Q:  The Earth orbits the Sun with a semi-major axis of a = 1.496 x 10^(11) m.
          The period is (of course) 1 year.  Convert the period into seconds,
          and use Kepler's Third Law to determine the combined mass of the
          Earth and the Sun.


          Re-arranging to solve for the combined mass,

                              4  pi^2  [1.496 x 10^(11) m]^3
           (m1 + m2)  =  -----------------------------------------------
                         6.67 x 10^(-11) N*m^2/kg^2 * (3.156 x 10^7 s)^2


                      =   1.99 x 10^(30) kg


          The great majority of this total mass belongs to the Sun.