M7 Ptolemy's Cluster, NGC6475, NGC6453, Tr30, NGC6444, B283, B286-7; Scorpius
Astro-Physics 155 EDF (155TCC) f/5.4 refractor
Total Exposure Time: 3.5 hours; LRGB
April 2010; RDO, Moorook, AU
Comments: Messier 7 (M7, NGC 6475) 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 and described it as the "nebula following the sting of Scorpius." The description may also include M6, but this is uncertain.
M7 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 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.(SEDS)
Dark clouds of opaque dust lay between us and the stars of the galactic central bulge. These appear to us as dark clouds against a prodigiously starry background. Many of these dark nebulae were imaged and cataloged by E. E. Barnard. This catalog begins with B1 and ends with B370. He published his initial list with the 1919 paper in the Astrophysical Journal, "On the Dark Markings of the Sky with a Catalogue of 182 such Objects". This image contains Barnard 283, 286-7.