| Summary: | Stars light up galaxies that are the major building blocks of the Universe. Throughout this thesis we cover the analysis of stars and stellar populations to gain knowledge on the formation of galaxies and the evolution of the Universe. Understanding massive earlytype galaxies is key to understand mass assembly and the formation and evolution of galaxies in the Universe. The spectra of stellar populations carry a wealth of information regarding galaxy formation and evolution. Absorption lines are particularly useful as they are tracers of galaxy formation epoch and time-scale. Models of stellar populations are important tools for the analysis of galaxies. The accuracy of such models are crucial for the accuracy of the derived results. The first step in my thesis is therefore to improve upon current single stellar population models of absorption line indices. Calibration of the models with galactic globular cluster is crucial, since these are known to be close to single stellar populations. The main aim of the thesis is to apply the models to the analysis of earlytype galaxies and Type Ia supernovae (SN Ia) host galaxies. The main novelty of the stellar population models are new empirical calibrations of absorption line indices. These are based on the most comprehensive stellar library available to date MILES. The stellar spectra of this library have been carefully flux-calibrated. The models are therefore applicable to data without Lick index calibrations. Based on the new stellar population models we have developed a method for deriving element abundance ratios, including [O/Fe], [C/Fe], [N/Fe], [Mg/Fe], [Ca/Fe] and [Ti/Fe]. The method is applied to galactic globular clusters and we find the models to be well calibrated. The pattern of derived element abundance ratios show strong evidence for self-enrichment within globular clusters. The method for deriving element abundance ratios is then applied to �4000 SDSS early-type galaxies. The element abundance ratios [O/Fe], [Mg/Fe], [C/Fe] and [N/Fe] show strong correlations with stellar velocity dispersion. Using the derived trends of element abundance ratios we constrain the lower time-scale limit of star formation and star-burst components in massive early-type galaxies to �0.4 Gyr. Both in the globular cluster and early-type galaxy study we find that the heavy �-elements Ca and Ti scale with Fe rather than with the lighter �-elements O and Mg. This implies that a significant contribution from SN Ia to the enrichment of heavy �-elements is universally found and puts strong constraints on supernova nucleosynthesis and models of galactic chemical evolution. SN Ia as standard candles connect luminosity distance to redshift space to constrain cosmology. We derive stellar population parameters for a quality selected sample of 84 SN Ia host galaxies. We find that the stretch factor of SN Ia light-curves are mainly dependent on stellar population age, indicating that SN Ia progenitor mass is the main driver of the peak luminosities. We do not find any significant dependencies on host galaxy properties for the scatter in the luminosity-distance relationship after light-curve corrections. This implies that the derived cosmological parameters from SN Ia peak luminosities are robust.
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