The nature of molecular clouds

Abstract

After a summary of the known phases of the ISM, a description is given of the hierarchy of molecular structures from the lowest to the highest densities (largest to smallest size scales). Despite the self-gravitating nature of nearly all molecular clouds, they do not collapse rapidly. Mechanisms of support (internal pressure), and the reasons for equipartition of kinematic, magnetic and gravitational energy densities are discussed. Then we procede to descriptions of (i) scaling laws for density, size, and velocity dispersion, and their physical reasons; (ii) ages of the various molecular entities; (iii) low-mass and massive star formation. We conclude with a description of the origin of molecular clouds, from the thermal instability that converts warm HI gas to cold HI gas, through the conversion of cold HI gas to diffuse molecular gas, and finally the transformation of the latter to molecular clouds of all sizes, via random collision buildup, the magnetic Rayleigh-Taylor instability, and gravitational instabilities on all scales.