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What makes supernovas "explode"?

Hello AskSciencers!

My question is: What makes supernovas explode? I understand the general process after the star stops fusing hydrogen (it starts to fuse heavier elements, like helium). When the nuclear fusion starts producing Iron, which ruins the star's hydrostatic equilibrium and gravity becomes the major force at work. At this point is what I want to know: When the star collapses due to lack of the hydrostatic equlibrium, what makes it explode?
gpt
There are numerous different types of supernovae, though generally they fall into a few broad categories: core collapse and fusion runaway. For core collapse supernovae the stellar remnant left behind is either a neutron star or a black hole. Fusion runaway supernovae typically obliterate their stars, leaving behind only the supernova explosion remnant nebula of gas.

The classic fusion runaway supernova is the Type Ia, which involves a white dwarf star (the core of a dead giant star made up out of typically Carbon/Oxygen matter under electron degeneracy pressures) acquiring more matter (such as from a companion star). This causes a sequence of different things to happen, some of which take a long time (hundreds or thousands of years) but the end result is that the temperature in the core rises high enough to enable fusion of the remaining elements there. But because the core is under electron degeneracy conditions it cannot go through the typical negative feedback process inside a normal star's core which causes increasing pressure to result in expansion which then counteracts the temperature increase and maintains a somewhat steady state of fusion. So instead there's a positive feedback loop, fusion releases energy which increases temperature, which increases the rate of fusion, etc. In a matter of seconds enough fusion reactions have occurred to release enough energy to gravitationally unbind the star, and it explodes, leaving only gas and dust behind. At first the supernova is bright due to the temperature of the core fragments, later it is bright due to energy from the radioactive decay of Nickel-56 (recently created via fusion) which has a half-life of only about 6 days.

There are other ways that this fusion feedback loop can be broken. In supermassive stars the temperature can get so hot that the light from the "glow" of the matter in the core (which is at millions upon millions of degrees) actually reaches into the gamma-ray spectrum. At a high enough temperature some of the thermal photons have enough energy to create electron-positron pairs inside the core, which they do. The positron is quickly annihilated, but the time the thermal energy spent as electron and positron particles reduced the photon pressure inside the core just slightly. Which means that as the core heats up from additional fusion reactions more and more of the thermal glow spends time as electron-positron pairs, further fighting against the thermal stabilization feedback loop and at some point a tipping point is reached where fusion reaction rates feed on themselves and then in a matter of only seconds enough energy is released to destroy the star (even though in this case it's a much more massive star). This is called a pair-instability supernova.

In intermediate mass stars (> 8 solar masses or so) there can be a more standard "Type II" core collapse supernova. At the end of the star's life it begins fusing Silicon into Nickel-56 (though not in a runaway process), which takes around 5 days or so. After which the core of the star is Nickel and Iron (from the decay of some of the Nickel-56). At this point no more fusion reactions can occur, so the opposite part of the feedback loop is broken compared to the fusion runaway supernovae. Normally increasing pressure in the core would raise fusion rates, which would release energy, raise temperatures, and cause an expansion that keeps collapse in check. The matter then progresses toward electron degeneracy conditions (the highest pressure normal atomic matter can hold). At some point the mass of the electron degenerate core crosses a threshold (the Chandresekhar limit of about 1.4 solar masses) and then collapses. Electrons are forced into fusing with protons, forming neutrons and releasing high energy anti-electron neutrinos. The Nickel/Iron core of the star collapses into a much denser neutron star. The surface of the core collapses at a significant fraction of the speed of light. The remaining envelope of the star follows the collapse like a landslide, then it rams into the surface of the neutron star (which is held up by neutron degeneracy pressure, and basically immovable to the envelope of the star) and then bounces off. This alone is not enough to cause a supernova explosion, but at the same time a ghostly wave of neutrino radiation emanates from the newly formed neutron star, the product of all those electron+proton fusion reactions. Neutrinos interact with matter only extremely rarely, but there are on the order of 10^57 neutrinos collectively carrying 10^46 Joules of energy (over a trillion quadrillion teratons of explosive power) or roughly as much as a trillion years' worth of the Sun's radiative power (even though it will live only a fraction as long). Only about 1% of this energy is deposited in the envelope of the dying star as the neutron wind passes through but this is more than enough to raise its temperature high enough so that its components have escape velocity. The result being that the envelope explodes outwards, and then will continue to glow for days as the radioactive decay energy from the Nickel-56 takes over. The remnant left behind can be a pulsar, a neutron star with hyper accelerated rotation (due to shrinking in size) and extremely strong magnetic fields which beams powerful radio signals across the Universe. Under various conditions the supernova can stall or fall back on the neutron star, leaving behind a black hole instead.

In the very most massive stars, even more massive than those which undergo pair-instability supernovae, the thermal glow inside the core can reach such an extreme that photons actually have enough energy to blast apart atomic nuclei in a process called photodisintegration. This process absorbs energy from the photons and again breaks that heat/radiation/pressure feedback loop which keeps fusion reactions and collapse in check. The core simply collapses, forming a black hole surrounded by extremely energetic and ultradense matter which attempts to fall into the black hole but is subjected to conditions which cause the formation of an accretion disk and relativistic jets along the axis of rotation of the black hole/star. These are called hypernovae and are responsible for some types of gamma ray bursts that can still be quite bright even across significant fractions of the entire visible Universe.