Sternevolution

Sternevolution refers to the changes a star undergoes over its lifetime, from birth in a molecular cloud to its end state. It is governed largely by the star’s initial mass and chemical composition, with gravity, energy generation, and energy transport determining each stage.

During the main sequence, hydrogen is fusing in the core. The dominant fusion channel depends on mass: the proton-proton chain in lighter stars and the CNO cycle in more massive stars. The balance between gravity and pressure sets the star’s structure, while luminosity increases with mass and main-sequence lifetimes range from millions to about ten billion years (the Sun’s lifetime is about 10 billion).

Low- to intermediate-mass stars evolve into red giants after core hydrogen runs out; hydrogen burning continues in a surrounding shell and the outer layers expand enormously. Helium ignition in the core leads to helium burning in the horizontal branch or red clump, followed by asymptotic giant branch evolution with repeated shell burning and heavy mass loss, ejecting a planetary nebula and leaving a white dwarf.

Massive stars (roughly above 8–10 solar masses) develop successive burning shells (H, He, C, Ne, O, Si) until iron-group elements form in the core. They end in core-collapse supernovae, leaving neutron stars or black holes and dispersing heavy elements into the interstellar medium.

Special outcomes include Type Ia supernovae from accreting white dwarfs in binary systems, and, at the highest masses, pair-instability supernovae. Binary interactions and strong winds can alter evolutionary paths and produce novae or other exotic remnants.

Astronomical evidence for stellar evolution comes from color–magnitude diagrams of star clusters, asteroseismology, and measurements of chemical abundances in stars and gas. Models incorporate convection, rotation, mass loss, and metallicity to predict observable properties.