M5 is, under extremely good conditions, just visible to the naked eye as a faint "star" 0.37 of a degree (22' (arcmin)) north-west ofstar5 Serpentis.Binoculars and/or smalltelescopes resolve the object as non-stellar; larger telescopes will show some individual stars, some of which are as bright asapparent magnitude 10.6.[8]M5 was discovered by German astronomerGottfried Kirch in 1702 when he was observing acomet.Charles Messier noted it in 1764 and—a studier of comets—cast it as one of his nebulae.William Herschel was the first to resolve individual stars in the cluster in 1791, counting roughly 200.[9] Messier 5 is receding from the Solar System at a speed over 50 km/s.[10]
Within M5, there are 105 knownvariable stars, 97 of them belonging to theRR Lyrae type.[11] RR Lyrae stars, sometimes referred to as"Cluster Variables", are somewhat similar toCepheid 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.[8]
The cluster contains twomillisecond pulsars, one of which is in a binary, allowing theproper motion of the cluster to be measured. The binary could help our understanding ofneutron degenerate matter; the current median mass, if confirmed, would exclude any "soft"equation of state for such matter.[12] The cluster has been used to test formagnetic dipole moments in neutrinos, which could shed light on some hypothetical particles such as theaxion.[13]
^Shapley, Harlow; Sawyer, Helen B. (August 1927), "A Classification of Globular Clusters",Harvard College Observatory Bulletin,849 (849):11–14,Bibcode:1927BHarO.849...11S.
