That could mean one of two things. Supernovas (and all other stars) do not make heavier elements or there is some mechanism by which heavier elements break down to simpler ones.
If stars do not make heavier elements then where do they come from?
Heavy elements like uranium break down in processes like radioactivity/fision but how does lighter ones do? Could it be that all elements (bigger than hydrogen) eventually break down? If so how long does it take? Surely it must be the same as the rate at which they are created or the universe will eventually fill up with heavier elements or run out of it. In an infinite age universe there must be some balance or the scales will tip towards the one side and stay there.
The generation of heavy elements is only guessed to be from what goes on in stars and SN events. The reverse process that transforms elements is not known either. All the modeling misleads everyong into believing we really know a lot about these processes when in truth all we really know is they happen.
Radioactive decay is responsible for breaking down larger elements into smaller one, i.e. Uranium to lead to Helium (with intermediaries). The process of decay is rather well documented as opposed to the creation of atoms larger than iron.
The fission decay process is all that is well known about how the elements transform and that is not a lot compared to what is not known. There is nothing known about how iron is made and all the modeling reguarding how iron gets that way is science fiction or silly science. The belief that fusion makes sense because it is about as logical a fiction story as anything writen. Where is the data that should be handy after all the time and money spent trying to get fusion to work?
