Messier 21
Young open cluster near the Trifid Nebula in Sagittarius.
Messier 21 (also known as NGC 6531 and Webb's Cross) is an open star cluster in the northeastern part of Sagittarius, situated near several other Messier objects—M20 through M25, with M24 being the exception. Charles Messier first recorded it on June 5, 1764. The cluster is both young and densely packed; while it contains a handful of blue giant stars, most of its members are faint and small. At magnitude 6.5, it cannot be seen without aid, but a pair of small binoculars reveals it easily on a clear, dark night. M21 lies close to the Trifid Nebula (NGC 6514) but is not physically linked to it, and it belongs to the Sagittarius OB1 association.
The cluster sits about 1,205 parsecs from Earth, with an interstellar extinction of 0.87. Its age is roughly 6.6 million years, and its total mass is 783.4 times that of the Sun. The tidal radius extends to 11.7 parsecs, while the nucleus spans 1.6 ± 0.1 parsecs and the corona 3.6 ± 0.2 parsecs. Within the coronal radius, at least 105 ± 11 members are known down to a visual magnitude of 15.5, many of them early B-type stars. An estimated 40 to 60 of the observed low-mass stars are likely pre-main-sequence objects, with 26 candidates identified through hydrogen-alpha emission and lithium in their spectra. The stars show little age variation, indicating that the cluster formed in a single, rapid burst of star formation. As of January 2022, M21 remains one of the few Messier objects never imaged by the Hubble Space Telescope.
Quick Facts
- Epoch
- J2000.0
- Ra
- 18 · 04 · 13.0
- Dec
- −22 · 29 · 24
- Dist Ly
- 1205 pc
- Appmag V
- 6.5
- Mass Msol
- 783.4
- Radius Ly
- 3.6 pc
- Names
- M21, NGC 6531, Cr 363, OCl 26.0
Facts from the source article.
Lore & Background
Messier 21 was discovered and catalogued by Charles Messier on June 5, 1764. It is positioned near the Trifid Nebula (NGC 6514) but is not associated with that nebulosity. The cluster forms part of the Sagittarius OB1 association. It is located 1,205 pc away from Earth with an extinction of 0.87. The cluster is around 6.6 million years old with a mass of 783.4 M☉. It has a tidal radius of 11.7 pc, 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, including many early B-type stars. An estimated 40–60 of the observed low-mass members are expected to be pre-main-sequence stars, with 26 candidates identified based upon hydrogen alpha emission and the presence of lithium in the spectrum. The stars in the cluster do not show a significant spread in ages, suggesting that the star formation was triggered all at once.
Reader's Guide
Messier 21 is notable as a relatively young and tightly packed open cluster, composed mainly of small dim stars with a few blue giants. Its proximity to the Trifid Nebula and its membership in the Sagittarius OB1 association place it in a rich region of the Milky Way. The cluster's age of about 6.6 million years and the lack of significant age spread among its stars indicate that star formation occurred in a single, rapid event. The presence of many early B-type stars and a substantial number of pre-main-sequence candidates makes it a valuable target for studying early stellar evolution. As of January 2022, Messier 21 is one of the few remaining objects within the Messier Catalog not to have been photographed by the Hubble Space Telescope, adding to its observational interest. Its visibility through small binoculars makes it accessible to amateur astronomers.
Did You Know?
- Messier 21 is also designated NGC 6531 or Webb's Cross.
- It was discovered by Charles Messier on June 5, 1764.
- The cluster is around 6.6 million years old and has a mass of 783.4 M☉.
- As of January 2022, it was one of the few Messier objects not photographed by the Hubble Space Telescope.
More in Messier Objects 1-24
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