NGC 6939 — Open Cluster in Cepheus

NGC 6939 is a rich, moderately old open cluster in Cepheus, about 4,000 light-years away, containing roughly 80 stars spread across 8 arcminutes. The cluster is well concentrated, with a bright core of about 30 stars and a more diffuse halo. Its age — roughly 1.3 billion years — places it among the older members of its class, with a well-developed red-giant branch visible on its colour-magnitude diagram. NGC 6939 is best known among amateurs for its proximity to the much larger Fireworks Galaxy NGC 6946, which sits just half a degree southeast and shares the same low-power eyepiece field.

A delightful target. A 4-inch at low power shows the cluster as a small bright patch with a dozen stars resolved at the edges; the same field includes the soft round glow of NGC 6946 to the southeast — a remarkable visual contrast between an embedded stellar cluster a few thousand light-years away and an entire galaxy 25 million light-years beyond. An 8-inch at 100x brings out 40-50 cluster members and reveals NGC 6946's mottled disk. The cluster-galaxy pair is one of the finest two-for-one views in the sky. Best observed June through October.

Discovered by William Herschel on 9 September 1798 — the same night as NGC 6946. Herschel apparently observed both objects in sequence, separated by only the few minutes it took to step his telescope across the half-degree gap.