M7 (NGC 6475)

Constellation: Scorpius

Distance: 980 light-years

M7 is the closest prototypical middle aged star cluster to earth; its age is approximately 200 million years. It consists of about 100 stars extending over about 25 light-years of space. The cluster is dominated by bright blue stars, set against a background of millions of unrelated and much more distant predominantly yellow stars towards the center of the Milky Way. M7 is an easy naked eye object, and has been known since antiquity; it was first recorded by the 2nd-century Greek-Roman astronomer Ptolemy, who described it as a nebula in 130 AD (hence the nickname, "Ptolemy's Cluster").

Stars are formed in groups from giant molecular clouds; once the ancestral cloud disperses, what is left is an "open" cluster, all of whose members are of the same age, same composition, same distance from earth and moving in the same approximate direction and speed. Eventually the cluster disbands. Our own Sun, like all stars, was born in such a cluster, which disbanded long ago.

(Some of this information from A Year in the Life of the Universe by Robert Gendler).