M9 - Globular Cluster in Ophiuchus
Globular cluster Messier 9 (M9, NGC 6333) is one of the original discoveries of Charles Messier, who cataloged it on May 28, 1764, and described it as "Nebula without star" of 3' diameter. It was first resolved into stars by William Herschel about 20 years later.
M9 is one of the nearer globular clusters to the nucleus of our Galaxy, with a computed distance of 5500 light-years from the Galactic Center (Burnham gives 7500, a slightly too high value). Its 12.0 arc minutes angular diameter corresponds to a linear extension of 90 light years at its distance of about 25,800 light years from our Solar system. However, it appears somewhat smaller visually, about 3 or 4', and on conventional photos can be traced to about 9.3 arc minutes. To the north and west, its light is significantly dimmed by interstellar dust, as it lies at the edge of a patch of dark nebula (Barnard 64); its light is probably weakened by at least one magnitude (a factor of about 2.5).