Messier 18 or M18 (also designated NGC 6613) is an open cluster in the constellation Sagittarius. It has an apparent visual magnitude of 7.5 and its angular diameter is 9 arc-minutes. M18 lies at an estimated distance of 4900 light years. The Equinox 2000 coordinates are RA= 18h 19.9m, Dec= -17° 08´ which makes M18 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 M18 through the Takahashi E-180 Astrograph (North is up). A 3x 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 M18 is 4220 light years and its diameter is 6 light years. It contains 40 stars and its estimated age is 50 million years.
Messier's Description of M18
June 3, 1764
`A cluster of small stars, a little below above nebula, No. 17, surrounded by slight nebulosity, this cluster is less obvious than the preceding, No. 16: with an ordinary telescope of 3.5 feet, this cluster appears like a nebula; but with a good telescope one sees nothing but stars.' (diam. 5')
- Object: M18
- Other Names: NGC 6613
- Object Type: open cluster
- Object Data: Apparent Magnitude = 7.5, Angular Size = 9 arc-minutes
- Object Position (Equinox 2000): RA= 18h 19.9m, Dec= -17° 08´, Constellation = Sagittarius
- Date/Time: 2011 May 06 at 08:52 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: 4 x 300s, f/2.8, ISO 800
- File Name: M18-01w.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.