= Solution
During <gravitational collapse>, nonspherical time-dependent multipoles can be lost in <gravitational radiation> or absorbed by the forming <black hole>. Matter remaining outside must ultimately escape or fall in if the final object is to be an isolated vacuum black hole. Assume that the classical exterior settles to a regular stationary asymptotically flat vacuum geometry, with a connected horizon and the regularity conditions of the <black-hole uniqueness theorem>. This settling assumption is physical input, rather than a consequence of charge conservation alone.
For a rotating remnant, the <black-hole rigidity theorem> supplies axial symmetry; the uniqueness theorem then identifies the exterior as <Kerr spacetime>. The nonrotating equilibrium case is <Schwarzschild spacetime>. Gravitational radiation carries energy and angular momentum but no net <electric charge>. In the intended neutral collapse, with no net charge carried away by expelled matter, charge conservation therefore leaves zero black-hole charge. Initial total neutrality alone would not prove this if oppositely charged matter were expelled; the two-parameter conclusion assumes the uncharged vacuum remnant just described. This leaves
$$
\boxed{M\text{ and }J,\qquad a=J/M,}
$$
as its two intrinsic continuous parameters. The final $M,J$ are what remain after escaping matter and radiation have carried off their shares; they need not equal the star's initial values. A spatial rotation may align the spin axis, and a translation or boost chooses the position and rest frame, so these do not add intrinsic parameters. All higher equilibrium multipoles are fixed by $M,J$; for example Kerr's mass quadrupole is $-J^2/M$. A regular Kerr black hole obeys $|J|\leq M^2$ in geometrized units. This is the content of the <black-hole no-hair theorem> in vacuum general relativity, with its stated hypotheses, rather than a statement about arbitrary matter theories or an unproved guarantee of dynamical settling. Hawking evaporation is ignored in this classical late-time equilibrium description.
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