  As a consequence of Birkhoff's theorem, the exterior gravitational field of a
spherically symmetric star or black hole is always given by the Schwarzschild
metric. In contrast, the exterior gravitational field of a rotating
(axisymmetric) star differs, in general, from the Kerr metric, which describes
a stationary, rotating black hole.
  In this paper, I discuss the possibility of a quasi-stationary transition
from rotating equilibrium configurations of normal matter to rotating black
holes.
