W. Baade and F. Zwicky predicted that this collapse strips the atoms of their electrons, packing the nucleii together as a neutron star . These are only 10-15 miles in diameter, with densities on the order of a billion tons per cubic inch. Such discoveries redirected attention to the Schwarzschild model of the exterior of a star. The families of radially ingoing and outgoing light rays in the Schwarzschild model show that it has what is now called Petrov type D . Petrov's classification was slow in making its way into the mainstream, but was crucial to the next major development.
All stars rotate.
For a very slowly turning star like our sun this is not very important, but when a star collapses, conservation of angular momentum implies that its rate of rotation increases. Thus a neutron star, for example, can be expected to rotate at a fantastic rate. It is believed that pulsars are rapidly rotating neutron stars. But the star producing Schwarzschild spacetime does not rotate. In l915 this static model had been found in only a few weeks, so it must have been considered fairly easy to set it spinning.