M23 is a pretty open cluster that's located in the rich starfields of the Sagittarius Milky Way. With an apparent magnitude of +6.9, it's beyond naked eye visibility but a nice binocular object and a glorious sight through small telescopes. This vast cloud of about 150 stars is located 2,150 light-years from Earth and has an actual diameter of about 20 light-years. With an estimated age of at least 220 million years old, it's one of the galaxy's oldest open clusters.

M23 was discovered by Charles Messier on June 20, 1764. It can be easily found just northwest of the teapot asterism of Sagittarius. The three stars that form the top of the teapot are φ Sgr (mag. +3.2), Kaus Borealis (λ Sgr - mag. +2.8) and Kaus Media (δ Sgr - mag. +2.7). Positioned 6 degrees northwest of Kaus Borealis is μ Sgr (mag. +3.8). M23 can be found 4.5 degrees northwest of this star and approximately 2/5ths of the way along a line connecting μ Sgr with ξ Ser (mag. +3.5). Located 5 degrees east of M23 is M24, the very large Sagittarius Star Cloud.

M23 Open Cluster (credit:- Sharp/NOAO/AURA/NSF)

Finder Chart for M23 (credit:- freestarcharts)

Finder Chart for M23 - 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)

When viewed through binoculars, M23 appears as a grainy smudge that covers 27 arc minutes. The brightest members are just about resolvable. A small 80mm (3.1-inch) telescope at low magnifications reveals the brightest stars, with many fainter background members visible using averted vision. There is a prominent mag. +6.5 white foreground star positioned at the northwest corner, about one third of a degree from the centre of the cluster.

Medium size 150mm (6-inch) or 200mm (8-inch) telescopes resolve M23 well, with tens of stars visible. The brightest member is an extremely luminous hot blue star of spectral type B9, that shines at mag. +9.2. Most stars are between 10th and 13th magnitude and there are about 100 stars brighter than mag. +13.5.

M23 Data Table

Object TypeOpen cluster
Distance (light-years)2,150
Apparent Mag.+6.9
RA (J2000)17h 57m 04s
DEC (J2000)-18d 59m 07s
Apparent Size (arc mins)27 x 27
Radius (light-years)10
Age (years)220 Million
Number of Stars150
Other NameCollinder 356

Sky Highlights - March 2017

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

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
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)
Southeast:- Jupiter (mag. -2.3 to -2.5)
Southwest:- Jupiter
Southeast:- Saturn (mag. +0.5)

Southern Hemisphere
West:- Venus (first half of month), Mars, Uranus
North:- Jupiter
East:- Saturn
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)

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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