M5 wide angle by Robert J. Vanderbei
|Observation data (J2000 epoch)|
|Right ascension||15h 18m 33.22s|
|Declination||+02° 04′ 51.7″|
|Distance||24.5 kly (7.5 kpc)|
|Apparent magnitude (V)||+5.95|
|Apparent dimensions (V)||23′.0|
|Estimated age||10.62 Gyr|
|Other designations||NGC 5904, GCl 34|
M5 is, under extremely good conditions, just visible to the naked eye as a faint "star" near the star 5 Serpentis. Binoculars or small telescopes will identify the object as non-stellar while larger telescopes will show some individual stars, of which the brightest are of apparent magnitude 12.2.
M5 was discovered by German astronomer Gottfried Kirch in 1702 when he was observing a comet. Charles Messier also noted it in 1764, but thought it was a nebula without any stars associated with it. William Herschel was the first to resolve individual stars in the cluster in 1791, counting roughly 200.
105 stars in M5 are known to be variable in brightness, 97 of them belonging to the RR Lyrae type. RR Lyrae stars, sometimes referred to as "Cluster Variables", are somewhat similar to Cepheid type variables and as such can be used as a tool to measure distances in outer space since the relation between their luminosities and periods are well known. The brightest and most easily observed variable in M5 varies from magnitude 10.6 to 12.1 in a period of just under 26.5 days.
A dwarf nova has also been observed in this cluster.