M 5

Messier 5 or M5 (also designated NGC 5904) is a globular cluster in the constellation Serpens. M5 is, under extremely good conditions, just visible to the naked eye as a faint "star" north-west of star 5 Serpentis (5 See). Binoculars and/or small telescopes resolve the object as non-stellar; larger telescopes will show some individual stars, some of which are as bright as apparent magnitude 10.6.

M5 was discovered by German astronomer Gottfried Kirch in 1702 when he was observing a comet. Charles Messier noted it in 1764 and William Herschel was the first to resolve individual stars in the cluster in 1791, counting roughly 200. Messier 5 is receding from the Solar System at a speed over 50 km/s.

M5 contains about 800.000 solar masses at a distance of about 25.000 light-years. Like other globular clusters in our Galaxy, M5 is very old: the age is estimated to about 11 Giga-years (1 Giga -year = 1000 x Million years, the age of the Universe is about 14 Giga-years). 

Within M5, there are about 100 known variable stars, most of them belonging to the RR Lyrae type, sometimes referred to as "Cluster Variables" which can be used as a tool to measure distances in outer space since the relation between their luminosities and periods are well known. 

The cluster contains two millisecond pulsars, one of which is in a binary, allowing the proper motion of the cluster to be measured.