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Open clusters, also known as galactic clusters, are relatively small groups of about 10 to several thousand stars. The stars in an open cluster typically formed at about the same place and time, and remain held together for some time by their mutual gravitational attraction.
Our galaxy and similar spirals are rich in open clusters,
and many relatively nearby examples are known to astronomers. The best known open cluster is the
Pleiades, which lies about 120 parsecs from Earth. Six of the stars in the Pleiades are visible
without optical aid. Here is a
link to clusters (both open and globular) that are Messier objects
and here is a set of links to
images of open clusters.
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Here we show an NOAO image of the bright young open cluster M6 in Scorpius, which at 5th magnitude is just bright enough to be visible to the naked eye (Ref). This cluster is at a distance of about 400 parsecs, about four times as far as the Pleiades, and has a diameter of about 5 parsecs. It has an estimated age of 100 million years, which is moderately young for an open cluster.