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]
^ abFrommert, Hartmut; Kronberg, Christine, "Messier 21", SEDS Messier pages, Students for the Exploration and Development of Space, retrieved 2018-11-12.
^ abcPark, 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 31 January 2024).{{citation}}: CS1 maint: DOI inactive as of January 2024 (link)
^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
^ abForbes, 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.