College Physics 10th Edition by Raymond Serway,Chris Vuille

Exercise 28

Imagine that the entire Sun collapses to a sphere of radius R g such that the work required to remove a small mass m from the surface would be equal to its rest energy mc 2. This radius is called the gravitational radius for the Sun. Find R g. (It is believed that the ultimate fate of very massive stars is to collapse beyond their gravitational radii into black holes.)