Open Cluster Messier 21

Object Description

The galactic star cluster Messier 21 is located about 0.7° northeast of the Trifid Nebula and is one of the discoveries made by Charles Messier in June 1764. Messier identified the star cluster as 11 Sagittari, which is a bit confusing as this star is about 2° to the southeast. Messier also believed he saw signs of nebula in the cluster, although there is no nebula there that can be seen with small telescopes.

M 21 is a less compact group with about six brighter stars in the center, surrounded by several dozen scattered, weaker stars. The brightest stars are of type B0. The distance is estimated to be about 2200 light years, making M 21 much closer than the Trifid Nebula. The diameter of the cluster is about 17 light years. In the center there are around nine stars per cubic parsec. [4]

«Revised New General Catalogue and Index Catalogue» Dr. Wolfgang Steinicke, 2021 [277]
DesignationNGC 6531
TypeOCL (I3m)
Right Ascension18h 04m 13.3s
Declination-22° 30' 00"
Diameter16.00 arcmin
Visual magnitude5.9 mag
Dreyer DescriptionCl, pRi, lC, st 9…12
IdentificationM 21, OCL 26, ESO 521-SC19

Finder Chart

If one extends the line φ - λ Sagittarii and positions the middle (2°) Telrad circle on it so that it is close to the star μ Sagittarii, the open star cluster M 21 should already be found in a large field eyepiece. The Trifid Nebula (M 20) lies about 0.7 ° to the southwest. Part of it can be seen in the 1° closeup on the location map. Both are best seen in the months of June to August.

Chart Messier 21
Chart created using SkySafari 6 Pro and STScI Digitized Sky Survey. [149, 160]

References

4«Burnham's Celestial Handbook: An Observer's Guide to the Universe Beyond the Solar System» by Robert Burnham; Dover Publications, Inc.; Voume I: ISBN 0-486-23567-X; Volume II: ISBN 0-486-23568-8; Volume III: ISBN 0-486-23673-0
149SkySafari 6 Pro, Simulation Curriculum; skysafariastronomy.com
160The STScI Digitized Sky Survey; archive.stsci.edu/cgi-bin/dss_form
277«Historische Deep-Sky Kataloge» von Dr. Wolfgang Steinicke; klima-luft.de/steinicke (2021-02-17)