How are shockwaves in a star that cause supernova created

Question

I know that during the collapsing of a star 8 times the mass of the Sun at the end of its life, a shockwave is generated that blows the star up. However, I do not understand how it is exactly generated. I would appreciate an answer.

Answer 1

You can use a rubber ball analogy.

At the end of a massive star's life the core consists mostly of inert nuclei of iron and nickel. The core is supported by electron degeneracy pressure - a quantum mechanical effect that prevents the electrons from having low kinetic energies. Their high kinetic energies are what provides the pressure.

If the core grows too massive and hot then an instability takes place - either it becomes energetically feasible for the electrons to combine with protons in the nuclei to produce neutrons, or the nuclei get photo-disintegrated and free protons combine with the electrons. Either way, the prop that has been holding up the core is removed and it collapses dramatically and suddenly - almost in free-fall.

A new equilibrium may be possible when the densities get so high that the free nucleons are "touching" - separated by around $10^{-15}$m. At this point there is a strong repulsive nuclear force that comes into play. This may be capable of stabilising the collapse and ultimately forming a neutron star, but not before the collapse has proceeded well past the equilibrium point. As such, there will be recoil, like a squashed rubber ball, and it is this recoil that powers the shock wave - this has become known as a "core bounce".

The mechanism by which the shock transfers energy to the envelope is still the source of debate. However it is strongly inferred that neutrinos are involved. These are produced in enormous quantities through the combination of protons and electrons during the collapse and end up possessing about 99% of the gravitational potential energy released by the collapse. But the matter is so dense that they cannot immediately escape. They are trapped for several seconds and this provides an enormous pressure that can propel material outwards.

Answer 2

After a massive star leaves the main sequence and becomes a supergiant, it begins fusing heavier elements, helium to carbon, carbon to oxygen and neon, neon to magnesium, etc. all the way up to iron. When the star starts fusing iron, no energy is generated. Instead, energy is taken away because more energy is needed to fuse iron than the energy released through the reaction. This loss of energy in the core causes it to collapse, as its mass is more than $1.4 M_\odot$, the Chandrasekhar limit. The core then starts to collapse in on itself.

Now, two things happen at once.

1. As neutron-degenerate matter forms inside the core, the remaining shell is blasted away to form a shock front.
2. As fusion stops in the core, the outer layers of the star collapse inward, heating up the core even more. This causes a runaway fusion reaction.

The runaway fusion reaction creates neutrinos, fueling the shockwave. The shockwave quickly propagates through the star, blowing it apart in a Type II supernova. The remnant is a neutron star, or a black hole (if the star is over $40M_\odot$).