NGC 6440 — Globular Cluster in Sagittarius

NGC 6440 is a bright globular cluster in Sagittarius, about 28,000 light-years away, projected against the rich star clouds of the Galactic bulge. It is moderately concentrated, with a small dense core surrounded by a tight halo of hundreds of resolved members. Heavy interstellar reddening — typical of bulge objects — dims and reddens its light, which is why a cluster of its intrinsic brightness still presents at only about magnitude 9.3 from Earth. NGC 6440 sits in a busy region of sky less than a degree from the bright planetary nebula NGC 6445, the two often visited together.

A satisfying telescope target. A 4-inch at moderate power shows a small bright round glow with a sharply concentrated core. An 8-inch at 200-300x begins to resolve the outer halo into individual stars and gives the cluster a clearly granular texture. A 12-inch under good skies fully resolves a hundred or more cluster members. NGC 6440 takes magnification well; atmospheric seeing usually limits the view rather than aperture. Star-hop from Mu Sagittarii about 4 degrees north. After observing, sweep less than a degree west to NGC 6445, a small bright planetary in the same eyepiece field. Best observed June through September.

Discovered by William Herschel on 28 May 1786. NGC 6440's bulge location and heavy reddening kept it relatively understudied until infrared surveys in the late 20th century, which provided cleaner photometry that established its age, distance, and chemical composition.