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M 7 - The Scorpions Tail Ptolemys Cluster
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M 7 - The Scorpion's Tail, Ptolemy's Cluster

February 18, 2005
Open Cluster M7 (NGC 6475), type 'e', in Scorpius

Right Ascension: 17 : 53.9 (h:m)
Declination: -34 : 49 (deg:m)
Distance: 0.8 (kly)
Visual Brightness: 3.3 (mag)
Apparent Dimension: 80.0 (arc min)

Known to Ptolemy 130 AD.

M7 is a large and brilliant group, easily detected with the naked eye. As Burnham describes it, "the cluster is seen projected on a background of numerous faint and distant Milky Way stars."

This splendid cluster was known to Ptolemy, who mentioned it about 130 AD, who described it as the "nebula following the sting of Scorpius". The description may also include M6, but this is uncertain. Because of this presumable discovery, the present author [hf] has proposed the name "Ptolemy's Cluster" for M7 some years ago, a proposition which has found some acceptance meanwhile.

M7 was observed by Hodierna before 1654 who counted 30 stars, and included in Abbe Lacaille's catalog of southern objects as Lac II.14. Charles Messier included it as No. 7 in his catalog on May 23, 1764.

M7 consists of about 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 or 20 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. Recent work suggests a slightly larger distance of 1000 light-years, which would increase the size to 25 light-years but would not affect the age.

Ake Wallenquist found that this is one of the clusters with the highest degree of concentration toward the center. Modern sources agrre on M7's integrated apparent visual brightness at magnitude 3.3, while older estimates, mostly from northern observers, had this southern cluster significantly underestimated at mag 4.1 to 5.0.


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