What is the primary difference in the mechanism driving energy output between a Main Sequence star and a Red Giant?

Answer

Main Sequence stars are core-driven fusion machines, whereas Red Giants are shell-driven expansion engines.

Main Sequence luminosity comes directly from stable hydrogen fusion in the core. Red Giant luminosity results from less efficient, voluminous burning in a shell surrounding the inert core, driving expansion.

What is the primary difference in the mechanism driving energy output between a Main Sequence star and a Red Giant?
astronomystarsstellar evolutionred giantsLuminositymain sequence stars