NGC 6681

Messier 70 (NGC 6681) is a globular cluster in the constellation Sagittarius, approximately 30,000 light-years from Earth and spanning about 65 light-years across. Charles Messier discovered it on August 31, 1780, the same night he found M69. NGC 6681 has a very dense, highly concentrated core — one of the hallmarks of a post-core-collapse globular cluster, where gravitational interactions have driven stars to sink toward the center over billions of years, creating an extraordinarily compact stellar nucleus. In 1995, M70 gained a brief brush with fame when Comet Hale-Bopp was discovered in photographs of its immediate vicinity, though the comet and cluster are entirely unrelated.

NGC 6681's extreme central concentration distinguishes it visually from the slightly looser M69 nearby. Like M69, it is relatively metal-rich for a globular cluster and sits in a similar direction toward the inner galaxy, embedded in rich Sagittarius starfields that make both clusters slightly challenging to locate despite their moderate brightness (magnitude 7.9). The cluster contains a modest number of variable stars, and its proximity to the galactic center means that tidal forces from the Milky Way's massive bulge play an ongoing role in shaping its dynamics and orbital evolution.

A small telescope shows M70 as a compact, concentrated glow; the dense core gives it a star-like appearance at low power that only a moderate aperture can resolve into individual stars at the edges. This composite image was made from exposures taken at the Kitt Peak National Observatory 0.9-meter telescope in July 1998 during the Research Experiences for Undergraduates (REU) program and again in October 1998. Dynamic range was carefully stretched to reveal both the crowded inner core and the sparser outer regions.