If the final element to be fused in a star is iron, where does everything after that come from?
In: 154
anything heavier than Iron will be the result of a supernova
when the core collapses from the attempt to fuse into Iron it momentarily becomes able to to restart fusion, however by then its too late and the reaction is extremely violent resulting in the supernova.
Iron is the end state for nuclear fusion in the core of a star. Every element beyond iron is formed in the supernova that happens as the core of the star collapses, or in the other kinds of supernova. Some heavy elements are also formed when 2 neutron stars enter into a death spiral and collide making yet another massive explosion.
To put it simply everything beyond iron is made in massive explosions as stars die.
So, the idea here is that iron (specifically iron-56) has the lowest energy of any atomic nucleus. In other words, trying to fuse iron will be endothermic (absorbing energy) rather than exothermic (releasing energy) as is the case for lighter elements. So, by the time a star reaches iron, its core is no longer able to produce energy and so it collapses.
It’s long been thought that supernovas produced elements heavier than iron. So, while producing these heavier elements does absorb energy, the supernova provides enough energy to do this. More recent discoveries have suggested that collisions between neutron stars, The remnants of stellar cores following a supernova, are responsible for many of the heavier elements in the universe. These collisions take place in binary system consisting of two neutron stars orbiting each other other. They slowly spiraling towards each other and eventually meet.
Iron is the final fusion event that **produces energy**. But further fusion does happen.
There are, broadly, two main processes that produce heavy elements.
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The first is the [*s-process*](https://en.wikipedia.org/wiki/S-process) (s for “slow”, in that it takes thousands of years).
Inside the core of an old star, there are a lot of extra neutrons flying around. Sometimes a nucleus absorbs one, raising its mass number by one. If the new nucleus is unstable, it undergoes beta decay, converting one of its neutrons into a proton and raising its atomic number by one. This new nucleus can then absorb another neutron and repeat the process.
This continues until you reach lead and bismuth. The next element, polonium, is so unstable that it decays faster than this process can proceed, so it does not make any heavier elements. (Specifically, polonium-210 is. Polonium-209 is more stable, but isn’t produced by the s-process.)
This process does not produce energy, however. In fact, it consumes a little bit of it, and these sorts of reactions drain the lifetime of the star and ultimately cause its collapse.
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The second is the [*r-process*](https://en.wikipedia.org/wiki/R-process) (for “rapid”). The r-process occurs in a matter of seconds in the extremely exotic environment of a supernova or neutron star merger. The exact degree to which each of those two events contributes is an active area of research, but current physics seems to be leaning towards neutron star mergers being more important than previously thought.
In the r-process, nuclei are bombarded with such an insane number of neutrons that there’s no time for beta decays on normal timescales. To put a sense of scale to this, you need an amount of free neutrons comparable to the density of water, which is absolutely insane – this is about 13 orders of magnitude more neutron bombardment than occurs inside an active nuclear reactor.
Rather than the slow absorb neutron -> beta decay -> absorb another neutron approach of the s-process, these nuclei absorb neutrons until they physically cannot absorb another (the “neutron drip line”, which is very far on the neutron-rich side, far beyond the point at which things are wildly unstable to beta decay under less exotic conditions). These wildly unstable nuclei undergo very rapid beta decay essentially as fast as new neutrons are added.
This process occurs so fast that even for the wildly unstable elements between lead and uranium, there’s no time for their typical forms of decay. A half-life of hours means nothing when you’re adding neutrons many times a second; you need extraordinarily unstable nuclei to get decays on those timescales. That means that the r-process can run up the periodic table incredibly fast, producing superheavy elements (possibly some beyond the reach of human particle accelerators).
Those superheavy elements rapidly decay in the aftermath of a supernova (or if blasted off during a neutron star merger), and that decay is responsible for a lot of the light produced by supernovae. None of the superheavy elements survive long enough to be incorporated into the next generation of stars or their planets, but some of their stable versions and decay products – like uranium – are long lived enough to do so.
The star may eventually go super-nova, exploding in a massive burst of energy. Iron is as far as fusion goes while producing energy, but an energy source (eg: supernova explosion) can push fusion further along the periodic table.