Messier 55 or M55 (also designated NGC 6809) is a globular cluster in the constellation Sagittarius. It has an apparent visual magnitude of 6.3 and its angular diameter is 19 arc-minutes. M55 lies at an estimated distance of 17,600 light years. The Equinox 2000 coordinates are RA= 19h 40m, Dec= -30° 58´ which makes M55 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 M55 through the Takahashi E-180 Astrograph (North is to right). A 3x enlargement of this image appears to the right.
In spite of its inclusion in the Messier Catalog, this globular cluster was actually discovered by N. L. Lacaille in 1752. According to Recio-Blanco et al.(2005), the distance of M55 is 19,300 light years and its diameter is 110 light years. Its estimated mass is 250,000 solar masses and it contains 40 variable stars.
Messier's Description of M55
July 24, 1778
`A nebula which is a whitish spot, of about 6' extension, its light is even and does not appear to contain any star. Its position has been determined from zeta Sagittarii, with the use of an intermediate star of 7th magnitude. This nebula has been discovered by M. l'Abbe de LaCaille, see Mem. Acad. 1755, p. 194 [Glyn Jones has erroneously 1775]. M. Messier has looked for it in vain on July 29, 1764, as reported in his memoir.'
- Object: M55
- Other Names: NGC 6809
- Object Type: globular cluster
- Object Data: Apparent Magnitude = 6.3, Angular Size = 19 arc-minutes
- Object Position (Equinox 2000): RA= 19h 40m, Dec= -30° 58´, Constellation = Sagittarius
- Date/Time: 2011 Oct 19 at 03:12 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: M55-01w.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.