The project STAREX has investigated the properties of stars showing extreme properties. We mean here stars showing the lowest initial metal content (down to zero metallicity), and/or the highest initial masses (up to one million solar masses), and or the highest initial rotations (up to rotations rates driving chemically homogeneous evolution), and/or by their initial content in the dark matter under the form of Weakly Interactive Massive Particles, and/or showing very high accretion rates during their formation process (up to 1 million solar masses per year). These properties are very relevant for describing the evolution of the first stellar generations in the Universe and, their radiative, chemical, and mechanical feedback in galaxies. Due to their short lifetimes (a few million years), the first massive, very massive, and supermassive stars have now disappeared from the present-day universe. Indirect observational constraints are however available as the observations of long-lived extremely iron-poor low-mass stars whose surface composition shows signs of the composition of the ejecta of these first stars, the constraints on reionization in the high redshift universe, the apparition of supermassive black holes (up to 1 billion solar masses) while the Universe was still very young (less than 1 billion years old), the observation of regions of high redshift galaxies with very high nitrogen-to-oxygen ratios indicating a rapid enrichment in a material having been processed at least by CNO burning. The project STAREX has addressed all these questions exploring the impact of normal and extreme stars. We have also improved the physics of our stellar models, exploring the consequences of new mass loss rate recipes, new nuclear reaction rates, new ways of implementing convection in stellar models based on 3D hydrodynamical modeling, new approaches for implementing the transport of the chemical species and of the angular momentum in massive stars based on the most recent constraints coming from asteroseismology. STAREX has produced new predictions for the impact of extreme stars on reionization, on the chemical evolution of galaxies, and on the formation of supermassive black holes at high-redshift. It has allowed 4 students to get their PhD. The STAREX project has produced more than 130 publications during the five years of its duration.
