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
Credit: NASA/STScI/WikiSky
Observation data (J2000 epoch)
ClassIX[1]
ConstellationAquarius
Right ascension20h 53m 27.70s[2]
Declination–12° 32′ 14.3″[2]
Distance54.57 ± 1.17 kly (16.73 ± 0.36 kpc)[3]
Apparent magnitude (V)9.35[4]
Apparent dimensions (V)6.6'
Physical characteristics
Mass1.68×105[5] M☉
Metallicity = –1.48 ± 0.03[3] dex
Estimated age9.5 Gyr[6]
Other designationsNGC 6981, GCl 118[7]

Observational history and guide

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.[4] 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 (died 1871) called it a bright "cluster of stars of a round figure". Astronomer Harlow Shapley (died 1972) noted a similarity to Messier 4 and 12.[8]

It is visible in a good night sky as if 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.[9]

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

References and footnotes
  1. Shapley, Harlow; Sawyer, Helen B. (August 1927), "A Classification of Globular Clusters", Harvard College Observatory Bulletin, 849 (849): 11–14, Bibcode:1927BHarO.849...11S.
  2. Goldsbury, Ryan; et al. (December 2010), "The ACS Survey of Galactic Globular Clusters. X. New Determinations of Centers for 65 Clusters", The Astronomical Journal, 140 (6): 1830–1837, arXiv:1008.2755, Bibcode:2010AJ....140.1830G, doi:10.1088/0004-6256/140/6/1830, S2CID 119183070.
  3. Figuera 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 Conferencias), 40, pp. 235–236, Bibcode:2011RMxAC..40..235F.
  4. Garfinkle, Robert A. (1997), Star-Hopping: Your Visa to Viewing the Universe, Cambridge University Press, p. 266, ISBN 978-0521598897
  5. Boyles, J.; et al. (November 2011), "Young Radio Pulsars in Galactic Globular Clusters", The Astrophysical Journal, 742 (1): 51, arXiv:1108.4402, Bibcode:2011ApJ...742...51B, doi:10.1088/0004-637X/742/1/51, S2CID 118649860.
  6. Sollima, A.; et al. (April 2008), "The correlation between blue straggler and binary fractions in the core of Galactic globular clusters", Astronomy and Astrophysics, 481 (3): 701–704, arXiv:0801.4511, Bibcode:2008A&A...481..701S, doi:10.1051/0004-6361:20079082, S2CID 3088769
  7. "NGC 6981". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2006-11-16.
  8. Burnham, Robert (1978), Burnham's Celestial Handbook: An Observer's Guide to the Universe Beyond the Solar System, Dover Books on Astronomy Series, 1 (2nd ed.), Courier Dover Publications, pp. 188–189, ISBN 978-0486235677
  9. Luginbuhl, Christian B.; Skiff, Brian A. (1998), Observing Handbook and Catalogue of Deep-Sky Objects (2nd ed.), Cambridge University Press, p. 25, ISBN 978-0521625562
  1. on August 29

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