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Iron killing fusion in stars

  1. Apr 15, 2012 #1
    I've just watched the Extreme Stars program in How the Universe Works and it states that when a massive star starts to make iron, the fusion is quenched and the core collapses. From the program, it sounds like the process is fast and will happen when a few grams of iron is formed.
    I find this difficult to believe because our sun formed from dust from a supernova explosion, which contains iron (and other heavier elements). The incorporated iron should have prevented the sun from maintaining the fusion of hydrogen.
    I will agree that a large portion of the sun mass could have been collected before the sun encountered supernova dust, which will dilute the amount of iron.

    Why am I wrong and the sun is still burning?
  2. jcsd
  3. Apr 16, 2012 #2


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    Either the program said something wrong or you didn't understand.

    Iron does not kill fusion in the sense that a little bit would make fusion stop. Don't think of it as a "poison".

    Iron is just the inert final end product of fusion in massive stars. It is not a prospective fuel for fusion, whereas lighter elements (H, He, C, N, O etc) are, as long as the temperature and pressure are high enough

    An iron nucleus cannot fuse with another iron nucleus in such a way as to release energy, no matter how high temperature and pressure in the core of the star.

    H can fuse to make Helium and some extra energy
    Helium can fuse to make stuff like Carbon Nitrogen Oxygen...plus extra energy (if the star core is hot enough to make it happen)
    Those can in turn fuse to make heavier nuclei, on up to Nickel and Iron (if the core temp and pressure is high enough) still with a net energy production.
    But after a substantial portion of the star core is deadweight non-fuel Iron then there are no more fusion options.

    all that really means is THE STAR HAS RUN OUT OF FUEL in its core. Trying to make Iron nuclei fuse takes up more energy than you get out. There is no gain. So the star's core is destined to cool down to where it no longer has the ability to support the outer layers which press in on it.

    Why the collapse that eventually occurs is SUDDEN is another question. That has to do with the ability of ordinary atomic matter (protons electrons and neutrons) to compress down to NEUTRON matter if you apply enough pressure. You know that neutron matter is amazingly dense. A given mass takes up much less volume, almost no space at all by comparison with ordinary atomic matter.
    In these very massive stars that are able to fuse all the way up to iron, the outer layer weighs so heavily on the core that eventually a threshhold density is reached where this change into neutron matter can occur. Then the collapse can be extremely fast.
    All the protons realize at once that they are able to absorb an electron and become a neutron. And matter suddenly can occupy a millionfold less volume. In effect the star's core is in free fall towards its center.

    The core does not have to be 100% iron for that to happen. The main thing is running out of fuel, so there is not enough energy-producing fusion occurring, and the core gets compressed to a point where whatever kind of atoms are in the core suddenly start converting to neutron matter.

    There should be a good wikipedia article describing this. Or some other online source.
    Last edited: Apr 16, 2012
  4. Apr 16, 2012 #3
    The amount of iron that needs to be built up before the core collapse is actually more than the mass of our Sun (about 1.4 times the mass). So, it's more than just a few grams, although yes, this buildup does happen quickly, astronomically speaking. Also, it needs to be at the core of the star (not all of the iron in the Sun would be near the center).
  5. Apr 16, 2012 #4
    I think iron absorb energy for further reaction
  6. Apr 16, 2012 #5
    basically, after a star finishes fusing H into He, it goes to the next element, and so on

    Fe can't fuse anymore, so the whole process sort of stops there
  7. Apr 16, 2012 #6
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