M5 is a superb globular cluster that's located close to the celestial equator in the constellation of Serpens (Caput). Shining at magnitude +5.7, it's visible to the naked eye under dark skies appearing as a faint "star". The cluster is large covering 23 arc minutes of apparent sky, which at a distance of 24,500 light-years corresponds to a spatial diameter of 160 light-years, making it one of the largest globulars in both apparent and actual size.
M5 was discovered by Gottfried Kirch and his wife Maria Margarethe on the May 5, 1702. At the time the couple were observing a comet when they stumbled across the cluster, recording it as a star with nebulosity. Charles Messier found it independently 62 years later on the May 23, 1764, describing it as a round nebula which "doesn't contain any stars". The first person to resolve the cluster into stars was William Herschel, who counted 200 of them using his 40-foot (12.2 meter) focal length reflector in 1791.
M5 is located 23 degrees southeast of orange giant star Arcturus (α Boo), which at mag. –0.04 is the fourth brightest star in the night sky. M5 is positioned 4 degrees east of 110 Her (mag. +4.4) and next to a small triangle of 6th magnitude stars. At the northern tip of the triangle is double star 5 Ser (mag. +5.0) with M5 located just northwest of this star.
Under very good viewing conditions, M5 can be just about glimpsed with the naked eye as a faint point of light. With binoculars, it's easily visible as small fuzzy patch. A small 80mm (3.1-inch) telescope reveals a bright glowing core wrapped inside a much fainter halo of nebulosity. A 100mm (4-inch) telescope under excellent skies will start to resolve individual stars, the brightest of which are 11th magnitude. A 150mm (6-inch) scope at high power resolves more of the outer edges of M5. Through larger scopes, it's a spectacular sight with thousands of stars coming into view, radiating like chains or legs outwards from the bright centre. The cluster appears noticeably elongated with a compact and somewhat clumpy core. Overall, M5 is a beautiful globular cluster that's best seen during the months of March, April and May.
With an estimated age of 13 billion years, M5 is one of the oldest Milky Way globular clusters. It's estimated to contain at least 500,000 stars. A total of 105 variable stars have been observed of which 97 belong to the RR Lyrae type. Also a dwarf nova has been observed in M5.
M5 Data Table
|Object Type||Globular cluster|
|RA (J2000)||15h 18m 34s|
|DEC (J2000)||02d 04m 58s|
|Apparent Size (arcmins)||23 x 23|
|Radius (light years)||80|
|Number of Stars||>500,000|
|Notable Feature||A dwarf nova has been observed in this globular|