Evolved descendants of Main Sequence O-types stars, B- and A-type supergiant stars are among the most luminous "regular" objects in galaxies with recent star formation. Quantitative spectroscopy by means of efficient multi-object spectrographs attached to 10m-class telescopes and modern non-LTE model atmosphere techniques enable us to gather an unprecedented wealth of information on distant galaxies.
In this talk, I will describe the spectroscopic diagnostics of blue supergiant stars, presenting recent results for several nearby galaxies. In this context, I will discuss the recently introduced Flux-weighted gravity -- Luminosity relationship (FGLR) as a reliable spectroscopic method to determine distances to galaxies up to 10 Mpc.
Finally, I will briefly discuss the exciting science that would be enabled by the up-coming generation of highly efficient wide-field multi-object optical spectrographs, such as WFMOS at Subaru/Gemini or SIDE at GTC.