A stellar core is the central region of a star, where nuclear fusion takes place.
Fusion reactants were components of a star's core. (DS9: "Meridian")
Doctor Timicin's 2367 experiment to revitalize Beta Praxillus involved detonating modified photon torpedoes within the star's core. Though initially successful in achieving a stable core temperature, neutron migration eventually destabilized core density and led to the star going nova. (TNG: "Half a Life")