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Do heavier elements break down during supernova?

  1. May 10, 2014 #1
    Heavier elements like gold, uranium etc. are formed at the end of a star's life. As the star explodes into a supernova, it gives rise to nebula which is the birthplace of new stars. But as the star has already fused lighter elements into heavier elements, where does the new hydrogen required for the formation of new stars come from? Do heavier elements breakdown during supernova or is it some other process?
     
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  3. May 10, 2014 #2

    UltrafastPED

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    The fusion cycle of a star stops with iron; there is no net gain of energy for iron fusion, nor of any heavier element.

    Fission is a net energy producer for every element heavier than iron.

    So if a star "does work" and produces a heavier element it will soon break it down.

    Thus the only time that the heavy elements are produced is during a cataclysmic event - then they are out of the "pressure cooker" of the stellar interior.

    No star burns all of its hydrogen - or any of the other elements that are created along the fusion ladder. So when a cataclysmic event occurs many different existing elements are also flung out, along with the newly created ones.

    So new star formation uses remnants from previous stars as well as the very large amounts of free hydrogen available between the stars. See "Stellar Nursery": http://en.wikipedia.org/wiki/Molecular_cloud
     
  4. May 10, 2014 #3

    phyzguy

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    Two points. First, most of the content of ordinary matter in the universe is still in the form of hydrogen gas. Only a small fraction (less than 10%) of it has collapsed to form stars. So the hydrogen to form new stars comes mostly from gas that has not yet been formed into stars, not from matter that was ejected from a supernova. Second, before a massive star goes supernova, it ejects its outer envelope, most of which is unburned hydrogen, out into interstellar space. Even a smaller star like the sun will eject on the order of half of its mass, a lot of which is hydrogen, before it contracts to form a white dwarf.
     
  5. May 10, 2014 #4
    If the star still has hydrogen left, then what is the need to go supernova? It can still fuse that hydrogen,can't it?
     
  6. May 10, 2014 #5

    phyzguy

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    The remaining hydrogen is in the outer envelope, where the temperatures are too low for fusion to occur. It collapses and goes supernova because it has consumed all of the nuclear fuel in the core, and can no longer generate energy to resist the pressure of the layers above.
     
  7. May 10, 2014 #6
    Can the hydrogen in the outer shell reach the core by some means?
     
  8. May 10, 2014 #7

    Nugatory

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    Some of it has to stay in the outer shell, because it's the weight of the outer shell that compresses the core enough for fusion to happen.
     
  9. May 10, 2014 #8

    Chronos

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    Only fully convective stars can replenish the hydrogen in their cores, and only red dwarf stars are fully convective. These low mass stars never develop degenerate cores and can fully utilize their hydrogen supplies. This allows them to be incredibly long lived - trillions of years by some estimates.
     
  10. May 11, 2014 #9
    I had read somewhere that when the star reaches the red super-giant stage, the inner layers start fusing carbon and the hydrogen in the outer layer also gets fused which causes the star to expand in size. Is that correct?
     
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