M7 · Ptolemy Cluster
| Chinese name | 托勒密星团 |
|---|---|
| Type | Open cluster |
| Constellation | Sco |
| RA | 17h54m |
| Dec | −35° |
| Apparent magnitude | 3.3ᵐ |
| Hemisphere | Southern |
| Best season | Summer |
| Difficulty | Easy |
| Focal length | 广角/中焦 135–400mm |
About
The Ptolemy Cluster lies about 980 light-years away in the tail of Scorpius and is a bright open cluster recorded as early as 130 AD by the Greek astronomer Ptolemy, giving it its name as one of the earliest star clusters ever identified. At magnitude 3.3 and spanning 1.3° (over twice the Moon's width), it is a conspicuous knot of stars to the naked eye in the Scorpion's tail. It contains roughly 80 stars about 200 million years old, dominated by blue-white luminaries. Embedded in the rich Milky Way toward Sagittarius, it is interwoven with dense star fields and dark dust. It is an excellent southern summer wide-field subject, suiting wide-angle to medium focal lengths that frame the cluster within the Milky Way.