The globular cluster Messier 5 (NGC 5904), in the constellation Serpens, is one of the finest in the sky.
Globular cluster M 5 was discovered by Gottfried Kirch and his wife Maria Margarethe in 1702, while observing a comet; he described it as a "nebulous star". Charles Messier found it independently in 1764, and described it as a round nebula which "doesn't contain any stars". William Herschel resolved individual stars in the cluster in 1791; he counted 200 of them with his 40-foot reflector, "although the middle is so compressed that it is impossible to distinguish the components."
Observing Messier 5
Visually, Messier 5 is nearly as impressive as the great Hercules Cluster, Messier 13. At magnitude 5.6, M 5 can just be seen with the naked eye on a clear night. To find M 5, first locate the nearby star 5 Serpentis. The globular cluster is easily visible as small fuzzy patch in good binoculars.
At low power, M 5 shows a well-concentrated core, and some resolution around the periphery of its halo. Its brightest stars, of magnitude 12.2, can just be resolved with a 4-inch telescope; they form curved patterns extending from the central part, suggesting a spider. Larger telescopes or photographs reveal thousands of stars, and a few less-populated gaps. At 100x the outer halo is well-resolved, and extends out to a diameter of 15'.
The nearby double star 5 Serpentis consists of components of magnitude 5 and 10 separated by 11". This star is mentioned in Messier's description. Also situated nearby, just about 40' south 5 Ser, is the faint and distant globular cluster Palomar 5 (magnitude 11.8; diameter 6.9').
Physical Properties
The distance to M 5 is about 24,500 light-years, and the cluster contains more than 100,000 stars - up to perhaps 500,000, according to some estimates. Spanning 165 light-years across, M5 is one of the larger globular clusters known. Its tidal radius, beyond which member stars would be torn away by the Milky Way Galaxy's tidal gravitational forces, is 202 light years. At 13 billion years old, it is also one of the older globulars in the Milky Way Galaxy. M 5 is receding from us at about 52 km/sec.
M 5 contains 105 known variable stars, of which 97 are the RR Lyrae type. RR Lyrae stars, sometimes referred to as "cluster variables", are somewhat similar to Cepheid variables, and can be used to measure distance since the relation between their periods and luminosities are well known. The brightest variable in M 5 varies from magnitude 10.6 to 12.1 over a period of 26.5 days. A dwarf nova has also been observed in this cluster.