Synthetic absorption lines in cool stars and normal galaxies.

The author first describes the basis of spectrum synthesis calculations where line lists with atomic and molecular constants are introduced. The accuracy of present-day available model atmospheres is also discussed. The application of synthetic spectra to reproduce the spectra of different objects is then described: the derivation of elemental and isotopic ratios is shown, including a discussion on results for the carbon, nitrogen and oxygen elements. The study of de-veiled spectra of T Tauri stars is described, where molecular bands can be used to check stellar parameters. A grid of 700 synthetic spectra together with reference stars, used through a perturbation method to study stellar populations is described. Finally the reproduction of strong features in composite spectra, and in particular, the Mg I triplet lines, the Ca II triplet lines, and TiO bands in metal-rich globular clusters and SO/E galaxies is given.