Stars more massive than 8 times the Sun's mass have core temperatures high enough to sustain nuclear fusion beyond the triple alpha process:

Additional ⁴He fusions can occur to make even heavier atoms.

However, fusion ceases to be an energy source for atoms heavier than iron, ⁵⁶Fe. Fusion of heavier atoms requires more energy than is released in the reaction.

When a massive star builds up an iron core (as a RED SUPERGIANT), it finally runs out of all available nuclear fuel to sustain the interior pressure.

Gravity causes a sudden and irrevocable inward collapse of the stellar core in which all the heavy atoms are broken apart until the remaining protons and electrons merge into neutrons.

The collapsing outer layers "bounce" off the incredibly dense neutron core to produce a Type II supernova explosion.