Studies of extremely young clusters. V. Stars in the vicinity of the Orion nebula.

Three-color photoelectric observations of stars in the vicinity of the Orion Nebula are presented. Most of the stars observed are brighter than V = 14, but a few fainter stars have been included to V = 18. The color-magnitude diagram for the Orion Nebula stars is very similar to that of NGC 2264, except that the point at which the fainter, gravitationally contracting stars depart from the zero-age main sequence is at (B - V)0 = -0.09, whereas in NGC 2264 it is at (B - V)0 = 0.00. According to the most recent theo- retical calculations, the gravitational contraction age corresponding to the observed turnoff point in Orion is 3 X 106 years and appears to be the same for stars in all parts of the Orion Nebula region. The present three-color photometric observations indicate that the ratio of total to selective absorption is normal in the vicinity of the Orion Nebula