The globular cluster Messier 5 (NGC 5904), in the constellation Serpens, is one of the finest in the sky.
M5 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.”
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.