Massive stars - Evolution
Massive stars display powerful stellar winds that combine large terminal velocities and important mass-loss rates, a fact that strongly affects their fate. Even during their short life-time, they lose a sizeable fraction of their initial mass. The Wolf-Rayet stage is indeed thought to result from the progressive evaporation of the star atmosphere.
Because of their extreme properties, early-type stars largely influence their surroundings. They are the first source of ionizing photons in the Universe and, as such, are thought to be at the origin of the large-scale HII regions. Through their winds and their dramatic end as supernovae, their influence is also mechanical. For example, their winds are so powerful that they can shape the ambient medium, resulting in spectacular wind-blown bubbles. The massive stars are also the main cause of chemical enrichment of the interstellar medium, especially in heavy elements that are mostly produced during the last stages of their evolution and during the supernova phase. Beyond their influence on their host galaxy, high-mass stars are further important for a number of extragalactic and cosmological issues. For example, they have been proposed to serve as cosmic candles thanks to the Wind-Momentum-Luminosity relation and the first generation of massive stars might have been responsible for the reionisation of the early Universe.
However, despite the progresses made over the last decades, many crucial issues are still lacking satisfactory answers. These unsolved questions range from the determination of the formation mechanism itself, to the upper-mass limit a massive star can have, and to their suspected link with the gamma-ray burst phenomenon. The exact distribution of their physical and wind parameters, the existence and role of magnetic fields, the detailed understanding of their final stage as supernovae and the subsequent formation of neutron stars or black holes are further topics that still challenge our understanding. Even a detailed comprehension of the winds of the O and WR stars is still missing.
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