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B Entropy change during the formation and death of stars

  1. Oct 6, 2016 #1
    The stars formation decreases the entropy but the radiations given out increases the overall entropy i am said ...but once the fusion processes are over the star no longer gives out radiations....and still the volume in which the matter particles of the star can be becomes less as the star shrinks! this lowers the entropy of the system doesn't it? (as the entropy is the volume of phase space available?)
  2. jcsd
  3. Oct 6, 2016 #2


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    Most entropy decrease conundrums are resolved by thinking about what constitutes a closed system. Within a closed system, some regions can decrease entropy but only by exchanging energy with other regions of that closed system. And for the entire closed system, entropy will always increase.

    A star is not a closed system, it is a region inside a closed system (the universe).

    I am not commenting on your statements about formation decreasing entropy etc.
  4. Oct 6, 2016 #3

    Thank you so much for your answer sir.
    But one more question ..If the universe is a closed system here and the star is a part of it (with nothing else close to it) , after its life time say it becomes a white dwarf with all its matter compressed to a much smaller volume thus decreasing the entropy...the star no longer can give out radiations as the fuel is over.Then is it right to say the expansion of the universe compensates for the decrease?
  5. Oct 6, 2016 #4


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    I don't know, but it sounds a bit out of left field to me.

    I would first look at the increase in the entropy of the universe caused by the totality of radiation emitted by the star over its lifetime and compare that with the decrease in entropy of the star itself due to its decreased volume and see how that tallies. I have no experience considering the entropy of stars, you are getting my first thoughts on the matter, so buyer beware.


    My next thoughts - for a closed system, given any time slice no matter how small, the total entropy will always increase. So, if there is a ever a small enough time slice in the life cycle of a star where energy emitted by a star cannot account for the entropy decrease caused by the star shrinking, then one must find another source of entropy increase if one is to confirm the 2nd law holds. I don't know if another mechanism would become necessary or what it might be if it were necessary (to balance the entropy books, I mean).
    Last edited: Oct 6, 2016
  6. Oct 6, 2016 #5


    Staff: Mentor

    This is not correct. The star is still at a nonzero temperature, so it will still radiate. It just won't radiate as much because its temperature will be lower once fusion has shut down. Even while fusion is going on, the star doesn't radiate because it has fusion going on; it radiates because it's at a certain surface temperature. Fusion is just the mechanism that maintains it at a certain surface temperature.

    Why will the star shrink? If you're thinking of it turning into a white dwarf, it radiates during this process.

    Incorrect. See above.
  7. Oct 6, 2016 #6


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    In order for a system to have a decrease in entropy, it has to have a big input of heat. This typically doesn't happen for stars.
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