M70 - NGC 6681

Messier 70 (NGC 6681) is a globular cluster located approximately 29,300 light-years from Earth in the constellation Sagittarius, situated close to the galactic center. It was discovered by Charles Messier in 1780 on the same night he cataloged the nearby cluster M69. M70 is a moderately concentrated globular cluster with a Shapley-Sawyer class of V, and it has a physical diameter of about 68 light-years. The cluster is believed to have undergone a core collapse at some point in its history, meaning that gravitational interactions between stars caused the core to contract dramatically to a very dense state. Core-collapsed globular clusters are distinguished by an extremely sharp, bright central peak in their brightness profile, and M70 displays this characteristic. Despite this dense core, M70 is not particularly massive compared to other Messier globulars. The cluster contains a modest number of known variable stars, including a few RR Lyrae types. An interesting historical footnote is that the comet Hale-Bopp was discovered in 1995 near M70 by Alan Hale and Thomas Bopp, who were both observing the cluster when they noticed the comet nearby. M70 lies about 2 degrees east of M69, and the two clusters make a natural observing pair during summer evenings. In amateur telescopes, M70 appears very similar to M69: a small, round, concentrated glow that is difficult to resolve into individual stars due to its compact nature and moderate distance. Telescopes of 8 inches or more can begin to show granularity at the edges under good conditions. Like M69, M70's low declination of about -32 degrees makes it better suited for observation from southern latitudes.

M70 spans about 68 light-years in diameter and lies approximately 29,300 light-years from Earth, having undergone core collapse.