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A god switchs off the nuclear fires in our sun tonight, do I have to worry?

  1. Oct 23, 2009 #1
    Say a god got mad and could and did turn off the nuclear fires in our sun. About how long till earth freezes?

    We read that it takes an incredibly long to for the average photon produced in near the center of the sun until the time it scatters for the last time and leaves the sun, does that mean the sun will take a while to cool down, that time lengthened somewhat by the slow gravitational collapse of a shut down sun?

    Say we look only at the volume of the sun where most energy is produced. Particles leave this volume (photons and anti neutrinos). The number of particles in this volume reduces, four protons and two electrons go to one helium nucleus. Does this change in particle number have thermodynamic consequences?

    We know that a large flux of neutrinos can blow a star apart that goes supernova?

    Does this flux of neutrinos tend to polarize the positively charged matter and the negativley matter? If so are large electric fields generated in a supernova?

    From this does it follow that the constant flux of neutrinos in our sun give rise to some very small but constant outward force to the matter in the sun?




    Thank you for your thoughts.
     
  2. jcsd
  3. Oct 23, 2009 #2
    Intuitively, it seems that something similar to a core-collapse supernova would happen. After all, those happen because the nuclear power "turns off" due to poisoning with iron and nickel.
     
  4. Oct 23, 2009 #3
    Yup. It will take several thousand years.

    Yes. Over time the core contracts and more energy goes into the envelope. Once it reaches a tipping point, it turns into a red giant.

    Yes.

    No. Neutrinos have no charge so they don't have any electromagnetic effects on the star.

    Yes. but I suspect if you run the numbers, you'll find the force too small to matter.
     
  5. Oct 23, 2009 #4
    Except that in the sun, there isn't enough mass or pressure to cause the star to collapse once everything in the center is compressed to white dwarf matter.

    In a supernova, the collapse is not caused merely by the existence of iron, it's that once you get past iron the nuclear reactions that happen actually absorb energy, so you get this feedback until everything is compressed to neutrons or possibly until you have a black hole.
     
  6. Oct 23, 2009 #5
    I know that, but I figured the sudden stopping of the nuclear reactions would cause the Sun to collapse somewhat dramatically into a white dwarf as the radiation pressure holding it up against gravity is suddenly removed. It's not going to collapse to a neutron star or anything, but that's why I said "like" a core-collapse supernova.
     
  7. Oct 23, 2009 #6

    marcus

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    A long long time. Someone already said "thousands of years". There would be no dramatic or sudden change as the core would still be hot---like 15 million kelvin--and would continue to radiate. And the thermal Xray from the core would continue to percolate gradually out. No noticeable change at first.
    At least thousands of years, might be one or more orders of magnitude more, before earth freezes.

    I think your guess is unphysical. Why would radiation pressure from the core suddenly be removed? The core, the inner 10 percent, is still hot. It still radiates.

    Two-fish quant already gave excellent response so it is superfluous for me to chime in. But just wanted to express agreement.
     
  8. Oct 23, 2009 #7
    I was indeed partially wrong. I forgot that the Sun will still have a tremendous amount of gravitational potential energy, and without fusion (I'm assuming higher-energy processes will still be forbidden somehow) the outer envelope will not suddenly be blown off as it would be if the Sun were a red giant. Thus, the Sun should still be able to radiate for perhaps millions of years. However, I still maintain that without fusion, the core will not be able to produce nearly as much radiation as it did and will therefore collapse inwards with the rest of the Sun until the overall Sun heats up enough to balance it's own weight with radiation pressure. It will necessarily be much denser than before, though maybe not to the white dwarf level.
     
  9. Oct 24, 2009 #8
    Without radiation pressure from nuclear reactions the Sun will collapse, but compression heating due to gravitational pressure will replace fusion as the heat source for millions of years. However the very outer layers which presently convect will collapse eventually and the Sun will probably shrink singificantly in the short term before eventually settling into a slow contraction. Since the Sun is below the Chandrasekhar mass limit it will only collapse to white dwarf density, becoming a sphere about 10,000 km in radius. If its heat dribbled out at its present luminosity then that process would take ~ 1 billion years. However there's no nuclear energy being generated so there's no "thermostat" like in Main Sequence stars, thus the Sun will probably brighten significantly. The Eddington Limit is the only constraint, which is at about 33,000 times present luminosity.

    Maybe turning off fusion is not such a good idea, if you're a god.
     
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