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Homework Help: Blackbody problem

  1. Oct 4, 2005 #1
    Hi, im having trouble with the last question in my assignment. It's on blackbody radiation and total power per area and whatnot. those questions i was able to do fine, however when i came to this problem i was a little lost..

    here it goes, i'll type the whole thing out..

    "The most important energy source in so called main sequence stars is fusion of four protons into He nuclei.
    A star has a radius r
    How many he nuclei does the star produce per second if He production is the dominant energy source?
    Use the Q value for the conversion 4H ->He to estimate the amount of raditation energy generated by hydrogen burning, and assume that the radiation energy output per second corresponds to the energy generated by hydrogen burning in a second."

    any help would be greatly appreciated. thanks

  2. jcsd
  3. Oct 5, 2005 #2

    Andrew Mason

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    Where did you get this problem? First of all, how do 4 protons fuse to become one He nucleus? Shouldn't it be 2 deuterons fuse to become a He nucleus?

    You appear to be missing an essential piece of information: the surface temperature of the star. Without that, it is not possible to answer the question.

  4. Oct 5, 2005 #3
    Sure that's possible 4 H fuse to form 4He2 .
    through Carbon Nitrigen Oxygen cycle
    Well I can't remeber whole thing but you can find it in any text book.
    Also a minimum temperature is required at core for this process ro take place
    But surface temperature would be much lesser
  5. Oct 5, 2005 #4
    sorry, i calculated surface temp to be 4990k

    the question says turns to He (of atomic mass # 4)

    i was just hoping for a formula i would be able to work with as i don't kow where to begin, but ill give the radius too if you want all the info. r=7.02*10^8 m

    anyone able to help this early in the mourning? :redface:
    thanks for the help, it is greatly apreciated :smile:

    Last edited: Oct 5, 2005
  6. Oct 5, 2005 #5


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    To be in equilibrium, the energy production in the star must equal the energy radiated, so

    Egen = Erad.

    Egen = Number of reactions * Qreaction.

    and one should have a formula for blackbody radiation, the Stefan-Boltzmann law E = ([itex]\sigma T^4[/itex]) *A.

    For He can be produced from 4 protons indirectly by the PP or CNO cycles.



    or more generally

  7. Oct 5, 2005 #6
    thanks man, im a little lost as to how i incorporate the radius, but ill figure it out :smile:

  8. Oct 5, 2005 #7


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    Surface area of a sphere, i.e. the star, of radius r is

    A = 4[itex]\pi r^2[/itex].
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