NGC 6637

Messier 69 (NGC 6637) is a compact globular cluster in the constellation Sagittarius, located approximately 25,000 light-years from Earth and spanning about 50 light-years across. It was first recorded by Nicolas Louis de Lacaille from the Cape of Good Hope in 1751 and subsequently catalogued by Charles Messier on August 31, 1780. NGC 6637 is one of the closer globulars to the galactic center — lying only about 6,000 light-years from the Milky Way's core — and as a result it sits in a rich, dense starfield of Sagittarius background stars that makes it visually challenging despite its proximity. At apparent magnitude 7.6 it is just within binocular range under dark skies.

NGC 6637 is one of the more metal-rich globular clusters in Messier's catalog — unusual for such an ancient system. Metal-rich globulars are thought to have formed closer to the galactic center, where earlier generations of supernova explosions enriched the available gas with heavy elements before the cluster assembled. M69 is strikingly similar in size, distance, and appearance to its Sagittarian neighbor M70, which lies just 1.8 degrees away — the two clusters form a visual pair that can be swept in a single binocular field. Very few variable stars have been identified in M69, which is consistent with its metal-rich population.

In binoculars M69 is a faint, compact glow in the rich Sagittarius starfields; a 150 mm telescope begins to resolve the outer halo into stars. This composite image was created from observations at the Kitt Peak National Observatory 0.9-meter telescope in May 1998 and July 1998 during the Research Experiences for Undergraduates (REU) program supported by the National Science Foundation.