Historically, it was pretty predictable: If there was a supernova in a given year, it was SN(that year). For example, SN1054 gave us the Crab Nebula. It wasn't like there were a lot of naked-eye supernovae (nor are there now) so the odds of seeing two in one year were low. read more
Eventually the implosion bounces back off the core, expelling the stellar material into space, forming the supernova. What's left is an ultra-dense object called a neutron star, a city-sized object that can pack the mass of the sun in a small space. There are sub-categories of Type II supernovas, classified based on their light curves. read more