Stars are classified as low, intermediate, or high mass according to the nuclear reactions they undergo. Low-mass stars are defined as those that develop electron-degenerate He cores on the red giant branch (RGB). If the He core grows to ∽0.45 M⊙, the star will undergo a core He flash until degeneracy is removed and quiescent He burning begins. Intermediate-mass stars can initiate core He burning under nondegenerate conditions and develop an electron-degenerate carbon-oxygen (C-O) core after core He exhaustion. At the completion of core He burning, low-mass stars also develop an electron-degenerate C-O core, and their subsequent evolution is similar to that of intermediate-mass stars. This is the beginning of the asymptotic giant branch. Stars that are massive enough can undergo He-shell flashes (also called thermal pulses) on the AGB. The AGB is terminated by either (a) complete removal of the hydrogen envelope by mass loss; or (b) ignition of carbon in the degenerate core. Massive stars are defined as those that develop a nondegenerate C-O core and therefore can ignite carbon nonviolently. They are able to go through a series of nuclear burnings (C, O, Ne, etc.), leading to the construction of the iron core followed by core collapse and supernova explosion.

The end products of evolution for low-, intermediate-, and high-mass stars are very different.