Messier 107 — Globular Cluster in Ophiuchus

M107 (NGC 6171) is a globular cluster in the constellation Ophiuchus, located approximately 20,900 light-years from Earth. It is one of the more loosely concentrated globular clusters in the Messier catalog, classified as Shapley-Sawyer concentration class X (on a scale of I to XII, where XII is the least concentrated). The cluster spans about 13 arcminutes across, corresponding to a true diameter of roughly 80 light-years. M107 contains several hundred thousand stars and has a relatively low luminosity compared to the great showpiece globulars like M13 or M5. It is notable for containing several dark voids or lanes within its structure — regions where interstellar dust partially obscures the cluster's stars, a feature unusual among globular clusters. M107 lies close to the galactic plane, which accounts for the reddening and absorption of its light by intervening dust. Its metallicity is moderate for a globular cluster, suggesting it may be somewhat younger than the most ancient halo globulars.

Located about 2.5 degrees south-southwest of the star Zeta Ophiuchi, near the bottom of the large constellation Ophiuchus. At magnitude 7.9, M107 is not visible to the naked eye but is an easy target for binoculars, appearing as a small, faint fuzzy patch. A 4-inch telescope at 100x shows a granular, loosely concentrated glow with little central condensation. An 8-inch telescope at 150-200x begins to resolve individual stars throughout the cluster, revealing its open, airy structure and the dark patches within. The cluster's loose concentration makes it easier to resolve than many brighter globulars. Compare it to the much denser M62 and M19 also in Ophiuchus to appreciate the diversity of globular cluster structures. Best observed in June and July when Ophiuchus is high in the southern sky.

Discovered by Pierre Mechain in April 1782, and later independently found by William Herschel, who cataloged it as H VI.40. It was one of the last objects added to the Messier catalog — Mechain reported it in a letter to Bernoulli in 1783, but it was not formally added to the catalog until 1947 by Helen Sawyer Hogg. M107 was one of the first globular clusters where distinct dark features within its structure were noted, leading astronomers to recognize that even globular clusters can contain or be partially obscured by interstellar dust.