Dissecting Galaxies with Quantitative Spectroscopy of the Brightest
  Stars in the Universe
  Measuring distances to galaxies, determining their chemical composition,
investigating the nature of their stellar populations and the absorbing
properties of their interstellar medium are fundamental activities in modern
extragalactic astronomy helping to understand the evolution of galaxies and the
expanding universe. The optically brightest stars in the universe, blue
supergiants of spectral A and B, are unique tools for these purposes. With
absolute visual magnitudes up to M_V = -9.5 they are the ideal to obtain
accurate quantitative information about galaxies through the powerful modern
methods of quantitative stellar spectroscopy. The spectral analyis of
individual blue supergiant targets provides invaluable information about
chemical abundances and abundance gradients, which is more comprehensive than
the one obtained from HII regions, as it includes additional atomic species,
and which is also more accurate, since it avoids the systematic uncertainties
inherent in the strong line studies usually applied to the HII regions of
spiral galaxies beyond the Local Group. Simultaneously, the spectral analysis
yields stellar parameters and interstellar extinction for each individual
supergiant target, which provides an alternative very accurate way to determine
extragalactic distances through a newly developed method, called the
Flux-weighted Gravity - Luminosity Relationship (FGLR). With the present
generation of 10m-class telescopes these spectroscopic studies can reach out to
distances of 10 Mpc. The new generation of 30m-class will allow to extend this
work out to 30 Mpc, a substantial volume of the local universe.
