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This is really bugging me. When you look up some educational text about stars life, this is what you find out:

  1. Gravity creates the temperature and pressure to start fusion reactions.
  2. The fusion proceeds to heavier and heavier cores ending with iron, which remains in the centre of the star.
  3. One moment, all light cores are depleted and the gravity wins over the power of the fusion reactions, now absent.
  4. The core of the star collapses into high density object, which may vary depending on the star mass.
  5. And the top layers of the star explode.

And I just cannot find clear explanation why. According to what I imagine, the top layers of the star should just fall into the collapsing core.

Is that because of the 3rd Newtons rule?

Or do the stars have some need to end with a cool boom?

Qmechanic
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Tomáš Zato
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    The external layers do fall into the collapsing core before exploding. I'm not really into this kind of subject, but since I know some people that are researching SN explosions, I can ask on a reference, if you like.

    As far as I could understand, it's highly non-trivial why the external layers ends up exploding, and it seem to involve lots of different mechanisms working together. So, I believe that you have all the right to be intrigued by that.

     – Hydro Guy May 06 '13 at 22:17
  • I believe the explanation comes from the creation of a shockwave originated by the collapse of the outer layers into the massive core. – PML May 06 '13 at 22:51
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    Has anyone managed to make a model that reliably explodes when an explosion is expected? I remember it being a major problem when I was an undergrad a decade ago. – Dan Is Fiddling By Firelight May 07 '13 at 13:59
  • The core collapse releases a lot of energy as photons and neutrinos. The envelope, which is in a far shallower gravity well, absorbs this energy and heats up enough for the atoms to escape. – Kevin Kostlan Sep 26 '23 at 03:28

4 Answers4

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There are lots of possible ways that stars can end their life, even in the subset of cases where the end is violent. Eloff has given an excellent answer, but I wanted to add a few points.

Summary (tl;dr):

You need the right conditions (mass, angular momentum, metallicity, etc) to produce a proto-neutron-star which is able to resist complete collapse to a black-hole. The bounce from hitting that proto-neutron-star surface, and the heating from neutrinos, is what drives the explosion of material. Radioactivity is eventually the source of the light we see from supernovae.

The basic picture for producing a supernova from a massive star1:

  1. The star burns progressively heavier elements on shorter timescales until producing iron (Fe) on the timescale of seconds.

  2. After iron, fusion in the core ceases, and pressure support is lost. Gravity is unhindered, and the star begins dynamical collapse.

  3. As the Fe-core contracts, electron-capture begins to convert protons + electrons into neutrons, emitting MeV neutrinos.

  4. The Fe-core, now largely composed of neutrons is stabilized to further collapse by neutron degeneracy pressure at nuclear densities.

  5. Material further out, which is still collapsing, hits the incredibly hard proto-neutron-star surface - causing a bounce (see video analog): the launch of a powerful shockwave outwards through the star.

  6. Because the neutrinos produced from electron-capture are so energetic (as dmckee points out), and because the densities are so high - the neutrinos are able to deposit significant amounts of energy into the outer-material, accelerating it beyond escape velocities. This is the supernova explosion.

  7. Due to the hot, dense, nucleon-rich nature of the ejecta, r(apid)-process nucleosynthesis produces radioactive Nickel (Ni) and Cobalt (Co).

  8. After roughly 10's of days, the expanding supernova ejecta becomes optically thin - allowing the radiation produced by Ni and Co decay to escape - this causes the optical emission we call a supernovae.

chart of supernova stages

from http://arxiv.org/abs/astro-ph/0612072

Why does a supernova explode?

All massive stars are not believed to produce supernovae when they explode. In the following figure (which is intended to convey the basic idea - but not necessarily the quantitative aspects), regions titled 'direct black-hole formation' are regions of initial mass where the neutron-degeneracy pressure (stage '4' above) is insufficient to halt collapse. The Fe core is massive enough that it continues collapsing until a black-hole is formed, and most of the material further out is rapidly accreted.

The region in this plot between about 8 and 35 solar masses is where the vast majority of observed supernovae are believed to come from.

