This is a thread worth resurrecting.
To expand on
@Funkworks's post #91, I want everyone to look at this graph. It was first introduced to me in 10th grade chemistry and is one of the most fascinating things I have ever seen.
The X-axis of the graph is the number of nucleons (meaning protons and neutrons) in the nucleus of a given atom. The Y-axis is the binding energy per nucleon in MeV. The higher the binding energy, the more stable the atom is. The most stable atom, at the peak of the curve, is iron-56. Atoms are always going to want to become more stable, and will release energy when they become more stable. Becoming less stable requires the addition of energy and only happens under special circumstances.
First, this graph shows why nuclear fusion is so much more efficient than fission. When one atom is split or fused into different atoms, the relative amount of energy released between two different reactions can be seen in the difference in the Y-axis positions of the material you started with and its products. Compare the Y-axis difference between uranium-235 and the products left behind when it splits, which would be somewhere around and between the marked positions of molybdenum-96 and xenon-131, to the difference between hydrogen and helium, which would be used for fusion.
Now, here's what this has to do with space: This graph shows why Type 2 supernovae occur and why heavy elements are only formed naturally in such an extreme event.
Stars, as we know, are giant fusion reactors. Their existence is a battle between the nuclear fusion explosion in their cores trying to blow them apart and gravity trying to hold them together. When a giant star runs out of hydrogen, it starts fusing helium. When the star runs out of helium, if it is sufficiently massive to squeeze it enough, it starts fusing carbon. Note that when you get past hydrogen to helium, according to the graph, fusing heavier things yields less and less energy. This is why giant red stars near the ends of their lives are cooler than young, hot, white stars.
A star that is sufficiently massive can keep fusing things heavier than carbon until the core is finally made of iron, the most stable atom. No more energy can be extracted from fusion at this point. Fusion stops and the star's core suddenly collapses under gravity, and through a process that is not fully understood, some of the inward-falling material bounces back, producing the supernova explosion.
Because it requires
adding energy equivalent to the energy released from fission to fuse iron and anything heavier, the gargantuan pressures and temperatures produced by a supernova explosion is one of the few, if not the only, natural phenomena capable doing this. This is why we know that Earth and the entire solar system is formed partially from a supernova remnant - if it was not, there would be no heavy elements in the solar system, let alone on Earth.
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