The gravitational potential energy U of a self-gravitating spherical body of mass M and radius R is a function of the details of the mass distribution. For the Sun, U_Θ = -2GM²_ΘR_Θ. What would be the approximate lifetime of the Sun, radiating at its present rate, if the source of its emitted energy were entirely derived from gravitational contraction? (M_Θ = 1.99 x 10³⁰ kg.)