Ptolemy Cluster
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| Observation data: J2000.0 epoch | |
|---|---|
| Class: | I,3,m |
| Constellation: | Scorpius |
| Right ascension: | 17h 53.9m |
| Declination: | −34° 49′ |
| Distance: | 0.8 kly (245 Pc) |
| Apparent magnitude (V): | 3.3 |
| Apparent dimensions (V): | 80.0′ |
| Physical characteristics | |
| Mass: | ( M☉) |
| Radius: | 20 ly |
| VHB: | |
| Estimated age: | 220 million years |
| Notable features: | |
| Other designations: | Messier 7, Ptolemy Cluster |
| See also: Open cluster, List of open clusters | |
The Ptolemy Cluster (also known as Messier Object 7 or NGC 6475) is an open cluster in the constellation Scorpius.
It was known to Ptolemy, who described it as a nebula, by 130 AD. Giovanni Batista Hodierna observed the cluster before 1654 and counted 30 stars in it. Charles Messier catalogued the cluster in 1764 and subsequently included it in his list of comet-like objects as M7. The cluster is easily detectable with the naked eye close to the "stinger" of Scorpius.
Telescopic observations of the cluster reveal about 80 stars within a field of view of 1.3° across. At the cluster's estimated distance of 800-1000 light years this corresponds to an actual diameter of 18-25 light years. The age of the cluster is around 220 million years while the brightest star is of magnitude 5.6.
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