Constraining stellar physics with the NLTE radiative transfer

All chemical elements in the Universe, except the very few lightest species, are produced in a nuclear fusion inside the stars. Following the stellar life cycle, these chemical elements are expelled into the interstellar medium where they proceed to contribute to the chemical enrich- ment of their surroundings. Spectroscopic observations are currently the only way to infer the chemical make-up of the stars. Combining those with the physical modelling of the radiation in the stellar plasma allows us to detect and measure the number of chemical elements we all are made of. The approach is generally applied to individual stars constituting the larger scale populations, from clusters to galaxies including our Milky Way and beyond. In this thesis I focus on the non-equilibrium modelling of stellar radiation and its influence on the measured chemical abundances of various elements. I provide a general overview of the methods and necessary information used to infer the stellar chemical composition. I then present developments in the non-equilibrium modelling and apply it to the analysis of a star cluster, our host star – the Sun, and eventually a broader Galactic population. I focus on the opportunities our modelling approach presents to observationally constrain the stellar evolution and consequential enrichment of the Galactic populations.