Hot Stars: Old-Fashioned or Trendy?

Spectroscopic analyses with the intention of the interpretation of the UV-spectra of the brightest stars as individuals ‐ supernovae ‐ or as components of starforming regions ‐ massive O stars ‐ provide a powerful tool with great astrophysical potential for the determination of extragalactic distances and of the chemical composition of star-forming galaxies even at high redshifts. The perspectives of already initiated work with the new generation of tools for quantitative UV-spectroscopy of Hot Stars that have been developed during the last two decades are presented and the status of the continuing effort to construct corresponding models for Hot Star atmospheres is reviewed. Since the physics of the atmospheres of Hot Stars are strongly affected by velocity expansion dominating the spectra at all wavelength ranges, hydrodynamic model atmospheres for O-type stars and explosion models for Supernovae of Type Ia are necessary as basis for the synthesis and analysis of the spectra. It is shown that stellar parameters, abundances, and stellar wind properties can be determined by the methods of spectral diagnostics already developed. Additionally, it will be demonstrated that models and synthetic spectra of Type Ia Supernovae of required quality are already available. These will make it possible to tackle the question of whether Supernovae Ia are standard candles in a cosmological sense, confirming or disproving that the current SN-luminosity distances indicate accelerated expansion of the universe. In detail we discuss applications of the diagnostic techniques by example of two of the most luminous O supergiants in the Galaxy and a standard Supernova of Type Ia. Furthermore, it is demonstrated that the spectral energy distributions provided by state-of-the-art models of massive O stars lead to considerably better agreement with observations if used for the analysis of H II regions. Thus, an excellent way of determining extragalactic abundances and population histories is offered. Moreover, the importance of Hot Stars in a broad astrophysical context will be discussed. As they dominate the physical conditions of their local environments and the life cycle of gas and dust of their host galaxies, special emphasis will be given to the corresponding diagnostic perspectives. Beyond that, the relevance of Hot Stars to cosmological issues will be considered.