NGC 6656

Messier 22 (NGC 6656) is the brightest globular cluster visible from the northern hemisphere and one of the closest, lying approximately 10,000 light-years away in the constellation Sagittarius. It spans roughly 65 light-years across and is a conspicuous naked-eye object in a dark sky, appearing as a fuzzy star in the rich Milky Way field just northeast of the bright star that marks the top of the Sagittarius "Teapot" asterism. Abraham Ihle first observed NGC 6656 in 1665, making it among the earliest-known globular clusters; it was later recorded by John Bevis and independently catalogued by Charles Messier in 1764. Johann Elert Bode resolved it into stars in 1777.

NGC 6656 is a notably rich cluster estimated to contain several hundred thousand stars with an age of around 12 billion years. It is slightly flattened — more obviously elongated than most globulars — possibly due to tidal interaction with the Milky Way's bulge, which lies relatively close in the same direction. M22 is one of only a handful of globular clusters known to contain a planetary nebula, and it harbors a small black hole at its core, inferred from radio observations of a compact radio source within the cluster. Because it lies close to the galactic plane, some of its light is dimmed by intervening dust, yet it still shines brilliantly enough to be visible well before full dark on a summer evening.

Binoculars reveal M22 as a large, bright, clearly non-stellar glow; a small telescope begins to sprinkle stars across the outer halo, and 150 mm aperture delivers a stunning, partially resolved view. This image was made from CCD exposures taken in June 1995 at the Burrell Schmidt telescope of Case Western Reserve University's Warner and Swasey Observatory on Kitt Peak, as part of the Research Experiences for Undergraduates (REU) program supported by the National Science Foundation.