Evolution of the Stars and Gas in Galaxies

A numerical computation of evolution starts from gas with Population I composition; then stars are formed at all times, at rates which are functions of stellar mass and mass of gas in the galaxy. Discrete time steps of 10° years are used, and 13 stellar masses. The stars are placed on the H-R diagram ac- cording to their masses and ages; each star ends as a white dwarf, while its excess mass enriches the interstellar gas. Different evolutionary sequences are constructed by adjusting four parameters of a stellar birth-rate function. Then "galaxies" resulting from each sequence of 10-12 X 1O~ years are com- pared with observed local galaxies with respect to colors, mass-to-light ratio, relative mass of gas, and types of stars contributing to the light. "Galaxies" closely resembling all normal types, Im to E, can be formed with a stellar birth rate pro- portional to the inverse square of stellar mass and to the mass of gas in the galaxy; the types differ in initial rate of gas consumption and in the birth rate of very low-mass stars. These types can all have the same age, and do not form an evolutionary sequence. It is shown that giant elliptical galaxies may have been so much brighter at short wavelengths a few billion years ago that the observed magnitude-redshift relation can be interpreted in terms of cosmological models that do not suffer from the high density and small age of the conventionally preferred model