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Fusion of nuclei inside stars switches from exothermic to endothermic for Iron and elements heavier than Iron. Supernovas not only achieve fusion of transuranides (albeit in trace relative quantities), but they also release a huge excess of energy that is comparable to the fusion energy generated during its whole lifetime

How well do we understand the mechanism that causes this huge mass-energy conversion?

Also, considering that fission releases 0.1% at most of rest mass, and fusion releases 1% at most, what is the fraction of mass-energy that is converted by the underlying mechanism in supernovas?

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The energy released in (mainly) neutrinos, along with light and the kinetic energy of the exploding envelope is around $10^{46}$ J. This is equivalent to 0.05 solar masses being converted into energy. This is less than a percent of the progenitor mass.

The energy source is gravitational potential energy. At the heart of a supernova, there was an Earth-sized ball of iron, of about 1.2 solar masses, that collapses to a radius of 10 km. This releases a few $10^{46}$ J, which is sufficient to power the supernova, dissociate the iron into its nucleons, endothermically make neutrons out of the protons, and have a tiny bit left over to make the traces of r-process elements you mention.

Most of the energy "lost" in a supernova is transferred to neutrinos during (well understood) beta decays and inverse beta decays taking place as the core collapses and then in the first few seconds after an extremely hot proto-neutron star is formed.

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  • $\begingroup$ the wiki article: (en.wikipedia.org/wiki/Supernova#Core_collapse) says that up to 10% of the rest-mass is converted during core collapse, but still 5% is 5 times more than nuclear fusion. Although if only 1% of that 5% energy in form of neutrinos interacts with matter and electromagnetism, so only $5 \times 10^{-4}$ of the star's mass is used to build heavy isotopes and power the explosion, it seems quite a low number $\endgroup$
    – diffeomorphism
    Commented Apr 3, 2016 at 2:00
  • $\begingroup$ where do I get 1% of neutrino absorption? from that same article: "A process that is not clearly understood is necessary to allow the outer layers of the core to reabsorb around $10^{44}$ joules[78] (1 foe) from the neutrino pulse, producing the visible explosion, although there are also other theories on how to power the explosion.[75]" $\endgroup$
    – diffeomorphism
    Commented Apr 3, 2016 at 2:02
  • $\begingroup$ @diffeomorphism Believe wikipedia if you wish. A supernova progenitor has a minimum 10 solar masses. So 10% of this is $2\times 10^{47}$J. This is one of a number of errors in the article. As to what causes the explosion, that is an entirely different (and unsolved) question to where does the energy come from. Indeed it is hard to get even 1% of the neutrino energy transferred to kinetic energy of the envelope. But nature finds a way apparently. $\endgroup$
    – ProfRob
    Commented Apr 3, 2016 at 7:17
  • $\begingroup$ I see. My question was related to the underlying mechanism, I know that gravitational collapse is what provides the energy/density/pressure/temperature for it to happen. Do we know what pressures/temperatures are required to trigger this unknown mechanism? $\endgroup$
    – diffeomorphism
    Commented Apr 3, 2016 at 14:54
  • $\begingroup$ @diffeomorphism Your question was clear enough. Why supernovae explode is covered by physics.stackexchange.com/questions/63558/… though you won't find an answer. $\endgroup$
    – ProfRob
    Commented Apr 3, 2016 at 15:37

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