Messier 72

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 . At 30 cm the core is clear: its 1.25 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.

Image Gallery

File:A glittering cluster shines again (potw2516a).jpg|M72 imaged by the Hubble Space Telescope

References and footnotes

References

1. "Messier 72".
2. (December 2010). "The ACS Survey of Galactic Globular Clusters. X. New Determinations of Centers for 65 Clusters". The Astronomical Journal.
3. "NGC 6981".
4. (August 1927). "A Classification of Globular Clusters". Harvard College Observatory Bulletin.
5. (October 2011). "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.
6. (1997). "Star-Hopping: Your Visa to Viewing the Universe". [[Cambridge University Press]].
7. (1978). "Burnham's Celestial Handbook: An Observer's Guide to the Universe Beyond the Solar System". [[Courier Dover Publications]].
8. (1998). "Observing Handbook and Catalogue of Deep-Sky Objects". Cambridge University Press.
9. (November 2011). "Young Radio Pulsars in Galactic Globular Clusters". The Astrophysical Journal.
10. (April 2008). "The correlation between blue straggler and binary fractions in the core of Galactic globular clusters". Astronomy and Astrophysics.