Messier 21 | |
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![]() Messier 21 imaged by the Vera C. Rubin Observatory | |
Observation data (J2000.0 epoch) | |
Right ascension | 18h 04m 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 | |
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. 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]
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: CS1 maint: DOI inactive as of November 2024 (link)