Messier 69 (NGC 6637) is a globular cluster in the constellation Sagittarius.
This globular was discovered by Charles Messier on August 31, 1780, the same night he discovered M 70. At the time, Messier was searching for an object described by LaCaille in 1752. But it is unclear if the object recovered by Messier is the same one LaCaille catalogued. M 69 is considerably fainter than any other Lacaille object, and could probably not be seen with Lacaille's telescope; moreover Lacaille's position deviates from M 69's by an unusually-large 1.2°.
Amateur Observation
Like its neighbor M 70, M 69 is one of the smaller and fainter globular clusters in Messier's catalog. It can just be seen in a dark night with pair of 10x50 binoculars only if the observing location is not too far north. From the latitude of Paris, Messier's observing site, it is a difficult object.
At magnitude 7.6, Messier 69 appears slightly brighter and more concentrated than M 70. It appears as a fuzzy patch in a 4" telescope, but larger instruments can resolve the cluster into individual stars. It has a diameter of about 9.8' and contains a a broadly concentrated, granular core that intensifies to a bright, nearly stellar core. Some very faint stars can be resolved around the highly irregular periphery. The eastern edge of the halo is especially ragged and is broken by a small notch.
Properties and Evolution
M 69 is about 30,000 light years away, and has a diameter of about 85 light-years. It is a close neighbor of the globular cluster M 70; about 1,800 light years separate them from each other. In contrast, the also-apparently-nearby globular M54 is about three times farther away.
As M 69 is only about 6,200 light years from the Galactic Center, its tidal radius is comparatively small (72 light years). Its stellar concentration is about average for a globular cluster; its central core has a diameter of roughly 6 light years, while its half-mass radius is about 7.2 light years.
M 69 is one of the most metal-rich globular cluster known. Nevertheless, its abundance of elements heavier than helium is still significantly lower than that of younger stars like our Sun. This indicates that the cluster was formed when the universe contained fewer heavy elements, as these elements had yet to be forged inside the stars.