Nuclear fusion ends in iron

An atom is composed of a nucleus and electrons, while a nucleus is composed of neutrons and protons. For an atom, the number of protons in its nucleus determines the type of element it is, for example, hydrogen with atomic number 1 has only one proton in its nucleus, helium with atomic number 2 has two protons in its nucleus, and so on for other elements.

So theoretically, as long as we can keep adding protons to a nucleus, it will become a heavier and heavier element, but of course, after all, protons are positively charged, and they do not like each other and will generate strong repulsive forces, so we need to add a certain number of neutrons to maintain the stability of the nucleus while adding protons.

This kind of thing is easy to say, but the actual operation is quite difficult, so much so that humans with modern technology can not do it, but humans can not do it does not mean that the universe can not do it, otherwise, the universe can not exist in a variety of elements, that the universe is how to do it? A common mechanism is a nuclear fusion.

In simple terms, nuclear fusion is the aggregation of lighter atomic nuclei into heavier nuclei at high temperatures and pressure.

The higher the atomic number of the nucleus, the higher the conditions for nuclear fusion, and the temperature and pressure of the core of the star are proportional to the mass of the star, so the lower mass of the stars in the universe is not fusion out of anything fancy.

For example, the star next door to our solar system, Proxima, is a very low mass red dwarf star, which can only fuse hydrogen into helium, and a yellow dwarf star like the Sun, which is no better, can only fuse oxygen with an atomic number of 8 for the lifetime of the Sun.

Only stars of sufficient mass can start round after round of nuclear fusion reactions at their cores to produce heavier and heavier elements, but even such stars cannot fuse all the elements known in the universe, because the fusion of stars ends at the atomic number 26 of the element iron.

In other words, at the core of the star, iron is the end of nuclear fusion. Why is this so? Because the fusion of iron nuclei does not release energy but absorbs it.

For those huge stars in the universe that can fuse iron, their gravity is very large, if the core of the fusion does not release energy, then these stars will be directly "collapsed" by their gravity, and then This is also known as a supernova explosion.

So the question arises since nuclear fusion ends at iron, how do elements heavier than iron in the universe come about? The answer is "neutron capture".

As the name suggests, "neutron capture" means that the nucleus of an atom captures neutrons, so for the sake of understanding, we can imagine the nucleus of an atom as an "eater". In an environment with neutron radiation, these eaters may "eat" some neutrons that come to them, but their "digestive capacity" varies from large to small, some can "eat" several neutrons, while some "eat" only one neutron and "eat" it. Some of them can "eat" several neutrons, while others "eat" only one neutron and then "indigestion".

For example, if a Fe-56 nucleus "eats" a neutron, it becomes Fe-57, and since Fe-57 is a stable isotope, it will be fine. The next time, if it "eats" another neutron, it becomes Fe-58, which is still a stable isotope, so it still doesn't matter.

If it "eats" another neutron, it becomes the unstable Fe-59, and then it has "indigestion". In this process, a neutron in the nucleus decays into a proton and releases an electron and an antineutrino, which adds 1 to its atomic number and turns into a cobalt-59 nucleus.

Compared to the iron-56 nucleus, the cobalt-59 nucleus is much less "digestible" and after "eating" a neutron, it becomes the unstable cobalt-60, so it also undergoes beta decay and its atomic number is added by 1 again, and then The nickel-60 nucleus has a strong "digestive capacity", and only after it "eats" three neutrons in a row will it undergo beta decay and become a copper-63 nucleus... ...

The "neutron capture" described above usually takes place in the interior of stars, where the neutron radiation is relatively weak and the efficiency of producing heavy elements is relatively low, so this is also called "slow neutron capture".

There is "slow neutron capture" in the universe, and of course, there is also "fast neutron capture", but, of all the elements known to be heavier than iron in the universe, the contribution of "slow neutron capture" is, of all the elements known to be heavier than iron in the universe, the contribution of "slow neutron capture" is not very large, and it is "fast neutron capture" that produces a lot of these elements.

When high-energy events such as supernova explosions and neutron star collisions occur in the universe, they create an environment of extremely high neutron radiation in a short period, which can be as high as 100 trillion neutrons per cubic centimeter per second.

In such a high neutron density environment, "fast neutron capture" occurs, where the lighter nuclei "eat up" and then suffer severe "indigestion", so that They then undergo various decays, and when everything subsides, a large number of elements heavier than iron appear in the universe.