Messier 21 or M21 (also designated NGC 6531) is an open cluster in the constellation Sagittarius. It has an apparent visual magnitude of 6.5 and its angular diameter is 13 arc-minutes. M21 lies at an estimated distance of 4250 light years. The Equinox 2000 coordinates are RA= 18h 04.6m, Dec= -22° 30´ which makes M21 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 M21 through the Takahashi E-180 Astrograph (North is to right). 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 M21 is 3930 light years and its diameter is 20 light years. It contains 105 stars and its estimated age is 4-8 million years.
Messier's Description of M21
June 5, 1764
`Star cluster, near the preceding [M20]: The nearest neighboring known star to these two clusters is 11 Sagittarii, 7 mag, according to Flamsteed. The stars of both these clusters are of 8-9 magnitude, enveloped in nebulosity.'
- Object: M21
- Other Names: NGC 6531
- Object Type: open cluster
- Object Data: Apparent Magnitude = 6.5, Angular Size = 13 arc-minutes
- Object Position (Equinox 2000): RA= 18h 04.6m, Dec= -22° 30´, Constellation = Sagittarius
- Date/Time: 2011 Aug 02 at 06:33 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: M20-01w.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.