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

Messier21
NGC6531
Object TypeOpen cluster
ConstellationSagittarius
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 - March 2017

Comet
Comet 41P/Tuttle-Giacobini-Kresak now visible with binoculars as it heads towards perihelion

Mercury
Mercury heading towards greatest elongation east

Minor Planet
Vesta now visible with binoculars and small telescopes.

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for March 2017

Northern Hemisphere
Evening
West:- Venus (mag. -4.8 to -4.1 - first half of month), Mars (mag. +1.3 to +1.5), Uranus (mag. +5.9), Mercury (mag. -1.5 to -0.4 - second half of month)
Midnight
Southeast:- Jupiter (mag. -2.3 to -2.5)
Morning
Southwest:- Jupiter
Southeast:- Saturn (mag. +0.5)

Southern Hemisphere
Evening
West:- Venus (first half of month), Mars, Uranus
Midnight
North:- Jupiter
East:- Saturn
Morning
West:- Jupiter
Northeast:- Saturn
East:- Neptune (mag. +8.0 - second half of month)

Deep Sky
Naked eye / binoculars:-
Melotte 111 - Mel 111 - The Coma Star Cluster (Open Cluster)
Messier 44 - M44 - The Praesepe (Open Cluster)

Telescopes:-
Messier 67 - M67 - Open Cluster
Messier 51 - M51 - The Whirlpool Galaxy (Spiral Galaxy)
Messier 97 - M97 - The Owl Nebula (Planetary Nebula)
Messier 101 - M101 - The Pinwheel Galaxy (Spiral Galaxy)
Messier 65 – M65 – Spiral Galaxy
Messier 66 - M66 - Intermediate Spiral Galaxy
Messier 95 - M95 - Barred Spiral Galaxy
Messier 96 - M96 - Intermediate Spiral Galaxy
NGC 4244 - Spiral Galaxy
NGC 4565 - Needle Galaxy - Spiral Galaxy

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