|Observation data (J2000.0 epoch)|
|Right ascension||17h 53m 51.2s|
|Declination||–34° 47′ 34″|
|Distance||980 ± 33 ly (300 ± 10 pc)|
|Apparent magnitude (V)||3.3|
|Apparent dimensions (V)||80.0′|
|Estimated age||200 Myr|
|Other designations||M7, NGC 6475, Ptolemy Cluster|
Messier 7 or M7, also designated NGC 6475 and sometimes known as the Ptolemy Cluster, is an open cluster of stars in the constellation of Scorpius. The cluster is easily detectable with the naked eye, close to the "stinger" of Scorpius (also called Scorpion). With a declination of -34.8°, it is the southernmost Messier object.
M7 has been known since antiquity; it was first recorded by the 1st-century Greek-Roman astronomer Ptolemy, who described it as a nebula in 130 AD. Italian astronomer Giovanni Batista Hodierna observed it before 1654 and counted 30 stars in it. In 1764, French astronomer Charles Messier catalogued the cluster as the seventh member in his list of comet-like objects. English astronomer John Herschel described it as "coarsely scattered clusters of stars".
Telescopic observations of the cluster reveal about 80 stars within a field of view of 1.3° across. At the cluster's estimated distance of 980 light years this corresponds to an actual diameter of 25 light years. The tidal radius of the cluster is 40.1 ly (12.3 pc) and it has a combined mass of about 735 times the mass of the Sun. The age of the cluster is around 200 million years while the brightest member star is of magnitude 5.6. In terms of composition, the cluster contains a similar abundance of elements other than hydrogen and helium as the Sun.
- "MESSIER 007", NASA/IPAC Extragalactic Database (NASA), retrieved 2012-04-19
- Villanova, S.; Carraro, G.; Saviane, I. (September 2009), "A spectroscopic study of the open cluster NGC 6475 (M 7). Chemical abundances from stars in the range Teff = 4500-10 000 K", Astronomy and Astrophysics 504 (3): 845–852,
- Piskunov, A. E.; et al. (January 2008), "Tidal radii and masses of open clusters", Astronomy and Astrophysics 477 (1): 165–172,
- Gendler, Robert; Christensen, Lars Lindberg; Malin, David (2011), Treasures of the Southern Sky: A Photographic Anthology, Springer, p. 139,
- Jones, Kenneth Glyn (1991), Messier's Nebulae and Star Clusters, The Practical astronomy handbook series (2) (2ns ed.), Cambridge University Press, p. 1,
- Messier 7, SEDS Messier pages
- Messier 7 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images
- APOD - 2012 September 12 - M7: Open Star Cluster in Scorpius
- New Horizons probe captures M7