M7 - Ptolemy Cluster
Messier 7 or M7 (also designated NGC 6475) is an open cluster in the constellation Scorpius. It has an apparent visual magnitude of 4.1 and its angular diameter is 80 arc-minutes. M7 lies at an estimated distance of 800 light years. The Equinox 2000 coordinates are RA= 17h 53.9m, Dec= -34° 49´ which makes M7 best seen during the summer. The Messier Summer Star Chart shows the position of all Messier objects visible during that season. As one of the more famous objects in the Messier Catalog, it is commonly known as the Ptolemy Cluster.
The image above shows the uncropped view of M7 through the Takahashi E-180 Astrograph (North is up). A 2x enlargement of this image appears to the right.
Known since antiquity, M7 is a large, naked eye star cluster that was first described by Ptolemy in 130 AD as the "nebula following the sting of Scorpius." According to Kharchenko et al. (2005), the distance of M7 is 980 light years and its diameter is 23 light years. It contains 750 stars and its estimated age is 220 million years.
Messier's Description of M7
May 23, 1764
`Star cluster more considerable than the preceding [M6]; this cluster appears to the naked eye like a nebulosity; it is little distant from the preceding, placed between the bow of Sagittarius and the tail of Scorpius. (Diam. 30')'
- Object: M7
- Other Names: NGC 6475, Ptolemy Cluster
- Object Type: open cluster
- Object Data: Apparent Magnitude = 4.1, Angular Size = 80 arc-minutes
- Object Position (Equinox 2000): RA= 17h 53.9m, Dec= -34° 49´, Constellation = Scorpius
- Date/Time: 2011 May 07 at 09:20 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: M7-01w.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.