Examining the M67 Classification as an Open Cluster

The cluster M67 ( = NGC 2682) in Cancer is a rich stellar cluster, usually classified as an open cluster. Using our own observations with the 0.4 m telescope, we show that M67 is a tight group of about 1200 stars. The actual radius of the cluster is about 3.1 pc and the average mass of a star in the system is about 1.33 M . We also show that the ratio of the mean kinetic energy of the cluster to its mean gravitational potential energy clust clust 0.3K U , while the value pre-dicted by the virial theorem is equal to. So the system is a gravitationally bound. This value of 0.5 clust clustK U is considered as an evidence of quasi-stability of the cluster and allows us to use the Chandrasekhar-Spitzer relaxation time for M67 chand 30 Myr as a characteristic dynamical relaxation time of the system under study. As the cluster is almost twice older its half-life time hal chandt f 70 0, it is argued that M67 was in the past (about 4 Gyr ago, close to its formation) a relatively small ( stars) globular cluster, but got “open cluster ” shape due to the dynamical evaporation of the majority of its stars