Messier 72

Messier 72 (also known as M72 or NGC 6981) is a globular cluster in the south west of the very mildly southern constellation of Aquarius.

Observational history and guide
M72 was discovered by astronomer Pierre Méchain in 1780. His countryman Charles Messier looked for it 36 days later, and included it in his catalog. Both opted for the then-dominant of the competing terms for such objects, considering it a faint nebula rather than a cluster. With a larger instrument, astronomer John Herschel called it a bright "cluster of stars of a round figure". Astronomer Harlow Shapley noted a similarity to Messier 4 and 12.

It is visible in a good night sky as a faint nebula in a telescope with a 6 cm aperture. The surrounding field stars become visible from a 15 cm-aperture device. One of 25 cm will allow measurement of an angular diameter of 2.5 arcminutes. At 30 cm the core is clear: its 1.25 arcminutes diameter, meaning a broad spread; and small parts scarcer in stars to the south and east.

Properties
Based upon a 2011 census of variable stars, the cluster is 54.57 +/- away from the Sun. It has an estimated combined mass of 168,000 solar masses and is around 9.5 billion years old. The core region has a density of stars that is radiating 2.26 times solar luminosity per cubic parsec. There are 43 identified variable stars in the cluster.