Messier 7, NGC 6475 - Ptolemy's Cluster

Messier 7, also designated NGC 6475, is a large and brilliant open cluster in Scorpius, easily detected with the naked eye.

This splendid cluster was known to Ptolemy, who mentioned it about 130 AD and described it as the "little cloud following the stinger of Scorpius." His description may also include M6, but this is uncertain.

M 7 was observed by Hodierna before 1654, who counted 30 stars. Edmond Halley listed it as No. 29 in his catalog of southern stars of 1678, and Nicholas Louis de Lacaille added it to his catalog of southern objects as Lac II.14. Charles Messier included it as the seventh object in his catalog in 1764.

Messier 7 is a huge open cluster, plainly visible to the naked eye as a concentrated patch in the Milky Way. Telescopic observations reveal about 80 stars within a field of view of 1.3° across. The cluster's brighter stars are near the cluster's center, with jagged star chains running generally east to west.

The cluster's estimated distance is 800-1000 light years, a little more than half as far as M 6. At this distance, it has an actual diameter of 18-25 light years. Its absolute magnitude is a rather modest -3.7, a luminosity of 2,500 suns, and it is approaching us at 14 km/sec.

Modern sources agree on M 7's integrated apparent magnitude at 3.3. Its brightest star is a mag. 5.6 yellow giant of spectral type G8; its hottest main sequence star is of spectral type B6. The age of the cluster is estimated at 220 million years.