M67 (NGC 2682)
Distance: 2600 Light Years

Right Ascension: 08 : 50.4 (hours : minutes)
Declination: +11 : 49 (degrees : minutes)

Unlike most open clusters M67 is truly ancient. Its roughly 500 members are estimated at 4 billion years old, almost as old as our solar system. Few open clusters are older than M67 with the exception of B-17, NGC 188, and NGC 6791. It is unclear how some rare clusters manage to survive for so long while most become dispersed after several hundred million years. Perhaps clusters with extreme longevity were initially formed in favorable orbits or by unusual events which have kept them isolated from the typical interactions which often shred galactic clusters. Observations of large numbers of open clusters have found two basic populations, a dominant population of clusters with a mean age of about 200 million years and a much older subset with an age of about 4 billion years. The older populations appear to be located in the outer disk of the galaxy and are more often further from the disk plane than the younger clusters. These unusual orbits may favor longer survival for these clusters. M67 is perched some 1400 light years above the galactic plane which likely protects the cluster from collisions with molecular clouds which tend to be sparser at higher galactic latitudes.


M67 is also known for its preponderance of variable stars especially W Uma stars. W Ursae Majoris type systems are eclipsing binaries with periods of 5 to 24 hours. W Uma stars are known as contact binaries where the two stars actually contact their photospheres resulting in mass transfer from the more massive component to the less massive one. They can eventually merge to form a single star. Roughly one in every thousand stars is a contact binary and they tend to be older stars which may explain the preponderance in M67.