To answer why supernovae explode: Consider the schematic process outlined above. The reason why some deaths-of-massive-stars explode and others don't, is that you need the right conditions (mass, angular momentum, metallicity, etc) to produce a proto-neutron-star which is able to resist complete collapse. The bounce from hitting that proto-neutron-star surface, and the heating from neutrinos is what drives the explosion of material. Radioactivity is eventually the source of the light we see from supernovae.

plot of relationship between initial and final stellar mass

from http://rmp.aps.org/abstract/RMP/v74/i4/p1015_1

Footnote

1: This discussion is constrained to 'core collapse' supernovae - the collapse of massive stars, observed as type Ib, Ic, and type II supernovae

Additional References

Basically any paper by or with Stan Woosley, e.g.
Woosley & Janka 2006 - The Physics of Core-Collapse Supernovae

Lecture Notes by Dmitry A. Semenov - "Basics of Star Formation and Stellar Nucleosynthesis"

DilithiumMatrix
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    There are a few problematic statements here: The neutrinos from supernovae have energies much less than 1 GeV - they are a fraction of the electron Fermi energy, more typically 10-30 MeV. The fusion process ends in Ni56. Fe56 is produced by electron captures in the core or later by radioactive decay. The neutron core is supported mainly by the strong nuclear force in asymmetric matter. Neutron degeneracy pressure is incapable of supporting a core bigger than 0.7 solar masses. Ni and Co decay produce neutrinos and positrons, not photons. – ProfRob May 13 '15 at 09:58
  • Thanks for your comments @RobJeffries, I've updated the neutrino energies --- my mistake. I tried to avoid the details of Ni56 vs. Fe56 production in the core. Can you provide some references for your last two points? – DilithiumMatrix May 13 '15 at 14:22
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    For the point about NDP only supporting a 0.7 Msun object - see Oppenheimer & Volkoff (1939) journals.aps.org/pr/abstract/10.1103/PhysRev.55.374 – ProfRob May 13 '15 at 14:49
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    I eat humble pie on the second point. Electron capture onto Ni and thence via Cobalt into Iron does result in gamma ray production: apologies. http://articles.adsabs.harvard.edu//full/1994ApJS...92..527N/0000527.000.html – ProfRob May 13 '15 at 14:55
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    Note though, these are gamma rays. The optical emission comes from thermalization in the expanding outer envelope. – ProfRob May 13 '15 at 15:08
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    Cessation of fusion is not what causes the collapse, since the pressure is due to electron degeneracy. It is either neutronisation or photodisintegration of the iron nuclei that removes the pressure support in a runaway fashion as the core approaches the Chandrasekhar mass for iron. – ProfRob Jan 16 '17 at 23:29
  • @DilithiumMatrix How can a "bounce" happen when the star collapses into a black hole, though? Wouldn't the material continue to accelerate into the singularity? – O. Aroesti Dec 17 '20 at 09:13
  • @O.Aroesti it is because the repulsive force between 2 neutrons grows rapidly as their separation decreases. The collapse has to be arrested before a black hole is formed. – ProfRob Oct 15 '21 at 11:49
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You're correct that when fusion reactions decrease past a certain point because the fuel is used up, the outward pressure created by the fusion no longer counteracts the gravitational forces and the star collapses (rapidly) in on itself. In stars of the right mass (smaller than about 15 solar masses, but large enough to collapse into a neutron star) the compacting core is heating up to insane temperatures of around 100 billion kelvin. This overwhelms the neutron degeneracy pressure that's preventing the star from collapsing further and a huge burst of neutrinos is released. About 10% of the star's mass/energy is released in around 10 seconds, which is a mind-boggling amount of energy.

This is where it gets fuzzy, according to wikipedia:

The suddenly halted core collapse rebounds and produces a shock wave that stalls within milliseconds in the outer core as energy is lost through the dissociation of heavy elements. A process that is not clearly understood is necessary to allow the outer layers of the core to reabsorb around 10^44 joules (1 foe) from the neutrino pulse, producing the visible explosion, although there are also other theories on how to power the explosion.

So it seems it's this sudden neutrino pulse that's reabsorbed by the outer-core that triggers the explosion.

Eloff
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    It is worth noting that the neutrinos produced in a supernova are mostly at a few GeV energies where their interaction cross-sections are roughly a factor of 1000 higher than those of typical solar neutrinos---that's enough that a small but significant fraction of the energy is deposited in the outer layers of the star. – dmckee --- ex-moderator kitten May 06 '13 at 23:29
  • @dmckee also I'm just speculating here, but I'd imagine the high density of the matter in the outer core would mean more neutrino interactions than with neutrinos passing through the earth? – Eloff May 07 '13 at 23:10
  • Yes, it goes essentially by the matter density too. – dmckee --- ex-moderator kitten May 07 '13 at 23:19
  • 10% of the star's rest mass energy would be $3\times 10^{47}$ J, and about 10 times more energetic than any known supernova and 10 times the energy released by the core collapse. You can't believe everything copied from wikipedia. – ProfRob Apr 03 '16 at 15:32
  • @dmckee I feel this is deja vu, but the neutrinos are MeV not GeV. – ProfRob Apr 03 '16 at 15:34
  • @RobJeffries I don't think that you've corrected me on this before. And I can't figure what I was thinking when I wrote that. Poking around now, I see that the sn1987a neutrino observations are mostly in the few 10s of MeV range, so you're right, though that is higher than the usual solar range (but only by one order of magnitude, not three). – dmckee --- ex-moderator kitten Apr 03 '16 at 15:45
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    @dmckee The neutrinos are of order the Fermi energy of the electrons. – ProfRob Apr 03 '16 at 15:53
  • Ah, yes. That makes sense. Nice. – dmckee --- ex-moderator kitten Apr 03 '16 at 15:54
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Here's my theory

