Postmainsequence

Postmainsequence is a term used in stellar evolution to describe all evolutionary stages that occur after a star has exhausted hydrogen burning in its core and leaves the main sequence. The path a star follows after the main sequence depends primarily on its initial mass and chemical composition, and it is characterized by changes in fusion processes, luminosity, temperature, and often significant mass loss.

In low- and intermediate-mass stars (roughly up to about 8 solar masses), the postmainsequence evolution proceeds through several distinct phases. After the subgiant stage, hydrogen burning moves to a shell around an inert helium core, and the star ascends the red giant branch. When conditions permit, helium ignites in the core; for the lowest masses this ignition is degenerate and occurs as a helium flash, while somewhat more massive stars ignite helium non-degenerately. The star may settle onto the horizontal branch (or red clump) with core helium burning, then ascend the asymptotic giant branch, where helium and hydrogen shell burning drive alternating thermal pulses and strong mass loss. This leads to the ejection of the outer layers as a planetary nebula, leaving behind a cooling white dwarf.

More massive stars follow a different route. After leaving the main sequence they may become blue or red supergiants and proceed through successive stages of heavier-element burning. The postmainsequence evolution of these stars often ends in a core-collapse supernova, leaving neutron stars or black holes, with the outer layers contributing to the interstellar medium.

Postmainsequence phases are key for understanding stellar populations, nucleosynthesis, and the chemical evolution of galaxies.