Morphology of Galactic open clusters

We analyzed the shapes of Galactic open clusters by the star-counting technique with the Two Micron All Sky Survey star catalog database. Morphological parameters such as the ellipticity and size have been derived via stellar density distribution, weighed by clustering probability. We find that most star clusters are elongated, even for the youngest star clusters of a few million years old, which are located near the Galactic disk. The shapes of young star clusters must reflect the conditions in the parental molecular clouds and during the cluster formation process. As an open cluster ages, stellar dynamics cause the inner part of the cluster to circularize, but the overall radius gets larger and the stellar density becomes sparser. We discuss how the internal relaxation process competes with Galactic external perturbation during cluster evolution.