Why is so much energy released in a supernova while so little in a star?

Question

There is a huge amount of energy released in a supernova. All fuel is used almost instantaneously. Why doesn't this happen in stars (like the sun)?

I am asking "Why does a large star spend millions of years slowly doing various fusion reactions? Why doesn't it explode in a supernova as soon as it's formed?"

If the hydrogen fuel (needed for fusion) is burnt up and the energy release will no longer be sufficient to withstand gravitational collapse the star will implode. When the volume is small enough (or the temperature or pressure high enough) new fusion reactions will start. Can't a new balance between energy production and energy radiation (into the void) form, like in the star before? Or will the vulume be too small? Has a critical mass density been passed, like in an atomic bomb (the fission one, though I'm not sure if there is a critical mass for the fusion one). Is the volume-surface ratio of importance?

Answer 1

There are two (basic) types of supernovae.

The type Ia thermonuclear supernova is caused by the rapid and complete nuclear fusion of (mainly) carbon and oxygen in a white dwarf.

A type II "core collapse" supernova occurs as the endpoint of stellar evolution for a massive star ($>8M_\odot$).

A core-collapse supernova is powered by gravitational energy, not nuclear fusion. The degenerate core of the star collapses very quickly, from something about the size of the Earth to something with a radius of about 10 km in less than a second. The vast amount of gravitational potential energy that is released goes chiefly into neutrinos that escape from the collapsed core. Normal stars do not do this because their cores are stable on long timescales and there are no collapse events. In a core-collapse supernova the collapse is triggered by very high photon and electron energies and these energies are just not reached until the endpoint of a massive star's life.

A type Ia supernova rapidly consumes all of its nuclear fuel because the whole white dwarf star structure is governed by electron degeneracy pressure. In these circumstances, energy released in nuclear fusion reactions (e.g. the fusion of two oxygen nuclei) can be used to raise the temperature of the non-degenerate ions in the gas, without increasing the electron degeneracy pressure very much. The result is a runaway reaction that spreads rapidly through the star and releases sufficient energy to gravitationally unbind the whole star.

Again, this doesn't happen in normal stars because most of the time the core or nuclear burning region is not electron-degenerate and the pressure of the gas responds quickly to a rise in temperature. The increased pressure expands the envelope, and the energy of the reactions is used to do work in the process. This regulates the nuclear burning allowing it to proceed in a smooth way. Where burning does take place in the core of a star - for instance in the He cores of stars at the tip of the red giant branch - then "explosive" burning does take place. But only the core of the star is degenerate and the rest of the star is able to respond in a way that dampens down the nuclear fusion rate again before it can spread significantly throughout the star.

Answer 2

Another approach to the question:

Converting hydrogen into helium is what powers the main sequence stars. It can convert something like 0.7% of the hydrogen mass into energy.

Going all the way from helium to nickel/iron releases even less energy. It still can be impressive (as in Ia supernovae), because it happens at many orders of magnitude shorter timescales.

On the other hand, simply throwing an amount of matter onto a neutron star or a black hole can be much more efficient. A fine-tuned accretion disk can convert as much as 20-40% of the infalling mass into energy. That's how quazars work.

A whole star falling onto itself can be as much capable - so in its final collapse it has an order of magnitude more energy to shine on us than in its whole main-sequence life.

...And supernova explosions would be even more spectacular if it wasn't for neutrinos carring away most of the energy.

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edit: Why these things happen at different timescales?

Because they have different mechanisms to self-regulate the energy release.

All energy release mechanisms in stars depend on the star core's density and the core can be prevented from becoming denser by its gas pressure (in proto-stars and smaller main-sequence stars), photon pressure (most main sequence stars), electron degeneracy pressure (in white dwarfs) or neutron degeneracy pressure (in neutron stars).

Should the employed self-regulation mechanism fail, some next mechanism kicks in and the star enters another more or less steady state. Failing that, the energy release (be it from thermonuclear reactions or a gravitational collapse) happens in a runaway fashion and everyone is happy that they observe the process from a distance.

Answer 3

EDIT: ProfRob has way more comprehensive answer, please see that instead!

For huge stars, the core gets saturated by heavier elements (up to iron, depending on the star size), and eventually energy released by fusion in the core cannot provide enough pressure to counter the gravitational pull of all the mass above it, causing it to shrink even more. This will in turn heat up the infalling gas, accelerating the fusion reactions, which in turn additionally adds more heat, thus resulting in a runaway process resulting in supernova explosion.

The exact details what happens depends on the size of the star, but this is the general idea how things proceed.