I've just watched the Extreme Stars program in How the Universe Works and it states that when a massive star starts to make iron, the fusion is quenched and the core collapses. From the program, it sounds like the process is fast and will happen when a few grams of iron is formed. I find this difficult to believe because our sun formed from dust from a supernova explosion, which contains iron (and other heavier elements). The incorporated iron should have prevented the sun from maintaining the fusion of hydrogen. I will agree that a large portion of the sun mass could have been collected before the sun encountered supernova dust, which will dilute the amount of iron. Why am I wrong and the sun is still burning?