This was the first first time modern instruments observed a core-collapse supernova. Neutrinos travel at essentially the speed of light but were observed from SN 1987a three hours before visible light. The difference is the time after iron core collapse for shock wave rebound to reach the surface of the star.
Neutrinos created during the iron core collapse fairly easily traverse through all the outer layers of the star, but light can not. Light is emitted only after rebound shock wave reaches the surface of the star.
Modeling indicates that 99% of the supernova explosion energy is radiated as neutrinos, but they are hard to detect. The "shower" observed on earth from three locations was a total of 25 neutrinos in two 2-3 second bursts.
In the three-ring picture shown here, the outer two rings were part of the progenitor star known as Sanduleak. It is the inner brighter ring that is being formed by shock wave collision of the recently expelled supernova materials hitting gasses out some distance from the star. This has been monitored for the 40 years since the explosion.
There is supposed to be a neutron star remnant left at the center of SN 1987a, but so far Hubble has not been able to detect any evidence of it. It's a mystery with only non-confirmable suggestions for an answer. See
Sky & Telescope