étoiles

Étoiles are self-luminous spheres of plasma held together by gravity that generate energy through nuclear fusion in their cores. They are mainly hydrogen and helium, with heavier elements in smaller amounts. The light they emit depends on surface temperature, producing colors from cool red to hot blue. The Sun is a typical étoile and serves as a reference point for stellar properties.

Stars form in giant molecular clouds when regions collapse under gravity, forming protostars that accumulate mass. When core temperatures reach the threshold for hydrogen fusion, the protostar becomes a main-sequence étoile. Lifetimes vary with mass: massive stars burn quickly, lasting millions of years, whereas low-mass stars can persist for hundreds of billions to trillions of years. After the main sequence, stars evolve into red giants or supergiants and end as white dwarfs, neutron stars, or black holes depending on initial mass.

Spectral classification groups étoiles by temperature into O-, B-, A-, F-, G-, K-, and M-type stars, with subdivisions, and luminosity classes. The Hertzsprung-Russell diagram plots luminosity against temperature to illustrate stellar evolution. Distances are measured by parallax; luminosities by absolute magnitude. Standard candles such as Cepheid variables and Type Ia supernovae help calibrate cosmic distances. Many stars are in binary or multiple systems, providing direct mass measurements that test theoretical models.

Étoiles are central to astronomy, accounting for most of the visible mass in galaxies and driving chemical enrichment through nucleosynthesis. They provide energy for planetary systems and influence galactic dynamics through winds and supernova explosions. Studying étoile populations across galaxies helps astronomers reconstruct star formation histories and the evolution of the universe.