Messier Index/M7
Messier 7 or M7, also designated NGC 6475 and sometimes known as the Ptolemy Cluster, is an w:open cluster of w:stars in the w:constellation of w:Scorpius.
| M7 | |
|---|---|
 | |
| Observation data (J2000.0 epoch) | |
| 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′ |
| Other designations | NGC 6475, Ptolemy Cluster |
The cluster is easily detectable with the w:naked eye, close to the "stinger" of Scorpius. It has been known since w:antiquity; it was first recorded by the 1st-century astronomer w:Ptolemy, who described it as a w:nebula in 130 AD. w:Giovanni Batista Hodierna observed it before 1654 and counted 30 stars in it. w:Charles Messier catalogued the cluster in 1764 and subsequently included it in his list of w:comet-like objects as 'M7'.
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 w: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.
