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Ptolemy’s Cluster

M7, NGC 6475

Even the brightest of the individual stars in this cluster are only on the verge of naked eye visibility, but in combination the entire cluster reaches an easily visible magnitude of +3.3. It was therefore recognised as a distinct object even in classical times, at least as early as Ptolemy in the second century, hence its common name of Ptolemy's Cluster. Some 1,500 years later, Charles Messier made it the seventh object in his Catalogue, giving the cluster its common alternative designation of 'M7'. M7, together with its close neighbour M6, are the first Open Clusters to appear in Messier's Catalogue.

In the sky, Ptolemy's Cluster falls within the band of the Milky Way in the far eastern parts of Scorpius, beyond the end of the Scorpion's tail on the borders of Sagittarius (and thus lies in the same general direction as the heart of the Milky Way Galaxy). It appears to fall very close in the sky to another Open Cluster (M6, or the Butterfly Cluster) though in fact M6 is about twice as far from Earth as M7.

Ptolemy's Cluster consists of about eighty blue stars occupying an region of space some twenty to twenty-five light years across, and the entire cluster lies a little less than 1,000 light years from the Solar System. The arrangement of stars is essentially random, but one feature stands out to an observer on Earth: one sequence of four brighter stars form a regular chain across the southern parts of the cluster. This is a simple line-of-sight effect, but is accentuated by the orange star HR 6658, which appears to fit neatly onto the end of this apparent line of stars. (In actuality this orange star is more than fifty light years beyond Ptolemy's Cluster, and completely unrelated.)


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