M12 is a magnitude +7.2 globular cluster in Ophiuchus that was discovered by Charles Messier on May 30, 1764. He was unable to resolve the cluster, describing it as "nebula without stars". It was William Herschel who first managed this in 1783. Through good binoculars, M12 appears as a faint hazy patch of light that's not well defined. Positioned nearby is the slightly brighter but similar looking globular M10. The two clusters are among the brightest of the seven Messier globulars located in Ophiuchus.
M12 is located in a barren area of sky that's devoid of bright stars and therefore finding it can require some patience. Start by locating Rasalhague (α Oph - mag +2.1) the brightest star in Ophiuchus. Join the stars of Ophiuchus in a curve heading westwards and southwards until arriving at two close together 3rd magnitude stars, Yed Prior (δ Oph - mag. +2.7) and Yed Posterior (ε Oph - mag. +3.2). M12 is located about 8 degrees northeast of the stars. Positioned 3.25 degrees southeast of M12 is M10, with the star 30 Oph (mag. +4.8) located one degree east of M10.
The best time of the year to observe M12 is during the months of May, June and July.
M12 is not a particularly concentrated globular cluster and was once believed to be an intermediate type object, something between a globular and a dense open cluster, such as M11. A small 80mm (3.1-inch) telescope reveals a fuzzy mottled ball that's obviously non-stellar. At medium to high powers, a 200mm (8-inch) scope resolves the clusters brightest stars with at least twenty pinpoints of light scattered throughout. Also visible are lines and branches of stars extending outwards from the diffuse center core. The core itself is not particularly bright or well defined. In total, the globular covers 16 arc minutes of apparent sky and on good nights, large amateur telescopes will show stars across the entire face of the cluster.
M12 is located at a distance of 18,000 light-years and is estimated to be 12.6 billion years old. It has a spatial diameter of 80 light-years and contains 70,000 stars. The brightest member star is of mag. +12.0.
M12 Data Table
|Object Type||Globular cluster|
|RA (J2000)||16h 47m 14s|
|DEC (J2000)||-01d 56m 52s|
|Apparent Size (arc mins)||16 x 16|
|Age (years)||12.6 Billion|
|Number of Stars||70,000|