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Homework Help: Astrophysics (Stars - PP chain)

  1. May 11, 2014 #1
    1. The problem statement, all variables and given/known data

    Estimate the lifetime of a proton against fusion to 4He in the center of a Zero-Age-Main-Sequence solar mass star. First calculate the energy generation, εpp in the center of the star from the p-p chain. Then convert this to the number of fusions (conversion of 4 protons to 1 4He) per unit volume per second. Finally compare this rate to the central number density of protons to estimate the lifetime.

    Constants: (C = stellar core values)

    TC = 13*106 K
    ρC = 78 g/cm3
    XH (Mass fraction Hydrogen) = 0.7

    2. Relevant equations

    εpp ~ 1.07*10-6*(ρC)*(XH)2*(T6)4

    3. The attempt at a solution

    This was an exam problem - I know how to find the energy generation rate as well as the number density of protons. Neither are difficult. I do not, however, have any idea how to convert the energy generated in to the number of fusions per volume per second.

    If anyone has a first step, that's really all I'm looking for but I genuinely have no idea how to even begin. I was thinking it might involve going through the PP-I -> PP-IV branches individually but that seems far too complicated.
  2. jcsd
  3. May 11, 2014 #2


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    The energy generation rate (ergs/sec) is just the energy generated per fusion(ergs/fusion) times the number of fusion reactions per second (fusions/second). Do you know how much energy is generated per fusion?
  4. May 11, 2014 #3
    Wooh, that was actually a huge help. I have arrived at an answer (though I do not know how correct it is). Here is the work (sorry, don't know Tex but I'll do my best to make it neat):

    To find the energy per fusion of 4 protons to 1 He, I just used Einstein's E=mc2:

    EHe ~ 3.72*109 ev

    E4 Protons ~ 3.752*109 ev

    ΔE = 32 MeV = 5.12*10-5 Erg = energy released per fusion (4 Protons -> 1 He)

    Then I found the number density using n = XH*ρ/mp

    n = 3.26*1025 protons or so

    Then finally, I divided number of protons by number of fusions/second, but I am thinking that I need to multiply number of fusions/second by 4 to get the number of protons/second at which point I arrived at an answer of approximately 1.14*10^13 years.

    This answer is quite high but we were asked on the exam if our estimate was reasonable and I know it shouldn't be all that close just taking the PP chain into account. Not sure how good my methods were but I'm at least thrilled to be able to arrive at an answer.
  5. May 11, 2014 #4


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    That sounds high. Is your εpp in ergs/sec for the whole star, ergs/sec/g, or ergs/sec/cm^3?
  6. May 11, 2014 #5
    Now that you mention it, I'm actually not entirely positive. My notes are not explicitly clear but I believe that it's ergs/s/g.

    Oooooohhhh, hold on. Let me do some stuff.
  7. May 11, 2014 #6
    Ok, it was definitely in ergs/sec/g. I converted it over and got εpp = 90 ergs/sec/c3 or so. Ended up with 1.46*1011 years which seems better.
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