1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Astro, flux & energy

  1. Nov 4, 2007 #1
    The neutrino flux from supernova 1987A was estimated to be 1.3e14 m^-2 at the Earth. SN 1987A occured in the Large Magellanic cloud, which is located at a distance of 50 kpc. If the average energy per neutrino was 4.2 MeV, estimate the amount of energy released in the form of neutrinos during the supernova explosion

    2. Relevant equations
    electron volt = 1.602e-19 J
    pc = 3.086e16 m

    3. The attempt at a solution
    Answer says: 2.6e45 J

    I can't seem to get the answer... :(
    any help will be appreciated :)
    i was thinking since flux falls off ~ 1/d².
    so i tried E = flux *d²*avg energy of neutrino
    =3.258e44 J.
    So i'm off by more than a factor of 10... :(
    What am i doing wrong??
  2. jcsd
  3. Nov 4, 2007 #2


    User Avatar
    Science Advisor
    Homework Helper

    the area of a spehere is [tex] 4 \pi r^2 [/tex]

    So you should multiply with 4pi = 12,.. =)

    also check the numericals, 1.3e14*(50000*3.086e16)²*4.2e6*1.602e-19 = 2.08*10^44 J
  4. Nov 4, 2007 #3
    oh!! Thank you
    (yeh i think i typed something wrong in the calculator before).
    Last edited: Nov 4, 2007
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook