M7 (NGC6475)

1. The history
2. To find M7
3. To observe M7
4. The stars

This fine galactic cluster is mentioned in the catalogue of Ptolemy, and in the 16th Century Latin translation of the Almagest appears as the Girus ille nebulosus, probably including both M7 and M6, although M6 is first described independently by Giovan Battista Hodierna before 1654.

Messier, in 1764, described M7 as "A cluster considerably larger than the preceding..."

M7 was also observed by Hodierna before 1654 who counted 30 stars, and included in Lacaille's catalog of southern objects as Lac II.14.

Easily visible to the naked eye, M7 lies about 4° northeast from l Scorpii in the Scorpion's tail, and about 3½° from M6.

M7 appears to the naked eye as a nebulosity, situated a short distance from the preceding M6, between the bow of Sagittarius and the tail of Scorpius. It is best observed, being so large, in good binoculars than in telescopes: it is at least 1° in diameter. Abbiamo già detto che M7 é facilmente osservabile ad occhio nudo, tanto che é uno degli oggetti di aspetto nebulare noti fin dall'antichità. E'uno dei pochi ammassi che può essere apprezzato in modo approfondito con un buon binocolo. Nel campo di M7 si trova anche un piccolo e debole ammasso globulare, NGC6453: si presenta come un puntino sfilacciato di mag. 11, di circa 1' di diametro. Circa 45' a sudest da M7 si trova l'evanescente ammasso galattico H18, formato da un'ottantina di deboli stelle.

Ake Wallenquist found that this is one of the clusters with the highest degree of concentration toward the center. According to him, M7 consists of 80 stars brighter mag 10 in a field of about 1.3 degrees apparent diameter which at its distance of perhaps 800 light years corresponds to a linear extension of 18 light years. It was classified as of Trumpler type I,3,m or I,3,r. This group is approaching us at 14 km/sec. The brightest star is a yellow giant (spectral type gG8, mag 5.6), the hottest main sequence star is of spectral type B6 (mag 5.89). M7's age was estimated at 220 million years, both according to the Sky Catalog 2000 and the new calculation of the Geneva Group of G. Meynet.

M7's integrated apparent magnitude is discordantly given at values between 3.3 and 5.0.