Like all stars, high mass stars start out on the Main Sequence, fusing H -> He in the core. As the core becomes full of He and the central star collapses, a shell H fusion to He begins and the star leaves the main sequence. High mass stars eventually evolve into the most luminous supergiant stars. First as Red Supergiants, then later moving back towards being Blue Supergiants. Deep inside a massive supergiant star, advancing stages of more complex fusion is progressing. A high mass star can reach the needed high temperature and densities required to fuse all the way creating IRON at its core.

Burning Stage Surface T (K) Radius Seconds (Yrs) Hydrogen 40,000 6 Rsun 2.3 x 1014 (7 Mill) Helium 16,000 70 Rsun 2.1 x 1013 (500,000) Carbon 4,300 100 Rsun 5.2 x 109 (100) Neon 4,300 100 Rsun 3.9 x 107 (1 yr) Oxygen 4,300 100 Rsun 1.6 x 107 (6 mos) Silicon 4,300 100 Rsun 1.2 x 105 (1 day) Iron 4,300 100 Rsun 0.35 seconds note: 3 x 107 second = 1 year

Once the star's central core is made up entirely of Iron, there is no more energy to be gained from fusion. This is because Iron is the most densly packed atom in the universe. With absolutely NO SOURCE OF FUEL LEFT, the central core and the rest of the star goes into a pure free fall as gravity finally takes over.