M39 is a very sparse cluster of 30 or so stars confined within a space of about 7 light years. Its members are all still on the main sequence but several of the brighter stars will soon leave and become red giants. The stars of M39 are about 250 million years old, which makes it a middle aged cluster. M39 is framed against the star clouds and dust lanes of the Cygnus Milky Way.

Open clusters are a diverse group with membership ranging from as few as ten members to as many as a few thousand. The stars are often confined to a space of between 3 and 20 light years but some clusters can be as large as 60 or more light years. The ages can vary dramatically. Most clusters are young, being less than a few hundred million years old although some are truly ancient having stayed together for several billion years. Most open clusters disperse after several hundred million years. The attrition starts with the very high mass stars which go supernova, and is followed by the very low mass stars which are often ejected as a result of tidal interactions with adjacent clouds and turbulence within the cluster itself. Our sun belonged to an open cluster some 5 billion years ago.