Messier 72 (NGC 6981) is a globular cluster in the western part of Aquarius.
Globular cluster M 72 was discovered by Pierre Mechain in August, 1780. Charles Messier found it the following October, and included it in his catalog.
Observing Messier 72
M 72 can be found 4° ESE of the 4th-magnitude star ε Aqr, or 3° WSW of the Saturn Nebula (NGC 7009). With a visual magnitude of 9.3 and an apparent diameter of 6.6', M 72 is one of the smaller and fainter globulars in Messier's catalog. Though the cluster is rather loosely concentrated, its 15th magnitude stars are quite difficult to resolve.
M 72 is a pale nebulous patch of light, very small and of grainy texture, in a 4-inch telescope; this only shows the 2'-diameter core region. In an 8-inch, only the extreme edges show suspicions of resolved stars. There are several field stars just to the south, a 9th magnitude star 5' ESE, and an abundance of scattered faint stars.
A 6th-magnitude star (HD 198431) is 40' to the west of M 72. Situated just about the same distance beyond this star is the very faint Aquarius Dwarf galaxy, which requires larger telescopes to observe.
About 1.5° to the east of M 72, Messier 73 can be found. This is a Y-shaped group of 4 stars which is generally considered to be an erroneous entry in Messier's catalog.
Physical Properties
M 72 is one of the more remote Messier globulars, about 55,000 light years away. It lies a considerable distance beyond the Galactic Center, and is actually one of the Milky Way's more intrinsically luminous globulars. Its diameter is about 106 light years, and is approaching us quite rapidly, at 255 km/sec.
Mesier 72 is not very concentrated; among Messier's globulars, only M 71 and M 56 are less concentrated. M 72 contains 42 known variable stars, mostly of the RR Lyrae type.