M72 was discovered by astronomer Pierre Méchain in 1780.[lower-alpha 1] His countryman Charles Messier looked for it 36 days later, and included it in his catalog.[8] 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.[9]
It is visible in a good night sky as a faint nebula in a telescope with a 6 cm (2.4 in) aperture. The surrounding field stars become visible from a 15 cm (5.9 in)-aperture device. One of 25 cm (9.8 in) will allow measurement of an angular diameter of 2.5 ′. At 30 cm (12 in) the core is clear: its 1.25 ′ diameter, meaning a broad spread; and small parts scarcer in stars to the south and east.[10]
Properties
Based upon a 2011 census of variable stars, the cluster is 54.57 ± 1.17 kly (16.73 ± 0.36 kpc) away from the Sun.[3] It has an estimated combined mass of 168,000[5]solar masses (M☉) and is around 9.5 billion years old. The core region has a density of stars that is radiating 2.26 times solar luminosity (Template:Lo) per cubic parsec.[6] There are 43 identified variable stars in the cluster.[3]
↑Shapley, Harlow; Sawyer, Helen B. (August 1927), "A Classification of Globular Clusters", Harvard College Observatory Bulletin849 (849): 11–14, Bibcode: 1927BHarO.849...11S.
↑ 2.02.1Goldsbury, Ryan et al. (December 2010), "The ACS Survey of Galactic Globular Clusters. X. New Determinations of Centers for 65 Clusters", The Astronomical Journal140 (6): 1830–1837, doi:10.1088/0004-6256/140/6/1830, Bibcode: 2010AJ....140.1830G.
↑ 3.03.13.23.3Figuera Jaimes, R. et al. (October 2011), Henney, W. J.; Torres-Peimbert, S., eds., "XIII Latin American Regional IAU Meeting: (item) The Globular Cluster NGC 6981: Variable stars population, physical parameters and astrometry", Revista Mexicana de Astronomía y Astrofísica, Serie de Conferencias40: pp. 235–236, Bibcode: 2011RMxAC..40..235F.
↑ 6.06.1Sollima, A. et al. (April 2008), "The correlation between blue straggler and binary fractions in the core of Galactic globular clusters", Astronomy and Astrophysics481 (3): 701–704, doi:10.1051/0004-6361:20079082, Bibcode: 2008A&A...481..701S
20h 53m 27.91s, −12° 32′ 13.4″
