Discovered 1764 by Charles Messier.

Open cluster M23 is another glorious sight for small telescopes and binoculars in the summer Milky Way. It is one of Charles Messier's authentical discoveries; he discovered this cluster on June 20, 1764.

At a distance of 2,150 light years, the apparent diameter of this 6-7th mag object, 27 arc minutes, corresponds to about 15 light years (Kenneth Glyn Jones' figure of 35' would correspond to about 20 light years). M23 was classified by Trumpler as I,2,r, while Götz assigns it to class II,2,r, and the Sky Catalog 2000 gives III,1,m. It contains at least about 150 proven member stars.

Already Wallenquist counted 129 probable cluster members within a diameter of 34', and gave the mean magnitude of the brightest five stars as 9.4. He considered it to be one of the older open clusters.

S.N. Svolopoulos, at Norman Lockyer Observatory, examined M23 in 1953. He found 149 member stars within 27.2', with the following distribution in magnitudes:

Mag 10: 12, mag 11: 17, mag 12: 24, mag 13: 20, and mag > 13.5: 96 stars.

The hottest stars in M23 are of spectral type B9, the brightest star is of magnitude 9.21. The age of M23 has been estimated at 220 million years (Sky Catalog 2000) and 300 million years (by G. Meynet's Geneva Team).

M23 can be easily found either 2.5 deg N and 3.5 deg W of Mu Sagittarii, approximately on the line to Xi Serpentis, or 0.5 deg S and 8 deg E of M9. It is a find open cluster of stars mainly between 10..13 mag. The cluster is not very sharply limited from a rich Milky Way background. A mag 6.5 white foreground star is quite prominent in the NW, about 18' from the cluster's center.