Which element synthesis capability distinguishes Red Supergiants from Red Giants?

Fusing elements all the way up to iron in the core.

The core fusion capabilities are a direct measure of the initial mass of the star and dictate its fate. A red giant, stemming from a Sun-like star, is generally massive enough only to complete hydrogen fusion in the core, followed by helium fusion (the helium flash). Once helium is depleted, the star lacks the necessary gravitational compression to trigger carbon fusion or anything heavier in its core, effectively ending its active nuclear life. In contrast, a red supergiant experiences such extreme gravitational forces that it can proceed through multiple, successive fusion stages—hydrogen to helium, helium to carbon and oxygen, and onward—continuously synthesizing elements up to and including iron in shells surrounding the inert center. This process is crucial as it is responsible for forging many of the heavy elements distributed throughout the universe.