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  phkahler
  Why is it said that it takes a supernova to make elements heavier than iron? You're not going to get iron-iron fusion, but what about proton-iron fusion or similar? Also, we can make reactors here on earth that convert Thorium into Uranium, and we can also make plutonium in a proper reactor. We mustn't confuse reactions useful for power production with reactions for element production right? Why can't a regular star produce some heavy elements?
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  jmyeet
  The Universe coalesced into hydrogen and helium from a quark-gluon plasma soon after the Big Bang. It's kind of staggering the sequence of events that occurred afterwards to bring us here.As many of us know, the fusion in stars produces elements as heavy as iron. It then takes explosions of those stars to scatter those elements into space, ultimately bringing them into the protoplanetary disc of a new star, such that it can form a planet in the right zone. That star then needs to live long enough and the system needs to be stable enough to produce complex life.But it gets worse because we obviously have elements heavier than iron. So stars of a sufficient size need to form such that when the stars die they do so in an even more violent fashion. The core needs to collapse into neutronium and the resultant supernova can produce heavier elements. They also come from neutron star mergers.So all the uranium we have on Earth came from such an event. Because of the nuclear decay chain we can estimate when this uranium was made and IIRC that's somewhere between 80 and 200 million years before the Earth formed.So this all had to happen sufficiently close to the Sun and that material had to be captured in the Sun's protoplanetary disc. We needed the right combination of elements to form a protective magnetic field and produce enough but not too much heat.We're going to keep discovering mechanisms like this and the importance of particular isotopes, events and things like how amino acids seem to form relatively easily (given the right elements are present), which itself is a consequence of CNO fusion.But also why did the Sun form at all? It has to be in a nebula of largely hydrogen and helium and something had to trigger that like the shock wave from a nearby supernova or neutron star or black hole merger.It's kind of why I think sentient life is incredibly rare.