Beyond binary How triple stars shape stellar evolution

Massive stars, born with at least 8–10 times the mass of the Sun, are rarely found alone. They commonly reside in multiple systems, and in triples, where a close binary is orbited by a distant third star, interplay between the components creates evolutionary pathways more complex than in isolated or binary stars. Interactions, such as tidal forces, mass transfer, and dynamical perturbations, can reshape these systems and the remnants they produce. Understanding triple evolution is therefore crucial for interpreting observations of massive stars. Yet most past studies have focused on binaries, neglecting the impact of a potential third companion. This thesis aims to improve our understanding of the evolution of massive stellar triples and to identify the statistical trends and observable signatures that arise from their interactions. Using fast analytical models, we study large populations of triples and find that interactions are integral to their lifetimes: mass transfer within the inner binary frequently governs their evolution, while supernova explosions often detach the third star from the system. We then study a rare interaction unique to triples: mass transfer from the outer star onto the inner binary. Developing an analytical model, we show that this process can shrink black-hole binaries and trigger gravitational-wave mergers, sometimes leaving signatures such as residual eccentricity or electromagnetic emission. Next, we study triples in which the inner stars merge after mass transfer. These mergers create rejuvenated stars whose apparent youth can betray their complex histories, offering a way to identify them observationally. Finally, we discuss directions for future research.