Open Cluster Messier 21

Messier 20
Messier 20: Trifid Nebula M 20 with Open Cluster M 21; Takahashi TOA 150/1100 APO Refractor f/7.27, TOA-67 Fieldflattener; SBIG STL-11000M, -25 °C; Astro-Physics 1200GTO; 32 x 5 min Baader-RGB-Filter (1x1); Namibia, Tivoli Southern Sky Guest Farm, 1360 m AMSL; © 28. 8. 2016 Manuel Jung [45]

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+Historic NGC/IC Version 22/9, © 2022 Dr. Wolfgang Steinicke [277]
Designation NGC 6531
Type OCL (I3m)
Right Ascension (J2000.0) 18h 04m 13.3s
Declination (J2000.0) -22° 30' 00"
Diameter 16 arcmin
Visual magnitude 5.9 mag
Metric Distance 1.205 kpc
Identification, Remarks h 1993; GC 4367; M 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 May to September.

Finder Chart Open Cluster Messier 21
Open Cluster Messier 21 in constellation Sagittarius. Charts created using SkySafari 6 Pro and STScI Digitized Sky Survey. Limiting magnitudes: Constellation chart ~6.5 mag, DSS2 close-ups ~20 mag. [149, 160]

Objects Within a Radius of 15°