4. (R9.3) The total luminosity of the stars in our galaxy is L 3 100 Lo. Suppose this has been constant for 100 yr. How much energy has our galaxy emitted in the form of starlight in that time? Most stars are powered by the fusion of H into 4He, with a release of 28.4 MeV for every 4He nucleus formed. If such fusion is the only significant energy source for the starlight, how many nuclei have been formed in the last 100 yr? The baryonic mass of our galaxy is 101 Mo. By what amount has the Helium fraction, Y, of our galaxy been increased from its primordial value, Yp 0.25?