M7: an open cluster in Scorpius

(POSS plate).

M 7 (NGC 6475), is an open cluster in Scorpius, type "e", named "The Scorpion's Tail"
Right Ascension 17h 53m.9
Declination -34� 49'
Distance 800 light years
Visual Brightness = 4.1 mag.
Apparent Dimension 80 arc min.


"M7 is easily detected with the naked eye,..., projected on a background of numerous faint and distant Milky Way stars." (Burnham).
It was known, probably, to Ptolemy who described it as the "nebula following the sting of Scorpius".
In this cluster 80 stars brighter mag 10 has been identified
The diameter is about 18 light years, resulting the cluster with the highest degree of concentration toward the center.
Its age is 220 million years
The brightest star is a yellow giant (gG8, mag 5.6)
the hottest main sequence star is of spectral type B6 (mag 5.9).