Messier Index/M7

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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

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.

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.

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