# Gravitational potential energy problem

1. Feb 9, 2008

### Tuugii

The gravitational potential energy W of a self-gravitating sphere of mass M and radius R depends on the detailed distribution of mass within the sphere, but it is generally of order of magnitude -GM^2/R. For the Sun, W_sun = -2G(M_sun)^2/R. What is the timescale
t = -(1/2)(W_sun)/(L_sun) over which gravitational contraction could have supplied the power radiated by the sun at it's present rate?

please give some hints, thanks a lot.

2. Feb 9, 2008

### Shooting Star

I am giving some answer because I think the question belongs to the Advanced Physics section.

There is something called a “virial theorem” which holds good for self gravitating bodies and many other systems. If the system is roughly in equilibrium, so that the time averages of kinetic and potential energies are changing slowly, the virial theorem implies that T = -(1/2)V. As the star shrinks, the energy is radiated away so that the above relation is valid. So, knowing the present luminosity of the sun, we can roughly find how long it can radiate at the present rate using this mechanism.

In fact, this was the theory proposed before nuclear reactions were known about.