Solar element abundances, non-LTE line formation in cool stars and atomic data

The composition of solar and meteoritic matter serves as a primary reference for cosmochemical studies. Element abundances in the Sun - and in cool stars in general - are usually determined by spectroscopy of a stellar atmosphere whose properties differ from most laboratory sources: (i) there are strong gradients of temperature and pressure, (ii) the main constituents are neutral hydrogen and helium, while the fractional abundance of electrons is only approximate to 10(-4), (iii) the plasma is immersed in an intense, anisotropic radiation field, and is highly turbulent. The present contribution discusses meteoritic and solar abundances and their suitability as cosmic reference data. Also addressed are mayor diagnostic problems encountered in the analysis of the photospheric line spectrum of cool stars - in particular line broadening and deviations from LTE in neutral iron. This paper is an update of a review [1] given at the 1987 Meudon symposium on high-S/N stellar spectroscopy.