All of electrostatics follows from the 1/r2 character of Coulomb's law, together with the principle of superposition. An analogous theory can therefore be constructed for Newton's law of universal gravitation. What is the gravitational energy of a sphere, of mass M and radius R, assuming the density is uniform? Use your result to estimate the gravitational energy of the sun (look up the relevant numbers). The sun radiates at a rate of 3.86 x 1026 W; if all this came from stored gravitational energy, how long would the sun last? [The sun is in fact much older than that, so evidently this is not the source of its power.]