Stellar Motions in the Orion Nebula Cluster.

This paper investigates the motions of the stars near the center of the Orion Nebula cluster. The observational material consists of plates taken with the Yerkes 40-inch refractor with an interval of fifty years between first- and second-epoch plates. A comparison of the plate constants for first- and second-epoch plates reveals that the reference frame is expanding, leading to an expansion age of the cluster equal to 3 X l0 years. Assuming the same velocity distribution along any axis within the cluster, the proper motions and the radial velocities lead to a distance of 520 parsecs for the cluster. The colormagnitude diagram of the cluster members shows the existence of a broad continuous sequence from O6 to K2 with stars as early as A0 departed from the standard main sequence, indicating that stars of later spectral types may still be in the process of gravitational contraction.