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.

Messier 72
M72 from Hubble Space Telescope; 3.44′ view
Observation data (J2000 epoch)
Class	IX[1]
Constellation	Aquarius
Right ascension	20h 53m 27.70s[2]
Declination	–12° 32′ 14.3″[2]
Distance	54.57 ± 1.17 kly (16.73 ± 0.36 kpc)[3]
Apparent magnitude (V)	9.3[4]
Apparent dimensions (V)	6.6'
Physical characteristics
Mass	1.68×105[5] M☉
Metallicity	${\displaystyle {\begin{smallmatrix}\left[{\ce {Fe}}/{\ce {H}}\right]\end{smallmatrix}}}$ = –1.48 ± 0.03[3] dex
Estimated age	9.5 Gyr[6]
Other designations	NGC 6981, GCl 118[7]
See also: Globular cluster, List of globular clusters

Observational history and guide

M72 was discovered by astronomer Pierre Méchain in 1780.^[a] 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 (L_☉) per cubic parsec.^[6] There are 43 identified variable stars in the cluster.^[3]

Map showing location of M72

Image Gallery

• 
  M72 imaged by the Hubble Space Telescope

See also

• List of Messier objects