The first stars, known as Population III stars, played a crucial role in the early evolution of the Universe by driving cosmic reionization and enriching the intergalactic medium with heavy elements such as carbon, nitrogen, oxygen, and fluorine. These elements are essential for the formation of subsequent generations of stars, planets, and galaxies. Understanding how Population III stars synthesized these elements is key to tracing the origins of chemical diversity in the Universe.

This thesis investigates the nucleosynthesis of light elements, particularly CNO elements and fluorine, in rapidly rotating massive Population III stars. By utilizing advanced stellar evolution models generated by the Geneva stellar evolution code (GENEC), we explore how rotational effects influence stellar yields and their contribution to the chemical enrichment of the Universe.

Rotation induces mixing between H- and He-burning zones in low-metallicity stars, opening pathways for the primary production of elements such as ¹³C, ¹⁴N, ¹⁹F, and ²²Ne. Our investigation reveals that increased initial rotation enhances this mixing, but it remains efficient only below a metallicity of about 0.0004 for stars with a rotational velocity on the zero-age main sequence equal to or larger than 40% of the critical one.

These models have been used to predict the evolution of nitrogen abundances in the Galactic halo, as well as the evolution of fluorine during the early phases of galactic evolution. Our results confirm the critical role of rotational mixing in explaining observed nitrogen and fluorine abundances at low metallicity. Note that the present computations use different diffusion coefficients than previous models computed for rotating Population III stars with GENEC.

Furthermore, we explore the impact of rotational mixing on the high N/O ratios observed in certain high-redshift galaxies. Our models suggest that rapidly rotating massive stars in starburst regions could increase the N/O ratio to values around -0.22, consistent with some observations in distant galaxies.

Lastly, this thesis revisits the origin of fluorine in massive stars. We evaluate whether fluorine abundances observed at the surface of red supergiants can be reproduced by our models. Additionally, we propose measuring the surface fluorine abundance in early WC stars (a type of Wolf-Rayet stars showing at the surface products of core He-burning) to verify whether the nuclear processes responsible for fluorine production during core He-burning occur in real stars. This study also highlights the significant role of rotational effects in enhancing fluorine synthesis in rapidly rotating, metal-poor stars, and examines the role of supernovae in dispersing fluorine, providing a comprehensive view of its production throughout cosmic history.