M12, Globular Cluster in Ophiucus

Image by Jim Dixon
M12 is nearly a twin of its apparent neighbor, M10, and is only slightly larger and an idea fainter. Nevertheless, it was once believed to be an intermediate type between globular and dense open clusters (like M11), as it is not very concentrated - Harlow Shapley included M12 in his concentration class IX. It is, e.g., notably much less concentrated toward the center than M10 (of class VII). At its distance of about 16,000 light years, the apparent diameter of M12 of 14.5 arc minutes corresponds to about 68 light years. This stellar swarm is approaching us at 16 km/sec.

Helen Sawyer Hogg determined the cluster's overall spectral type as F7 and gives its color index as 0.0, and the mean magnitude of the 25 brightest stars as 13.97. The brightest stars of M12 are about mag 12.0, its horizontal branch level (of giant stars) is magnitude 14.9, according to the Deep Sky Field Guide to Uranometria 2000.0. Alan Sandage has found 13 variables in M12.

M12 is one of Charles Messier's original discoveries, found on May 30, 1764. Like many other globular clusters, Messier described it as "Nebula without stars", as did Bode 10 years later; a consequence of the modest resolving power of their instruments. William Hersechel was the first to resolve it into stars in 1783.

Globular cluster M12 is easily found either 2 deg N and 2 deg W of M10, or 2 deg N and 8.5 deg E from Delta Ophiuchi.