First, the assumptions:

  1. Matter is displacement of space*
  2. Matter moves towards higher concentrations of space**

These 2 assumptions can explain why objects in motion stay in motion**, why gravity exists, why the max speed is light-speed, +electromagnetic forces, +the strong/weak force, etc.).

How this applies to supernovae:
At the last stages before the supernova, the matter collapse is no longer in check, and so it begins collapsing to the point where there is no longer space between the matter composing the star. When that happens, matter on the surface has little to no space on the inside and a great concentration of space on the outside. Based on our assumptions, the surface matter will explode outwards at near light-speed due to this imbalance. Space will fill in to the next layer, and this process will continue downwards towards the center of the star, and BOOM...SUPERNOVA!!!


*Picture a ping-pong ball (mass) placed inside of a foam mattress (space). Space is compressed most near the surface of the mass and progressively less as you go out out

**Let's say matter moves at a velocity of ++c(ρf-ρb)/(ρf+ρb), where c is the speed of light (equivalent to how fast space can respond to changes), ρf is the average density of space in front of the mass (in the direction of travel), and ρb is the net density of space behind the mass. As the object moves, space in front responds at light-speed to get out of the way and fill in behind. In this way, the front and rear densities are maintained and constant velocities are maintained. For example, if space is on one side of a mass but none on the other, then the mass would travel at light speed. Because the mass is traveling at light-speed, the space behind it will not be able to catch up to touch the rear of the mass (which would slow it down based on our assumptions), and the space in front will just be able to keep up but not get out of the way, and so the imbalance will remain and the mass will continue on at light-speed (whiche explains why it is so hard to accelerate things as they approach c).

+requires positive/negative directional understanding of space which I won't get into right now.

++This is not for sure the equation, but represents an approximation knowing that an object at rest relative to space would have equal densities on both sides and an object with space on only one side would travel at light-speed. The true equation would be based on what would keep the relative densities of space constant for any possible velocity.

Briguy37
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    You say it's a theory, so what experiments are backing it? – Tomáš Zato Feb 10 '17 at 00:08
  • @TomášZato: Any theory about the fundamentals of the universe has the entire history of experiments to test against. Whether it can correctly explain everything it supposes about is the main test (though even in that case it could be incorrect about the "why" but still yield correct results). If you suppose the 2 assumptions, it can explain the following and more: Why do objects in motion stay in motion? Why is there gravity? Why do supernovae explode? Why does the strong force overcome the electromagnetic at about twice the radius of a proton (hint: That's where they begin to touch)? – Briguy37 Feb 10 '17 at 15:14
  • Why does it get harder and harder to accelerate as you approach the speed of light? Why don't electrons and protons bond together? – Briguy37 Feb 10 '17 at 15:22
  • @TomášZato But why would you replace an existing theory? This happens when the new theory predicts something unexpected by other theories and is found to be true. For example, this happened with the Michelson–Morley experiment for classical physics and relativity. That was thought by many to disprove the existence of aether. However, classical mechanics assumed that space is stationary, whereas this theory assumes that matter can move, distort, and affect space, and so it is entirely possible that space moves with the matter closest to it. – Briguy37 Feb 10 '17 at 15:40
  • Thus, a modified Michelson–Morley experiment could be performed where, all other things being equal, one of the paths of light goes through standing water and the other goes through a current of moving water. This theory would predict that light would speed up or be slowed based on the direction the water is moving, whereas relativity would predict that light would be constant. – Briguy37 Feb 10 '17 at 15:40
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This "bounce" theory isn't logical. 1. How strong that bounce must be so it can escape such strong gravity and eject very heavy outer layers of the core? 2. For something to bounce it must be elastic in some way or hit elastic surface. What is elastic in this case? 3. Another thing that's bothering me is what happens with bouncing when the core is much denser and shrinks to black hole? Where is bounce in that case?

So, I think that supernova is nothing but colossal neutrino explosion with totaly insignificant or in best case very small bounce effect.

Misa Jankovic
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    You know, people didn't just sit around and say "hey, I bet it bounces" without doing a lot of theoretical work and simulations. You've thrown out the bounce idea based on what evidence or simulation or alternative? – Brandon Enright Feb 18 '14 at 23:07