Ptolemy's Cluster

Messier 7 (NGC 6475), sometimes called Ptolemy's Cluster, is one of the most ancient recorded deep-sky objects in the sky. The Greek-Egyptian astronomer Claudius Ptolemy noted it in 130 AD as "a nebula following the sting of Scorpius," making it one of the earliest catalogued celestial objects outside of stars and planets. Charles Messier added it to his list in 1764. The cluster lies in the constellation Scorpius, projected against a dense backdrop of Milky Way starfields that makes for one of the most photogenic open cluster views in the sky — a sparkling foreground swarm set against a glittering river of background stars.

NGC 6475 contains around 80 stars spread across roughly 20–25 light-years; its distance is estimated at 800–1,000 light-years. The stars are estimated to be in excess of 200 million years old, considerably more evolved than its neighbor M6, and several bright yellow and orange giants among the membership signal that the cluster is gradually losing its hottest, most massive stars to age. Compared with the tighter Butterfly Cluster nearby, M7 is loose and sprawling — best appreciated through binoculars or a rich-field telescope rather than a high-power eyepiece.

M7 is visible to the naked eye as a broad misty patch low in the southern sky; binoculars immediately resolve it into dozens of stars scattered across a field wider than the full Moon. This color composite was assembled from CCD images taken in June and July 1995 at the Burrell Schmidt telescope of Warner and Swasey Observatory on Kitt Peak, as part of the Research Experiences for Undergraduates (REU) program supported by the National Science Foundation.