Messier 23 or M23 (also designated NGC 6494) is an open cluster in the constellation Sagittarius. It has an apparent visual magnitude of 6.9 and its angular diameter is 27 arc-minutes. M23 lies at an estimated distance of 2150 light years. The Equinox 2000 coordinates are RA= 17h 56.8m, Dec= -19° 01´ which makes M23 best seen during the summer. The Messier Summer Star Chart shows the position of all Messier objects visible during that season.
The image above shows the uncropped view of M23 through the Takahashi E-180 Astrograph (North is up). A 2x enlargement of this image appears to the right.
This open cluster was discovered by Messier in 1764. According to Kharchenko et al. (2005), the distance of M23 is 2050 light years and its diameter is 20 light years. It contains 177 stars and its estimated age is 300 million years.
Messier's Description of M23
June 20, 1764
`A star cluster, between the end of the bow of Sagittarius and the right foot of Ophiuchus, very near to 65 Ophiuchi, according to Flamsteed. The stars of this cluster are very close to one another. Its position was determined from Mu Sagittarii.' (diam. 15')
- Object: M23
- Other Names: NGC 6494
- Object Type: open cluster
- Object Data: Apparent Magnitude = 6.9, Angular Size = 27 arc-minutes
- Object Position (Equinox 2000): RA= 17h 56.8m, Dec= -19° 01´, Constellation = Sagittarius
- Date/Time: 2011 Apr 12 at 10:46 UTC
- Location: Bifrost Astronomical Observatory, Portal, AZ
- Mount: Astro-Physics 1200GTO
- Telescope: Takahashi Epsilon 180 Hyperbolic Astrograph
- Camera: Canon EOS 550D (Rebel T2i) (modified with a Baader UV/IR filter)
- Field of View: 1.70° x 2.56° at 1.7 arc-sec/pixel (web version: 10.0 arc-sec/pixel)
- Exposure: 2 x 300s, f/2.8, ISO 800
- File Name: M23-01w.jpg
- Processing (Adobe Camera Raw): Graduated Filter, Vignetting Correction, Noise Reduction, White Balance, Curves
- Processing (Photoshop CS5): Average Images, Curves, Noise Reduction
- Original Image Size: 3454 × 5179 pixels (17.9 MP); 11.5" x 17.3" @ 300 dpi
- Rights: Copyright 2011 by Fred Espenak. All Rights Reserved. See: Image Licensing.