The Universality of the Initial Luminosity Function.

The universality of an initial luminosity function based upon the galactic clusters and the solar vicinity is examined with regard to its implications for the mass-to-light ratios for stellar systems. For this purpose the times that stars spend on and near the main sequence are derived as a function of stellar mass from a discussion of the available data for individual stellar models. These results are then used to calculate upper limits to the mass-to-light ratios for stellar systems as functions of the ages of the systems-on the assumption that an initial luminosity function of the usual form has been operative. The observational data relating to the mass- to-light ratios for galaxies and globular clusters are then analyzed, and the corresponding values for their mass-to-light ratios are obtained. These observational values are compared with the calculated upper limits, and it is found that ages greatly in excess of 12 billion years would be required for many galaxies in order for the data to be consistent with the adopted form of the initial luminosity function. The evidence appears to strongly support the view that the initial luminosity function has at times differed in a very significant way from its assumed form. It follows that the form of the initial luminosity function is sensitive to at least certain of the physical parameters that describe the pre-stellar medium-parameters such as density, temperature, turbulent and magnetic states, and chemical composition.