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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.<ref name=Adam2018/> 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 the Trifid nebula (NGC 6514), but is not associated with that nebulosity.<ref name=Park2001/> It forms part of the Sagittarius OB1 association.<ref name=Ancker2000/>
This cluster is located Template:Val<ref name=kharchenko2005/> away from Earth with an extinction of 0.87.<ref name=Fedele2010/> Messier 21 is around 6.6 million years old with a mass of Template:Val.<ref name=Piskunov2008/> It has a tidal radius of 11.7 pc,<ref name=Piskunov2008/> with a nucleus radius of Template:Val and a coronal radius of Template:Val. There are at least Template:Val members within the coronal radius down to visual magnitude 15.5,<ref name=Forbes1996/> including many early B-type stars.<ref name=Park2001/> An estimated 40–60 of the observed low-mass members are expected to be pre-main-sequence stars,<ref name=Park2001/> with 26 candidates identified based upon hydrogen alpha emission and the presence of lithium in the spectrum.<ref name=Fedele2010/> The stars in the cluster do not show a significant spread in ages, suggesting that the star formation was triggered all at once.<ref name=Forbes1996/>
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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
GalleryEdit
- M21map.png
Map showing the location of M21