Tweedledum Cluster

NGC 6633 falls just inside Ophiuchus' border with Serpens. This is a comparatively close star cluster, at a distance of some 1,300 light years from the Solar System.

A small but relatively prominent open cluster some 1,300 light years from the Sun, standing out against the densely starred backdrop of the band of the Milky Way. This cluster lies in the north of the constellation Ophiuchus, while nearby across the border in Serpens is another open cluster of almost identical brightness, IC 4756. Together this pair of clusters are sometimes known as the Tweedledee Cluster (IC 4756) and the Tweedledum Cluster (NGC 6633).

The stars of the cluster, as seen from Earth, approximate a curving, tapering shape, and it is for this reason that NGC 6633 is occasionally known by another name, the Captain Hook Cluster. The entire cluster spreads across an area of sky approximately equivalent to that of the Moon's disc. None of the individual stars in the cluster exceed eighth magnitude, but the cluster as a whole has a visual magnitude of +4.59 (and so is visible to the naked eye under good seeing conditions).

The brightest star within the area of the cluster is HD 169959, which lies near the base of the cluster's 'hook' shape. This star is in fact not part of the cluster at all, but a white giant lying nearly four times farther from the Solar System, whose light, despite its much greater distance, outshines the foreground stars that truly belong to the Tweedledum Cluster.