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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?

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    $\begingroup$ Have you read any resources on what actually happens in supernovae? This question might be better if it could show some traces of prior research and maybe more specific questions related to it $\endgroup$
    – tuomas
    Jun 23 at 8:38
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    $\begingroup$ This question does not show much effort for prior research on the topic $\endgroup$
    – tuomas
    Jun 23 at 8:39
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    $\begingroup$ Are you 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?" $\endgroup$
    – PM 2Ring
    Jun 23 at 9:30
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    $\begingroup$ A useful approach might be to actually do some simple research about supernovae, and then maybe come back and ask a similar question about Type Ia supernovae (which actually do involve nuclear fusion, unlike core-collapse supernovae). $\endgroup$ Jun 23 at 11:21
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    $\begingroup$ Since this question has generated (at least) one high quality answer already I don't see how the site is best served by blocking further answers. Voting to keep open! $\endgroup$
    – uhoh
    Jun 25 at 4:28
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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.

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  • $\begingroup$ It looks as if you are a supernova yourself! $\endgroup$ Jun 23 at 12:03
  • $\begingroup$ So the energy released (the flash) in the first type is due to gravitation while for the second type fusion is responsible? In the second type only a flash will be seen? So you can use them as candles, so to speak. $\endgroup$ Jun 23 at 12:13
  • $\begingroup$ @Deschele Schilder - In a core-collapse supernova, most of the gravitational potential energy is released as neutrinos from the core, but there is also a shockwave and a slew of associated nuclear reactions in the outer layers of the collapsed star, so those supernovae as well become very bright. $\endgroup$
    – antlersoft
    Jun 23 at 15:56
  • $\begingroup$ @antlersoft Will it be comparable in light strength to a type 2 supernova? Can it be used as a standard candle for measuring the expansion rate of the universe in the past (if their distances are known)? $\endgroup$ Jun 23 at 16:23
  • $\begingroup$ @Deschele You got that backwards. A type Ia supernova is caused by fusion, from material falling onto a white dwarf. They are standard candles because it takes a certain amount of total mass to initiate the fusion reaction (remember, a white dwarf has stopped doing fusion). A type II supernova is caused by core collapse. They aren't standard candles because they can occur over a very large mass range. $\endgroup$
    – PM 2Ring
    Jun 23 at 21:34
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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.


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.

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  • $\begingroup$ Now that my answer is choosen as an "accepted" answer, I can see that it completely misses the question in its current form (after the 2 edits). I'll try to add some clarifications. $\endgroup$
    – fraxinus
    Jun 27 at 11:12
  • $\begingroup$ In high mass stars, i.e. $m > 30$~M$_{\odot}$, are there other dominant "energy release mechanisms" than photon pressure? $\endgroup$ Jun 30 at 17:18
  • $\begingroup$ If you talk about "regulation mechanisms" that govern the energy release - well, main sequence stars are all (independent of their mass) regulated by the combination of gas pressure and photon pressure. The big stars (more than 30x solar mass) are photon pressure domain because the photon pressure is proportional to T ^ 4 and gas pressure to T ^ 1. Older, post-main-sequence stars play with electron degeneracy pressure as well. If we talk about the energy release mechanisms, they can be groupped as thermonuclerar and gravitational and they change roles few times over the star evolution. ... $\endgroup$
    – fraxinus
    Jun 30 at 20:43
  • $\begingroup$ ... Main sequence stars are powered by thermonuclear fusion of hydrogen. Younger, pre-main-sequence stars are powered by Kelvin-Helmholtz mechanism (gravitational collapse). Old, post main-sequence stars show some diversity depending on their mass and evolution stage. $\endgroup$
    – fraxinus
    Jun 30 at 20:48
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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.

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    $\begingroup$ Nuclear fusion is not what powers a (type II) supernova. $\endgroup$
    – ProfRob
    Jun 23 at 10:32
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    $\begingroup$ A lot of the energy released right before and during the Supernova is through electron capture which is the reaction $p^+ + e^- \rightarrow n + \nu$. These ultra-relativistic neutrinos carry huge amounts of energy which is far more than the energy produced during normal nuclear fusion. @ProfRob please correct me if I am wrong. $\endgroup$ Jun 23 at 10:41
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    $\begingroup$ @AryanBansal electron capture by protons requires energy, in the sense that kinetic energy of protons and electrons is turned into the larger rest mass of a neutron. $\endgroup$
    – ProfRob
    Jun 23 at 11:32
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    $\begingroup$ @AryanBansal The bulk of the neutrinos produced in a core-collapse supernova -- the ones which restart the stalled shock wave and drive the explosion -- are actually thermal emission from the superhot proto-neutron-star, after the electron-capture process has induced the core collapse. See e.g. en.wikipedia.org/wiki/Supernova#Core_collapse $\endgroup$ Jun 23 at 11:35
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    $\begingroup$ @Aryan Yes, you're still a bit confused, but there's no shame in that, it's a complex topic! ;) (And I'm certainly no expert on it). Neutrinos do carry some energy out of the star. But more importantly, the thermal neutrino (& antineutrino) flux allows energy to be rapidly conducted through the star, heating it on a much faster timescale than what would be possible without neutrinos. $\endgroup$
    – PM 2Ring
    Jun 23 at 22:53

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