Stellar Evolution

Abstract

Stellar masses range (at least) from some 0.07 M_⊙ to some 60 M_⊙, i.e. through almost three orders of magnitude, with luminosities ranging from 10⁻⁶L_⊙ to almost 10⁷L_⊙, through almost 13 orders of magnitude. Their radius and surface temperature tend to grow with their mass, and even more so their luminosity, as L ~ M ^3±2. All stars burn hydrogen (partially) to helium; the hotter ones further to C, N, O, or even all the way to iron, thereby liberating ≲ 10 MeV per nucleon = 1% of their rest energy. Burning hydrogen to helium is controlled by a weak nuclear reaction and hence proceeds at the slow, steady main-sequence rate, in contrast to more advanced burning which leads to rapid expansions, along the giant branch in the Hertzsprung-Russell diagram, and to oscillatory instabilities. Whenever radiative heat transport from the burning core to the cooling surface falls short of the needs, convectiveheat transport takes over, which can be radius dependent, and lead to chemical mixing. Deviations from spherical structure arise through the stars’ spin, through the presence of binary companions with mass transfer, and through the companions’ passing to a late stage of evolution, via a (super-) nova explosion.