Progress in Stellar Spectral Line Formation Theory

Spectral line formation theory is at the heart of astrophysical diagnostic. Our knowledge of abundances, in both stellar and interstellar contexts, comes almost enti- rely from line analysis, as does a major fraction of our ability to model stellar atmospheres. As new facets of the universe become observable so the techniques of high reso- lution spectroscopy are brought to bear, with great reward. Improved instruments, such as echelle spectrographs, employ- ing detectors of high quantum efficiency, have revolutioned our ability to observe high quality line profiles, although until now this ability has been confined to the brightest stars. Fabry-Perot interferometers and their modern deriva- tives are bringing new ranges of resolving power to studies of atomic and ionic interstellar lines, and of course radio techniques imply exceedingly high resolution for the cool interstellar medium of molecules and radicals. Telescopes in space are extending the spectral range of these types of observations. Already the Copernicus and IUE high resolution spectrographs have given us a tantalizing glimmer of what it will be like to obtain ultraviolet spectra with resolution and signal to noise ratio approaching those obtainable on the ground. Fairly soon Space Telescope will be producing high resolution spectroscopic data of unparal- leled quali ty and distance range. As often happens in astro- physics the challenge is now coming from the observers to the theorists to provide interpretational tools which are adequate to the state of the data.