18h04m36s, −22° 30′00″| Messier 21 | |
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Open cluster Messier 21 in Sagittarius
Credit: John Saunders | |
| Observation data (J2000.0 epoch) | |
| Right ascension | 18h04m 13.0s[1] |
| Declination | −22° 29′ 24″[1] |
| Distance | 3,930 ly (1,205 pc)[2] |
| Apparent magnitude (V) | 6.5[3] |
| Apparent dimensions (V) | 14.0′[4] |
| Physical characteristics | |
| Mass | 783.4[5] M☉ |
| Radius | 12 ly (3.6 pc)[5] |
| Estimated age | 6.6×106 years[5] |
| Other designations | M21, NGC 6531, Cr 363, OCl 26.0[6] |
| Associations | |
| See also: Open cluster, List of open clusters | |
Messier 21orM21, also designated NGC 6531orWebb's Cross, is an open clusterofstars located to the north-east of Sagittarius in the night sky, close to the Messier objects M20toM25 (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]
{{citation}}: CS1 maint: DOI inactive as of January 2024 (link)
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