M21 - NGC 6531
DSS image of M21
Overlaid DSS image of M21, 30' x 30' with north at top and west to the right

Aladin viewer for the region around M21
Bode 50, h 1993, GC 4367, Mel 188, Cr 363, Lund 806, ESO 521-SC019, OCl 26

Type  Open Cluster
Magnitude  5.9
Size  13'
Right Ascension  18h 4' 13.4"  (2000)
Declination  22° 29' 24" S
Constellation  Sagittarius
Description  Cl, pRi, lC, st9...12
Classification  I 3 m
Observing Notes

Andrew Cooper
Aug 10, 2020    Waikoloa, HI (map)
20cm f/6 Newtonian, Cave Astrola @ 76x

Big, bright, coarse, sparse, about 15' in diameter, several dozen 7-10th magnitude stars with a notable concentration at the center

Andrew Cooper
Jun 27, 2020    Waikoloa Quarry, HI (map)
8x42mm Nikon Prostaff 3S Binoculars @ 8x

A cluster of 7-9th magnitude stars 15' in diameter and just north of M20, bright, rich, resolved, in binoculars the M20-M21 region appears as a contiguous object

Andrew Cooper
Aug 11, 2017    Waikoloa, HI (map)
Hodgepodge w/TV-76mm f/6 APO @ 30x

A bright coarse cluster in a nice field with M20, a notable 9th magnitude star dominates the group with the remaning members fading in magnitude to beyond the reach of the 76mm

Andrew Cooper
Apr 19, 2012    Hale Pohaku, HI (map)
46cm f/4.5 Newtonian, Deep Violet @ 175x

An obvious cluster at low power, at moderate power the cluster is a sparse object, notable only because of the brightness of its membership, a couple dozen stars across a 10' area, coarse, arranged in clumps with a distinctive circlet at the western end

Andrew Cooper
Jun 6, 1999    TIMPA, Avra Valley, AZ (map)
20cm f/10 SCT

Sparse open cluster, in same field as the Trifid Nebula M20, a couple dozen stars visible, hard to determine the borders of the cluster

Rev. T.W. Webb
May 19, 1885    Hardwick, Herefordshire, England (map)

In a lucid region
― Celestial Objects for Common Telescopes, T. W. Webb, 1917

John Herschel
Jul 28, 1830    

A tolerably rich, scattered, coarse cluster; one star of 9th magnitude, the rest of 10th to 12th magnitude.

Charles Messier
Jun 5, 1764    

Star cluster, near the preceding [M20]: The nearest neighboring known star to these two clusters is 11 Sagittarii, 7 mag, according to Flamsteed. The stars of both these clusters are of 8-9 magnitude, enveloped in nebulosity."
― Connaissance des Temps, 1781
Other Data Sources for M21
Nearby objects for M21

Drawings, descriptions, and CCD photos are copyright Andrew Cooper unless otherwise noted, no usage without permission.

A complete list of credits and sources can be found on the about page