Why should stars explode?

  1. Okay, so so you got two forces acting on a star. Gravity trying to compress it, and the internal pressure, caused by fusion, pushing it out. For the star's life they are at equilibrium, but the star dies when it runs out of nuclear fuel.

    Then why do some stars explode? Shouldn't they all collapse?
     
  2. jcsd
  3. Chronos

    Chronos 10,046
    Science Advisor
    Gold Member

    The collapse induces enormous temperatures the star cannot contain.
     
  4. During collapse (and during slow shrinkage too) temperatures and pressures inside the star rise. If at some point temperature and pressure enable a previously impossible nuclear reaction which releases a lot of energy and has a large dependence on temperature, then this may trigger a runaway burn. This is especially likely if matter is degenerate - because degenerate matter does not noticeably expand when heated. (Expansion may quench the reaction).
     
  5. Also as the core collapses into a neutron star or blackhole the outer layers are falling in the density rises to point where these gases see a brick wall and rebound, in addition to nuclear reactions creating heavy elements, that what I though at least
     
  6. Huge stars (with one exception) both collapse and explode. The cores of these stars have the greatest pressure and temperature. Hydrogen is fused into helium, carbon, oxygen and other elements. When the core fuses into a nickel isotope (IIRC nickel 59), the nickel quickly decays into iron. Elements lighter than iron can fuse into heavier elements and thereby release energy. Elements heavier than iron can split into lighter elements and again release energy. Iron is exceptional in that neither fusion nor fission of this element releases energy. The core has reached a dead end. The mass of the outer layers of the star are no longer held up by the release of core energy. So the outer layers collapse onto the iron core at about a quarter of the speed of light. These still contain hydrogen and other lighter elements in layers like an onion. The pressure of the collapse causes these elements to fuse at an incredible rate and the resulting explosion blows the outer layers away and crushes the core even further. You get a super nova event and the core is reduced to neutrons or a black hole.

    Really unique giant stars don’t collapse. The pressures at the cores of these stars are so great that anti-matter is created. A chain reaction of matter -antimatter collisions causes the star’s core to erupt and the star is totally ripped apart in a hyper nova.
     
  7. In some sense they all do, but what happens with core collapse supernova is that the inner parts of the star collapse which releases enough energy to blow away the outer layers. With other supernova, you have enough energy with nuclear reactions that the whole star goes kaboom.
     
  8. I don't seem to be able to edit my earlier post. Anyway, it's Nickel 56 decays to Cobalt 56 and then to Iron 56 (then kaboom).
     
  9. Nope. The kaboom happens before the nuclear reactions. What happens with core collapse supernova is that the core of the star collapses into a neutron star. This collapse releases 10^53 ergs of energy (most of which gets emitted in the form of neutrinos) and by some magic process which we don't understand, 1% of that energy gets deposited into the outer layers and that goes kaboom.

    So for core collapse supernova, you do get a collapse, but the energy from that collapse generates an explosion.
     
  10. Nope, it’s iron core first, then neutron core, then kaboom.

    “Within a massive, evolved star (a) the onion-layered shells of elements undergo fusion, forming an iron core (b) that reaches Chandrasekhar-mass and starts to collapse. The inner part of the core is compressed into neutrons (c), causing infalling material to bounce (d) and form an outward-propagating shock front (red). The shock starts to stall (e), but it is re-invigorated by a process that may include neutrino interaction. The surrounding material is blasted away (f), leaving only a degenerate remnant.”

    http://en.wikipedia.org/wiki/Supernova
     
  11. Drakkith

    Staff: Mentor

    Incorrect. The silicon burning process that produces nickel takes approximately 5 days in a 25 solar mass star. See the table here: http://en.wikipedia.org/wiki/Type_II_supernova

    Edit: I will say that a lower mass star burns at a slower rate and would take longer, but I don't know how to calculate the time. So I would guess that it would be possible for stars near the lower limit to survive in the silicorn burning phase for long enough for some nickel to decay into cobalt and then iron over a few months. For higher mass stars the process is too quick for much iron to accumulate.
     
    Last edited: Jan 4, 2012
  12. Okay, so all elements from the periodic table up to and including iron are created within the cores of large stars, how are the heavier elements assembled if the star explodes at the production of iron?
     
  13. Drakkith

    Staff: Mentor

    The supernova process forces nickel, cobalt, iron and heavier elements to fuse together in the explosion, forming the rest of the elements.
    See here: http://en.wikipedia.org/wiki/Supernova_nucleosynthesis
     
  14. Drakkith

    Staff: Mentor

    Been doing some looking around and according to the following link, I believe the core consists of a mix of different elements and isotopes near Iron.

    http://www.ucolick.org/~woosley/ay220-09/lecture12/lecture12.09.pdf

    It looks to me like most of the core is 54Fe, which would be a mostly iron core. (If those numbers are a measure of the fraction of each element in the core.)
     
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