A star with mass 2.0 x 1028 kg which is 5.0 x 1020 m from the center of a galaxy revolves around that center once every 5.0 x 108 years. Supposing that this star is basically at the edge of the galaxy, each of the stars in the galaxy consists of a mass identical to that of the star and the stars are distributed equally in a sphere about the galactic center, approximate the number of stars in the galaxy.