Messier 21

Messier 21 or M21, also designated NGC 6531, is an open cluster of stars in the north-east of Sagittarius, angularly close to the Messier objects M20 to M25, save for 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]

Messier 21
Open cluster Messier 21 in Sagittarius
Credit: John Saunders
Observation data (J2000.0 epoch)
Right ascension18h 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☉
Estimated age6.6×106 years[5]
Other designationsM21, NGC 6531, Cr 363, OCl 26.0[6]

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]

  • Map showing the location of M21

References
  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, arXiv:0909.3737, Bibcode:2009MNRAS.399.2146W, doi:10.1111/j.1365-2966.2009.15416.x, S2CID 6066790.
  2. Kharchenko, N. V.; et al. (2005), "Astrophysical parameters of Galactic open clusters", Astronomy and Astrophysics, 438 (3): 1163–1173, arXiv:astro-ph/0501674, Bibcode:2005A&A...438.1163K, doi:10.1051/0004-6361:20042523, S2CID 9079873.
  3. Frommert, Hartmut; Kronberg, Christine, "Messier 21", SEDS Messier pages, Students for the Exploration and Development of Space, 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, arXiv:1310.2612, Bibcode:2013A&A...560A..76M, doi:10.1051/0004-6361/201321626, S2CID 118422539.
  5. 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.
  6. "M 21". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved November 11, 2018.
  7. Adam, Len (2018), Imaging the Messier Objects Remotely from Your Laptop, The Patrick Moore Practical Astronomy Series, Springer, pp. 128–130, Bibcode:2018imor.book.....A, ISBN 978-3319653853.
  8. Park, Byeong-Gon; et al. (December 2001), "The Galactic Open Cluster NGC 6531 (M21)", Journal of the Korean Astronomical Society, 34 (3): 149–155, Bibcode:2001JKAS...34..149P, doi:10.5303/JKAS.2001.34.3.149 (inactive 2021-01-11).CS1 maint: DOI inactive as of January 2021 (link)
  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, arXiv:astro-ph/0006283, Bibcode:2000A&A...362..580V
  10. Fedele, D.; et al. (February 2010), "Timescale of mass accretion in pre-main-sequence stars", Astronomy and Astrophysics, 510: 7, arXiv:0911.3320, Bibcode:2010A&A...510A..72F, doi:10.1051/0004-6361/200912810, S2CID 118520073, A72.
  11. Forbes, Douglas (September 1996), "Star Formation in NGC 6531-Evidence From the age Spread and Initial Mass Function", Astronomical Journal, 112: 1073, Bibcode:1996AJ....112.1073F, doi:10.1086/118079.

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