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R.-P. Kudritzki (Inst. for Astronomy, U. of Hawaii)
The spectral analysis of the most luminous stellar objects as individuals provides a unique tool to investigate the properties of young populations in galaxies. In this way, kinematics, abundances, abundance gradients and distances of galaxies can be determined. Two groups of objects, blue supergiant stars and Planetary Nebulae are discussed.
It is shown how the parameters of the most luminous blue supergiants (temperature, gravity, radii, luminosities, masses, mass-loss rates, wind velocities, abundances) in different galaxies are determined using spectral information from the UV to the IR and hydrodynamic NLTE model atmospheres including the effects of stellar winds and spherical extension. Recent results for O, B, A-supergiants in galaxies of the Local Group and in spirals out to 7 Mpc distance are presented. Special emphasis is given to the Wind Momentum -- Luminosity Relationship as a tool for the determination of extragalactic distances out to the Virgo and Fornax clusters of galaxies.
The new hydrodynamic NLTE model atmospheres can also be used to predict the ionizing properties, line spectra and winds of massive stars in the early universe observable at high redshift. We present new results for massive stars at extremely low and zero metallicity and applications of these calculations for the population synthesis interpretation of extreme Lyman-alpha emitting galaxies at high redshift.