M21 is a compact open cluster in the constellation of Sagittarius, which is positioned less than one degree northeast of the Trifid Nebula (M20). It contains about 60 stars, mostly of them white but a sprinkling of blue giants, set in a tightly packed area covering 13 arc minutes. With an apparent mag. of +6.5, M21 is a nice sight in binoculars and small telescopes but compact enough that larger scopes also show it well, especially at low magnifications.

M21 was discovered - along with M20 - by Charles Messier on June 5, 1764. It's best seen from southern and equatorial regions during the months of June, July and August.

M21 Open Cluster (credit:- Siegfried Kohlert - www.astroimages.de)

Finder Chart for M21 (credit:- freestarcharts)

Finder Chart for M21 - pdf format (credit:- freestarcharts)

Finder Chart for M9 (also shown M4, M8, M19->M21, M23, M80 and M107) (credit:- freestarcharts)

Finder Chart for M9 (also shown M4, M8, M19->M21, M23, M80 and M107) - pdf format (credit:- freestarcharts)

The ideal starting point to find M21 is the teapot asterism that forms the core star grouping of Sagittarius. The top three stars of the teapot are Kaus Borealis (λ Sgr - mag. +2.8), Kaus Media (δ Sgr - mag. +2.7) and φ Sgr (mag. +3.2). Just over 6 degrees north of an imaginary line connecting φ Sgr and Kaus Borealis is M20, with M21 located 0.75 degrees northeast of M20.

M21 is a fine binocular object with the brightest stars resolvable. It appears compact and misty with a sprinkling of starlight, particularly when using averted vision. Through a 80mm (3.1-inch) scope, the cluster is a wonderful sight with many stars revealed. In total, there are some 35 stars between magnitudes +8 and +12 and a medum size 150mm (6-inch) or 200mm (8-inch) scope will show them all. The stars are packed tightly together.

M21 is a relatively young cluster at only 4.6 million years old. It's located 4,250 light-years from Earth and has an actual diameter of 16 light-years.

M21 Data Table

Object TypeOpen cluster
Distance (light-years)4,250
Apparent Mag.+6.5
RA (J2000)18h 04m 13s
DEC (J2000)-22d 29m 24s
Apparent Size (arc mins)13 x 13
Radius (light-years)8.0
Age (years)4.6 Million
Number of Stars60

Sky Highlights - July 2017

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for July

Meteor Shower
Southern Delta Aquariids (Aquarids) meteor shower peaks on July 29

Northern Hemisphere
West:- Mercury (mag. -0.5 to +0.3) (second half of month)
Southwest:- Jupiter (mag. -2.0)
South:- Saturn (mag. +0.2)
West:- Jupiter
South:- Saturn
East:- Neptune (mag. +7.8)
Southwest:- Saturn
South:- Neptune
Southeast:- Uranus (mag. +5.8)
East:- Venus (mag. -4.1)

Southern Hemisphere
West:- Mercury (second half of month)
Northwest:- Jupiter
East:- Saturn
West:- Jupiter
North:- Saturn
East:- Neptune
West:- Saturn
North:- Neptune
Northeast:- Venus, Uranus

Deep Sky

Small telescopes:-
Messier 13 - M13 - Great Hercules Globular Cluster
Messier 92 - M92 - Globular Cluster
Messier 11 - M11 - The Wild Duck Cluster (Open Cluster)
Messier 7 - M7 - The Ptolemy Cluster (Open Cluster)
Messier 6 - M6 - The Butterfly Cluster (Open Cluster)
Messier 4 - M4 - Globular Cluster
Messier 8 - M8 - Lagoon Nebula (Emission Nebula)
Messier 16 - M16 - Eagle Nebula (Emission Nebula with Open Cluster)
Messier 20 - M20 - Trifid Nebula (Emission and Reflection Nebula)

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