Stars in the upper main sequence have too much mass to die as white dwarfs.
Just like other stars on the main sequence, they consume hydrogen in their cores and ignite hydrogen shells and as a result become giants or for more massive stars, supergiants.
Their cores contract and fuse helium, first in the core and then in a shell producing a carbon/oxygen core.
Unlike smaller massed stars, upper main sequence stars have enough mass to ignite carbon fusion, which produces a neon and magnesium core, and then ignite the carbon shell.
This pattern of burning fuels in the core and then in the shell continues until silicon fuses to make iron.
Iron is the most tightly bound of all atomic nuclei, there are no further reactions that can combine with and release energy from these iron nuclei.
Eventually the core is converted entirely into iron which is surrounded by layers of shell fusion that consume the stars remaining fuel reserves.
Because no energy is being emitted in the core, the electrons in the core must support the weight of the outer layers by the strength of electron degeneracy alone.
Continued deposits of iron from shell fusion cause the core to exceed the Chandrasakar limit of 1.4 Solar Masses.
The star becomes a supernova.
Electron degeneracy in the core fails to support the stars weight and the core collapses.|
The collapsing core generates staggeringly enormous heat which breaks apart the iron nuclei back into elemental protons, neutrons and electrons.
As the density of the core climbs the electrons combine with protons to form more neutrons and releases a flood of neutrinos.
When the neutron rich material of the core reaches nuclear density it stiffens and resists the collapse of the core so abruptly that it rebounds.
The outward flowing neutrinos and rebounding core slam into the inward flowing shell-fusing matter.
This impact stops the core’s rebound and causes the infalling matter to reverse course.
This matter accelerates rapidly as it encounters less resistance and soon forms an outgoing shock wave.
After a few hours this shockwave reaches the stars surface and blasts away the outer layers in a spectacular explosion.