The Trapezium Cluster of the Orion Nebula

A color-magnitude diagram (V, IC) for 68 stars in the Trapezium cluster has been determined from narrow-band CCD photometry, and has been combined with spectroscopic information on the brighter cluster members. Most of the stars are no older than 1 x 10 to the 6th yr, with little indication of a significant population of much older stars. The star density in the immediate vicinity of the Trapezium is higher than in any other known region of star formation, 2200 stars per cu pc brighter than M(IC) = +6.0, corresponding to at least 1800 solar masses per cu pc, or at least 3000 solar masses per cu pc if the components of Theta Ori are included. This latter value corresponds to 0.6 x 10 to the 5th H2 molecules per cu cm, showing that the cluster must have formed from a very dense subregion of the molecular cloud. Reasonable assumptions as to the depth and opacity of the cloud, combined with the 2.2 micron luminosity function of the cluster, indicate that these high star densities do not permeate the entire cloud, but rather are confined to the region of the cavity in the front surface formed by the Trapezium OB stars.