Astronomy:Messier 21

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Short description: Open cluster in the constellation Sagittarius
Messier 21
Messier 21.jpg
Open cluster Messier 21 in Sagittarius
Credit: John Saunders
Observation data (J2000.0 epoch)
Right ascension 18h 04m 13.0s[1]
Declination−22° 29′ 24″[1]
Distance3,930 ly (1,205 pc)[2]
Apparent magnitude (V)6.5[3]
Apparent dimensions (V)14.0′[4]
Physical characteristics
Mass783.4[5] M☉
Radius12 ly (3.6 pc)[5]
Estimated age6.6×106 years[5]
Other designationsM21, NGC 6531, Cr 363, OCl 26.0[6]
See also: Open cluster, List of open clusters

Messier 21 or M21, also designated NGC 6531 or Webb's Cross, is an open cluster of stars located to the north-east of Sagittarius in the night sky, close to the Messier objects M20 to M25 (except M24). It was discovered and catalogued by Charles Messier on June 5, 1764.[7] This cluster is relatively young and tightly packed. A few blue giant stars have been identified in the cluster, but Messier 21 is composed mainly of small dim stars.[3] With a magnitude of 6.5, M21 is not visible to the naked eye; however, with the smallest binoculars it can be easily spotted on a dark night. The cluster is positioned near the Trifid nebula (NGC 6514), but is not associated with that nebulosity.[8] It forms part of the Sagittarius OB1 association.[9]

This cluster is located 1,205 pc[2] away from Earth with an extinction of 0.87.[10] Messier 21 is around 6.6 million years old with a mass of 783.4 M☉.[5] It has a tidal radius of 11.7 pc,[5] with a nucleus radius of 1.6±0.1 pc and a coronal radius of 3.6±0.2 pc. There are at least 105±11 members within the coronal radius down to visual magnitude 15.5,[11] including many early B-type stars.[8] An estimated 40–60 of the observed low-mass members are expected to be pre-main-sequence stars,[8] with 26 candidates identified based upon hydrogen alpha emission and the presence of lithium in the spectrum.[10] The stars in the cluster do not show a significant spread in ages, suggesting that the star formation was triggered all at once.[11]

As of January 2022, Messier 21 is one of the few remaining objects within the Messier Catalog to not have been photographed by the Hubble Space Telescope.[12]

Gallery

  • Map showing the location of M21

See also

References

  1. ↑ 1.0 1.1 Wu, Zhen-Yu et al. (November 2009), "The orbits of open clusters in the Galaxy", Monthly Notices of the Royal Astronomical Society 399 (4): 2146–2164, doi:10.1111/j.1365-2966.2009.15416.x, Bibcode: 2009MNRAS.399.2146W. 
  2. ↑ 2.0 2.1 Kharchenko, N. V. et al. (2005), "Astrophysical parameters of Galactic open clusters", Astronomy and Astrophysics 438 (3): 1163–1173, doi:10.1051/0004-6361:20042523, Bibcode: 2005A&A...438.1163K. 
  3. ↑ 3.0 3.1 Frommert, Hartmut; Kronberg, Christine, "Messier 21", SEDS Messier pages (Students for the Exploration and Development of Space), http://messier.seds.org/m/m021.html, retrieved 2018-11-12. 
  4. ↑ Morales, Esteban F. E. et al. (2013), "Stellar clusters in the inner Galaxy and their correlation with cold dust emission", Astronomy & Astrophysics 560: A76, doi:10.1051/0004-6361/201321626, Bibcode: 2013A&A...560A..76M. 
  5. ↑ 5.0 5.1 5.2 5.3 5.4 Piskunov, A. E. et al. (January 2008), "Tidal radii and masses of open clusters", Astronomy and Astrophysics 477 (1): 165–172, doi:10.1051/0004-6361:20078525, Bibcode: 2008A&A...477..165P. 
  6. ↑ "M 21". SIMBAD. Centre de données astronomiques de Strasbourg. http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=M+21. 
  7. ↑ Adam, Len (2018), Imaging the Messier Objects Remotely from Your Laptop, The Patrick Moore Practical Astronomy Series, Springer, pp. 128–130, ISBN 978-3319653853, Bibcode: 2018imor.book.....A, https://books.google.com/books?id=7nNUDwAAQBAJ&pg=PA128. 
  8. ↑ 8.0 8.1 8.2 Park, Byeong-Gon et al. (December 2001), "The Galactic Open Cluster NGC 6531 (M21)", Journal of the Korean Astronomical Society 34 (3): 149–155, doi:10.5303/JKAS.2001.34.3.149, Bibcode: 2001JKAS...34..149P. 
  9. ↑ van den Ancker, M. E.; Thé, P. S.; de Winter, D. (June 2000), "The central part of the young open cluster NGC 6383", Astronomy and Astrophysics 362: 580, Bibcode: 2000A&A...362..580V 
  10. ↑ 10.0 10.1 Fedele, D. et al. (February 2010), "Timescale of mass accretion in pre-main-sequence stars", Astronomy and Astrophysics 510: 7, doi:10.1051/0004-6361/200912810, A72, Bibcode: 2010A&A...510A..72F. 
  11. ↑ 11.0 11.1 Forbes, Douglas (September 1996), "Star Formation in NGC 6531-Evidence From the age Spread and Initial Mass Function", Astronomical Journal 112: 1073, doi:10.1086/118079, Bibcode: 1996AJ....112.1073F. 
  12. ↑ "Explore - the Night Sky | Hubble's Messier Catalog". 28 August 2017. https://www.nasa.gov/content/goddard/hubble-s-messier-catalog. 

External links

Coordinates: Sky map 18h 04m 36s, −22° 30′ 00″



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