| Messier 12 | |
| Epoch: | J2000 |
| Class: | IX |
| Constellation: | Ophiuchus |
| Appmag V: | 6.7[1] |
| Size V: | 16.0 |
| Radius Ly: | 37.2 ly[2] |
| Metal Fe: | –1.14 |
| Age: | 13.8 ± 1.1 Gyr |
| Names: | NGC 6218 |
Messier 12 or M 12 (also designated NGC 6218) is a globular cluster in the constellation of Ophiuchus. It was discovered by the French astronomer Charles Messier on May 30, 1764, who described it as a "nebula without stars". In dark conditions this cluster can be faintly seen with a pair of binoculars. Resolving the stellar components requires a telescope with an aperture of 8inches or greater. In a 10inches scope, the granular core shows a diameter of 3 (arcminutes) surrounded by a 10 halo of stars.
M12 is roughly 3° northwest from the cluster M10 and 5.6° east southeast from star Lambda Ophiuchi. It is also located near the 6th magnitude 12 Ophiuchi. The cluster is about 16400ly from Earth and has a spatial diameter of about 75 light-years. The brightest stars of M12 are of 12th magnitude. M10 and M12 are only a few thousand light-years away from each other and each cluster would appear at about magnitude 4.5 from the other. With a Shapley-Sawyer rating of IX, it is rather loosely packed for a globular and was once thought to be a tightly concentrated open cluster. Thirteen variable stars have been recorded in this cluster. M12 is approaching us at a velocity of 16 km/s.[3]
A study published in 2006 concluded that this cluster has an unusually low number of low-mass stars. The authors surmise that they were stripped from the cluster by passage through the relatively matter-rich plane of the Milky Way.