Messier 21 orM21, also designatedNGC 6531 orWebb's Cross, is anopen cluster ofstars located to the north-east ofSagittarius in the night sky, close to the Messier objectsM20 toM25 (exceptM24). It was discovered and catalogued byCharles 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. 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 theTrifid Nebula (NGC 6514), but is not associated with that nebulosity.[8] It forms part of the Sagittarius OB1 association.[9]
This cluster is located1,205 pc[2] away from Earth with anextinction of 0.87.[10] Messier 21 is around 6.6 million years old with a mass of783.4 M☉.[5] It has a tidal radius of 11.7 pc,[5] with a nucleus radius of1.6±0.1 pc and a coronal radius of3.6±0.2 pc. There are at least105±11 members within the coronal radius down to visual magnitude 15.5,[11] including many earlyB-type stars.[8] An estimated 40–60 of the observed low-mass members are expected to bepre-main-sequence stars,[8] with 26 candidates identified based uponhydrogen alpha emission and the presence of lithium in thespectrum.[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 theHubble Space Telescope.[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 12 July 2025).{{citation}}: CS1 maint: DOI inactive as of July 2025 (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.
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