OBSERVATIONAL TESTS FOR THE EVOLUTION OF MASSIVE STARS IN NEARBY GALAXIES

Population synthesis calculations applicable to the massive stellar content in nearby galaxies are presented. We combine stellar evolution calculations with mass loss, model atmospheres with line blanketing, and a spectral type calibration to compute observable parameters of massive stars as a function of the star formation rate and the initial mass function (IMF) slope. The number of O stars of given spectral types, the number of W-R stars, supernova rates, and fluxes of ionizing photons are predicted. Important constraints for the theories of stellar atmospheres and stellar evolution can be derived from observations if stellar number counts and ionizing flux data are available. The influence of mass loss on stellar evolution calculations is discussed, and our current knowledge of the post-main-sequence evolution of massive stars in external galaxies is evaluated.