The Universe is primarily hydrogen and helium: cosmic abundances. The Big Bang produced mainly hydrogen and helium with trace amounts of some lighter elements with no appreciable production of carbon, nitrogen, oxygen, iron, magnesium, silicon, ... , and other elements heavier than iron. So, where did these elements come from?
Nucleosynthesis
The pre-supernova star has an onion-skin structure -- an iron core surrounded by various layers of material. The prediction is that:
What about elements heavier than iron? Well, the SN outburst is a strong source of neutrons. This is a key point since there is no electrical barrier for the addition of neutrons to nuclei. This means that one can build up very massive elements (through the r-process and s-process) if there are sufficient neutrons. SN are good sites for high neutron fluxes.
Test of the Picture
SN1987A offered a nice test of this picture. It used to be thought that the light curves for Type I and Type II SN were grossly different in that the luminosity of Type II SN dropped off like bricks after the star had expanded greatly and cooled. It turns out that our ideas are changing (in no small part due to SN 1987A). We now envision that:
SN1987A is not the typical Type II SN, but it is clearly a Type II SN. However, it went into a state where it continued to shine much longer than one would have expected if it exploded (and was formed hot) and then just cooled and marched outward. The SN 1987A light curve looked like
A natural source for this energy is nuclear power. In particular, tapping the energy contained in the decay of radioactive elements. This picture works very nicely. We have that
An interesting by-product of this exercise is that it makes firm predictions for how much nickel (an iron peak element) and eventually iron is produced in the explosion. This calibrates the model and detailed predictions on the elemental yield of the explosion can be made! The results of SN1987A corrborate many assumptions we made about nucleosynthesis and also helped steer us into other promising directions.