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This thesis is a compendium of three published articles (plus an additional unpublished work) that represents the fruits of research focused on the massive Cygnus OB2 star-forming region. It is one of the nearest, most massive and youngest Galactic OB association and it provides an excellent target to study massive star properties, formation and

Context. Very metal-poor halo stars are the best candidates for being among the oldest objects in our Galaxy. Samples of halo stars with age determination and detailed chemical composition measurements provide key information for constraining the nature of the first stellar generations and the nucleosynthesis in the metal-poor regime. Aims: Age

This paper presents stellar mass functions and i-band luminosity functions for Sloan Digital Sky Survey (SDSS) galaxies with i 21 using clustering redshifts. From these measurements, we also compute targeting completeness measurements for the Baryon Oscillation Spectroscopic Survey (BOSS). Clustering redshifts is a method of obtaining the redshift

Part of the input data required for all NLTE codes include information of the atomic species of interest via the so called model atoms. These are often not public, making difficult the comparison between different codes. Here I present the IDL/GDL code maKe Atoms Simple ( KAS), a code that collect, combine and reorganize atomic data in order to