NGC 6864

Messier 75 (NGC 6864) is a remote, highly concentrated globular cluster in the constellation Sagittarius, lying over 61,000 light-years from Earth — with some estimates placing it as far as 100,000 light-years, depending on the reddening correction applied. It was discovered by Pierre Méchain on August 27, 1780, and catalogued by Charles Messier two months later. Despite its great distance, NGC 6864 is a genuinely enormous and luminous system: it spans more than 100 light-years and shines with a luminosity roughly 200,000 times that of the Sun. Its intrinsic brightness is among the highest of all Milky Way globular clusters, a consequence of its large physical size and high stellar density.

NGC 6864 is a highly concentrated cluster with a very dense core — one of the most centrally condensed globulars in the Milky Way — giving it an almost star-like appearance in small telescopes. Its concentration class (XII on the King scale) is among the highest possible, suggesting a long history of core-collapse dynamics or an inherently compact initial configuration. The outer halo fades rapidly into the background sky, with relatively few stars in the outer regions compared with less concentrated globulars. Because of its extreme distance, M75 sits far from the disrupting tidal forces of the galactic center and bulge, having spent much of its life in the outer halo.

A small telescope shows M75 as a bright but compact, almost star-like glow with very little obvious extent; only larger apertures (250 mm or more) begin to resolve the outer halo into individual stars. This image was made from data taken in October 1998 at the Kitt Peak National Observatory 0.9-meter telescope.