Typically, they occur when a massive star - more than eight times the mass of the Sun - runs out of fuel and collapses to form a hot relic called a neutron star.
Their extreme brightness allows them to be seen in distant galaxies.
But observers cannot pick up this optical emission until several hours or days after the explosion, so a supernova's first moments are shrouded in mystery.
I thought Proxima Centauri was the closest star at 4.2 light years. If it were to pop, it would take 4.2 years before we knew it, right?