Stars will continue to forge elements into heavier elements in their cores via nuclear fusion until it is no longer energetically favorable for them to do so. The heaviest element that massive stars can create via nuclear fusion is iron. Once the core of a massive star has been entirely converted to iron, nuclear fusion cannot proceed further and there is no source of energy great enough to keep the star from collapsing under its own gravity. As a result, the outer layers of the star fall inward on the core and rebound, flying outward in a massive explosion called a supernova. Elements heavier than iron can be formed during this violent release of energy. The supernova generates huge shockwaves that propagate through the surrounding ISM, sweeping up material in front of them and heating and ionizing gas. Material from the outer layers of the star is also ejected into the ISM, enriching it in the heavier elements produced by nuclear fusion over the star’s lifetime. The next generation of stars to form in this region will therefore start out slightly richer in these heavy elements than their predecessors — so by measuring the amount of heavy elements present in stars, astronomers can infer the existence of prior generations of massive stars. All of the elements on Earth besides hydrogen (and some helium) were formed and released in this manner — so it is literally true that, as Carl Sagan said, “we are made of starstuff.”


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