Birkhoff's Theorem proves qualities about General Relativity
that demonstrate its relation to Newtonian Gravity,
i.e., conditions under which the two converge.
One consequence is that under GR, isolated, non-rotating,
spherically-symmetric masses are equivalent to gravity
from a point mass, analogous to what Isaac Newton
proved about Newtonian Gravity to make it practical to apply
it to the dynamics of the Solar System.
A corollary to this is that a spherically symmetric pulsation
(e.g., a star's pulsating that happens to retain perfect
spherical symmetry of its mass) creates no Gravitational Waves.