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.
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 Type||Open cluster|
|RA (J2000)||17h 57m 04s|
|DEC (J2000)||-18d 59m 07s|
|Apparent Size (arc mins)||27 x 27|
|Age (years)||220 Million|
|Number of Stars||150|
|Other Name||Collinder 356|