters. At its distance of 24,500 light years, this diameter is about 23 minutes of arc. Visually it appears somewhat smaller, about 10 or 12 arc minutes, and on typical phitigraphs, it can be traced to 17 arc minutes (corresponding to the inner 125 light-years of the cluster). Its tidal radius, beyond which member stars would be torn away by the Milky Way Galaxy's tidal gravitational forces, is 28.4 arc minutes, or 202 light years, so that the cluster gravitationally dominates a spherical volume of over 400 light years diameter. It has a compressed central core of 0.84' angular, or roughly 6 light years diameter, and its half-mass radius is estimated at 2.11', corresponding to a linear radius of 15 light years.