Messier 7

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Messier 7
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Observation data (J2000.0 epoch)
Right ascensionTemplate:RA[1]
DeclinationTemplate:DEC[1]
Distance980 ± 33 ly (300 ± 10 pc)[2]
Apparent magnitude (V)3.3
Apparent dimensions (V)80.0Template:Prime
Physical characteristics
Mass735[3] M☉
Radius25 ly
Estimated age200 Myr[2]
Other designationsPtolemy Cluster, M7, NGC 6475, Cr 354
Associations
ConstellationScorpius
See also: Open cluster, List of open clusters

Messier 7 or M7, also designated NGC 6475 and sometimes known as Ptolemy's Cluster,[4][5] 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. With a declination of −34.8°, it is the southernmost Messier object.

Observations

M7 has been known since antiquity; it was first recorded by the 2nd-century Greek-Roman astronomer Ptolemy, who described it as a nebula in 130 AD.[6] 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".[4] In Chinese astronomy it is identified as Yú, the Fish (simplified Chinese: 鱼; traditional Chinese: 魚).[5][7]

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.[3] The age of the cluster is around 200[2] 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.[2]

On August 29, 2006, Messier 7 was used for first light image of the Long Range Reconnaissance Imager (LORRI) telescope on the Pluto-bound New Horizons spacecraft.[8]

As of January 2022, Messier 7 is one of the few remaining Messier objects not photographed by the Hubble Space Telescope.[9] This is mainly due to those objects' angular diameter or lack of scientific significance. Most such objects are open clusters of large angular diameter that would require thousands of photos due to Hubble's small field of view. (For comparison, Hubble's well known panoramic photo of the Andromeda Galaxy, covering less than half of our galactic neighbor, required approximately 400 individual movements and 7400 exposures.)[10]

Brightest members

Messier 7 contains over a thousand likely members stars spread across several degrees. Members brighter than about 8th magnitude have evolved away from the main sequence, with the most luminous main sequence stars around spectral class A0. The brightest stars with a membership probability higher than 75%:[11]

Designation Right ascension Declination Apparent magnitude Spectral type
HD 162391[12] Template:RA Template:DEC 5.53 G8/K0III[13]
HD 162496[14] Template:RA Template:DEC 5.68 K1III[15]
V957 Sco[16] Template:RA Template:DEC 5.86 B5IIIp[17]
HD 162587[18][a] Template:RA Template:DEC 5.98 K1III[20]
HD 162515[21] Template:RA Template:DEC 6.53 B9III[22]
HD 163274[23] Template:RA Template:DEC 6.65 B9III/IV[24]
HD 163139[25] Template:RA Template:DEC 6.73 B9.51III[26]
HD 162780[27] Template:RA Template:DEC 6.86 B9.5III+n[28]
HD 162888[29] Template:RA Template:DEC 6.92 A0IIIn[30]
HD 161575[31] Template:RA Template:DEC 6.93 B9III(pSi)[32]
V958 Sco[33] Template:RA Template:DEC 6.94 B8/9III[34]

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  1. ^ Not in Gaia DR2, so doesn't appear in Cantat-Gaudin & Anders, but generally considered to be a member.[19]

See also

References

  1. ^ a b Page Module:Citation/CS1/styles.css has no content."MESSIER 007", NASA/IPAC Extragalactic Database, NASA, retrieved 2012-04-19
  2. ^ a b c d Page Module:Citation/CS1/styles.css has no content.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, arXiv:0906.4330, Bibcode:2009A&A...504..845V, doi:10.1051/0004-6361/200811507, S2CID 17534818
  3. ^ a b Page Module:Citation/CS1/styles.css has no content.Piskunov, A. E.; et al. (January 2008), "Tidal radii and masses of open clusters", Astronomy and Astrophysics, 477 (1): 165–172, Bibcode:2008A&A...477..165P, doi:10.1051/0004-6361:20078525
  4. ^ a b Page Module:Citation/CS1/styles.css has no content.Gendler, Robert; Christensen, Lars Lindberg; Malin, David (2011), Treasures of the Southern Sky: A Photographic Anthology, Springer, p. 139, ISBN 978-1461406273
  5. ^ a b Page Module:Citation/CS1/styles.css has no content.Ridpath, Ian. "Star Tales - Scorpius". Retrieved 21 July 2025.
  6. ^ Page Module:Citation/CS1/styles.css has no content.Jones, Kenneth Glyn (1991), Messier's Nebulae and Star Clusters, The Practical astronomy handbook series (2ns ed.), Cambridge University Press, p. 1, ISBN 978-0521370790
  7. ^ Page Module:Citation/CS1/styles.css has no content."AEEA 天文教育資訊網" [Activities of Exhibition and Education in Astronomy] (in 中文). Archived from the original on 30 September 2007.
  8. ^ Page Module:Citation/CS1/styles.css has no content."LORRI's First Light1". pluto.jhuapl.edu. Retrieved 2018-10-24.
  9. ^ Page Module:Citation/CS1/styles.css has no content."Explore - the Night Sky | Hubble's Messier Catalog". 28 August 2017.
  10. ^ Page Module:Citation/CS1/styles.css has no content."Hubble's High-Definition Panoramic View of the Andromeda Galaxy". 24 March 2015.
  11. ^ Page Module:Citation/CS1/styles.css has no content.Cantat-Gaudin, T.; Anders, F. (2020). "Clusters and mirages: Cataloguing stellar aggregates in the Milky Way". Astronomy and Astrophysics. 633: A99. arXiv:1911.07075. Bibcode:2020A&A...633A..99C. doi:10.1051/0004-6361/201936691.
  12. ^ Template:Cite Gaia DR3
  13. ^ Template:Cite XHIP
  14. ^ Template:Cite Gaia DR3
  15. ^ Template:Cite XHIP
  16. ^ Template:Cite Gaia DR3
  17. ^ Template:Cite XHIP
  18. ^ Template:Cite Gaia DR3
  19. ^ Page Module:Citation/CS1/styles.css has no content.Reddy, Arumalla B. S.; Lambert, David L. (2019). "Comprehensive abundance analysis of red giants in the open clusters Stock 2, NGC 2168, 6475, 6991, and 7762". Monthly Notices of the Royal Astronomical Society. 485 (3): 3623. arXiv:1902.02939. Bibcode:2019MNRAS.485.3623R. doi:10.1093/mnras/stz468.
  20. ^ Template:Cite XHIP
  21. ^ Template:Cite Gaia DR3
  22. ^ Template:Cite XHIP
  23. ^ Template:Cite Gaia DR3
  24. ^ Template:Cite XHIP
  25. ^ Template:Cite Gaia DR3
  26. ^ Template:Cite XHIP
  27. ^ Template:Cite Gaia DR3
  28. ^ Template:Cite XHIP
  29. ^ Template:Cite Gaia DR3
  30. ^ Template:Cite XHIP
  31. ^ Template:Cite Gaia DR3
  32. ^ Template:Cite XHIP
  33. ^ Template:Cite Gaia DR3
  34. ^ Template:Cite XHIP

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