Q:  What is a reasonable rough guess for the mean molecular weight mu
             at the center of the Sun?  (Hint: assume pure hydrogen)


           Let's assume pure hydrogen.  The center of the Sun is hot enough
                that it will be completely ionized.  Therefore, each hydrogen
                atom will give rise to 2 particles: an electron and a proton.

 
                The mean molecular mass will be roughly 0.5, since there are
                two particles which add up to the mass of one hydrogen atom.


                          mu = 0.5