When this happens, it happens fast . Collapse halted by neutron degeneracy . The collapse releases an enormous amount of energy. We have a supernova . As the core collapsed, so did the envelope initially.
Core collapse generates a shock wave, which hits the envelope and generates neutrinos.
In about 2 hours the envelope itself gets ejected and expands away from the core, leaving the bare neutron star . Sometimes mass of core is so large that when it collapses gravity can overcome even neutron degeneracy and we wind up with a black hole. We see supernovae in other galaxies. The supernova is so bright it will actually outshine the combined luminosity of the galaxy sometimes. Fades with time, over a week or two until it fades back to nothing.