Messier 23, NGC 6494

The open cluster Messier 23 (also catalogued as NGC 6494) is a glorious sight for small telescopes and binoculars, located in the rich starfields of the Sagittarius Milky Way.

This stunning 7th-magnitude cluster, discovered by Charles Messier in June 1764, fills an area the size of the full Moon. Messier 23 contains 150 stars in an area 27' across; the brightest is of magnitude 9.21. A magnitude 6.5 foreground star is quite prominent, about 18' northwest of the cluster's center. Star chains curve around the periphery like gems around the cornice of a crown. The cluster is narrower east to west on its north than on its south, where its full width approaches one degree.

Messier 23 lies 2,100 light years away, and has a diameter of approximately 20 light-years. It has an absolute magnitude of -4.7, and a luminosity of 6,300 suns. The hottest stars in M 23 are of spectral type B9, making the cluster 300 million years old.