Globular Star Cluster M-72 (NGC 6981)

Globular Cluster, M-72
a d A 30 minute exposure with an SBIG STL-1301E camera thru Kopernik's 20-inch RC Cassegrain working at F/5.3. The field of view is about 20x25 arc minutes with North at the top.

Charles Messier: (Oct. 4th, 1780) 'Nebula seen by M. Mechain on the night of 29-30 August 1780, above the neck of Capricorn. M. Messier looked for it on the 4th and 5th of October following. The light is as faint as the preceding M-71. Near to it is a little telescopic star…. The diameter is about 2 arc minutes.'

Description of M-72 in The Deep Sky Field Guide to Uranometria 2000 (c) Willmann-Bell, Inc.:

Globular Star Cluster M-72 was discovered by Pierre Mechain on August 29th 1780 and confirmed by Messier on October 4th of that year. Both considered it as a faint nebula rather than a cluster, but it was resolved into a cluster in October 1810 by William Hershel. In deed this is a difficult cluster to resolve visually, and as a result, M-72 has not been one of the more “popular” objects at amateur star parties. Robert Burnham Jr. noted in Burnham’s Celestial Handbook that he could only see slight mottling in the outer edge using his 10 inch reflector. Long time Sky & Telescope 'deep sky' columnist Walter Scott Houston reported the same for his observations with a 13.1 inch telescope. However, the cluster is resolvable in Kopernik’s 20 inch. Although the cluster is large, bright, and of only moderate concentration, even its brightest stars are rather faint, at 14.2 magnitude. The bulk of the bright stars (horizontal branch) are as faint as 17th magnitude. A globular cluster is only resolved if the telescope used can detect the brightest stars in the cluster.

M-72’s estimated distance is 62,000 light years, with a diameter of 42 light years. There are at least 100,000 stars with a total mass of 3 to 5 hundred thousand times the mass of our sun. (But Click here for the latest news on Globular Star Cluster distances!!)

Globular star clusters contain some the oldest stars known. However, strangely, M-72 and other globulars each contain a few blue giants, which are typically very young stars. Since there is no sign of star formation in globular clusters, astronomers have been puzzled by the “blue cluster stars” for years. However, recent observation and theory suggests that the blue giants in globular clusters are the result of the merging of two old stars, and are not recently formed young stars.