The Orion Nebula: structure, dynamics, and population

Abstract

The physical nature and evolution of the Orion Nebula has begun to be revealed by calibrated emission line images and high resolution spectroscopy. We review the evidence that the nebula is a thin wall of emission on the near side of the Orion Molecular Cloud and that its separation from the dominant ionizing star is about 0.3 pc. The density of the nebula decreases rapidly away from the ionization front and the ionized gas is moving at 8 km s⁻¹ away from the front. A three dimensional model of the surface is presented and its peaks and valleys interpreted as due to irregularities in the density of the molecular cloud. The front is moving rapidly into the molecular cloud, so that objects previously shielded from ionization are continuously being revealed. Recent Hubble Space Telescope images are reviewed and they indicate that protoplanetary disks around pre-main sequence stars are both common and rendered visible by ionization and projection against the bright nebula. A large body of velocity data is discussed and it is seen that Kolmogorov type turbulence seems to only apply to material in the ionization front and the statistical fluctuations become less correlated away from the front.