efore I sketch the proof of the virial theorem, let's consider the simplest possible case: a single light particle in circular orbit around a heavy one. Say the light one has mass m and the heavy one has mass M. And suppose the orbit has radius R. Then the potential energy is
V = -GmM/R (1)
where G is Newton's constant. To figure out the kinetic energy, remember that the gravitational force is
Fgrav = -GmM/R2
while the centrifugal force is
Fcentrif = mv2/R
In a circular orbit these counteract each other perfectly, so we must have
mv2/R = GmM/R2
Thus the kinetic energy of the light particle is
T = mv2/2 = GmM/2R (2)
while the kinetic energy of the heavy one is negligible. Comparing (1) and (2), we see that
T = -V/2
just as the virial theorem says!
The virial theorem lets us generalize this fact to arbitrary gravitationally bound systems. Of course, in a more general system of this sort - even a particle in an elliptical orbit - the kinetic and potential energy change with time. That's why the virial theorem refers to time averages of the kinetic and potential energy. But the basic idea is the same. And the proof is surprisingly simple.