Messier 23 is a large, bright, open cluster located in the constellation of Sagittarius 4.5 degrees northwest of Mu Sagittarii. It can be seen here at the bottom center of the frame.

A complex of dark nebulae, the most distinctive of which is Barnard 84a, lies just to the north of M23. B84a is at the top of the frame with a 9.5 magnitude star in the middle of it. Just above M23 are LDN 288 and LDN 285.

M23 shines at magnitude 5.5 and lies 2,150 light-years away. It subtends an apparent size of 25 arcminutes, which corresponds to 15-20 light-years in real size. It contains 150 member stars, the brightest of which is magnitude 9.2. It is estimated to be 220 to 300 million years old.

M23 was discovered by Charles Messier on June 20, 1764.

North is to the top in the above image.

Messier 23 Catalogs: M23, NGC 6494 Object Type: Open Cluster Magnitude: 5.5v Size: 25' Constellation: Sagittarius Image Field Centered At: RA: 17h 57m 01s Dec: -18° 24' 28"