As the center of the massive star collapses down, creating more complex atoms through continued fusion of nuclei, the outer atmosphere becomes ENORMOUS, expanding to a radius the size of Jupiter's orbit. This is a RED SUPERGIANT.

At the center, the core develops an "Onion Skin" composition. Each progressive layer of the onion reflects the subsequent series of burning elements. Hence in this diagram, working out from in, we see the order in which the elements were burned:

Hydrogen --> Helium --> Carbon --> Oxygen --> Neon --> Silicon --> Iron

However: once the center is made up of Iron, there is NOTHING left to fuse.. Fusion reactions of elements more massive than Iron do not LIBERATE energy, they instead REQUIRE energy. The star has lead itself into a very precarious situation!