Messier 21 information

Messier 21
Messier 21
	Open cluster Messier 21 in Sagittarius ; Credit: John Saunders
Observation data (J2000.0 epoch)
Right ascension	18h 04m 13.0s
Declination	−22° 29′ 24″
Distance	3,930 ly (1,205 pc)
Apparent magnitude (V)	6.5
Apparent dimensions (V)	14.0′
Physical characteristics
Mass	783.4 M☉
Radius	12 ly (3.6 pc)
Estimated age	6.6×106 years
Other designations	M21, NGC 6531, Cr 363, OCl 26.0
Associations
	See also: Open cluster, List of open clusters

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.^[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]

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^ ^a ^b Cite error: The named reference kharchenko2005 was invoked but never defined (see the help page).
^ ^a ^b Cite error: The named reference SEDS was invoked but never defined (see the help page).
^ Cite error: The named reference Morales was invoked but never defined (see the help page).
^ ^a ^b ^c ^d ^e Cite error: The named reference Piskunov2008 was invoked but never defined (see the help page).
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^ ^a ^b ^c Cite error: The named reference Park2001 was invoked but never defined (see the help page).
^ Cite error: The named reference Ancker2000 was invoked but never defined (see the help page).
^ ^a ^b Cite error: The named reference Fedele2010 was invoked but never defined (see the help page).
^ ^a ^b Cite error: The named reference Forbes1996 was invoked but never defined (see the help page).
^ "Explore - the Night Sky | Hubble's Messier Catalog". 28 August 2017.

[Wu2009-1] Cite error: The named reference Wu2009 was invoked but never defined (see the help page).

[kharchenko2005-2] Cite error: The named reference kharchenko2005 was invoked but never defined (see the help page).

[SEDS-3] Cite error: The named reference SEDS was invoked but never defined (see the help page).

[Morales-4] Cite error: The named reference Morales was invoked but never defined (see the help page).

[Piskunov2008-5] Cite error: The named reference Piskunov2008 was invoked but never defined (see the help page).

[SIMBAD-6] Cite error: The named reference SIMBAD was invoked but never defined (see the help page).

[Adam2018-7] Cite error: The named reference Adam2018 was invoked but never defined (see the help page).

[Park2001-8] Cite error: The named reference Park2001 was invoked but never defined (see the help page).

[Ancker2000-9] Cite error: The named reference Ancker2000 was invoked but never defined (see the help page).

[Fedele2010-10] Cite error: The named reference Fedele2010 was invoked but never defined (see the help page).

[Forbes1996-11] Cite error: The named reference Forbes1996 was invoked but never defined (see the help page).

[12] "Explore - the Night Sky | Hubble's Messier Catalog". 28 August 2017.

Messier 21 information

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Messier 21
Open cluster Messier 21 in Sagittarius Credit: John Saunders
Observation data (J2000.0 epoch)
Right ascension	18^h 04^m 13.0^s^[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