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The so called Ptolemy’s Cluster was already known to ancient Greeks. Ptolemy described it without doubt. It has been catalogued by many astronomers, including Charles Messier, Edmund Halley and Nicolas Louis de Lacaille. This is a large cluster, visible with the naked eye and located at about 1,000 light years. More than 80 stars belong to this cluster. Its brightest stars are HR 6658 (a double: 5.6-7.9), HR 6657 (also double: 6.1-6.9) and HR 6662 (variable: 6.0-7.3).
Additional Information
Object
Name(s): M7, NGC 6745, Ptolemy’s Cluster
Type: Open Cluster
RA: 17h 53m 49.9s
Dec: -34º 46’ 54”
Constellation: Scorpius
Size (arcmin): 80×80
Magnitude: +3.3
Distance: 1,000
Image
Date: 2014-08-22
Location: SSO near Coonabarabran, NSW Australia
Size (arcmin): 230×150
Telescope: Takahashi FSQ 106 f/5
Camera: SBIG STL-11000M (4008x2672pix)
Guiding: not recorded
Total exposure: 100 min (L: 25min; RGB: 75min)
Processing: PixInsight