Nuclear fusion ends at iron

An atom is made up of a nucleus and electrons, while the nucleus is made up of neutrons and protons. For an atom, the number of protons in its nucleus determines the type of element it is. For example, the element 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. Of course, after all, protons are positively charged, and they don't like each other, which will create a strong repulsive force, 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, 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? One common mechanism is nuclear fusion.

In simple terms, nuclear fusion is lighter atomic nuclei under high temperature and pressure polymerization of heavier nuclei, each star in the universe is a natural "nuclear fusion reactor", under the squeeze of its gravity, the core of the star will form a high temperature and pressure environment, thus providing the conditions for nuclear fusion.

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, our solar system "next door" Proximal, is a very low mass of red dwarf stars, such stars can only fusion of hydrogen into helium, and the sun is like a yellow dwarf, not much better, the sun's life, it can only fusion of an atomic number of 8 oxygen elements.

Only stars of sufficient mass can start rounds 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 iron.

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

The energy released by nuclear fusion is an important basis for stars to maintain their stability, and for those huge stars in the universe that can fuse iron, their gravity is very large. 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" is the capture of neutrons by the atomic nucleus, for easy understanding, we can imagine the nucleus as a "food", in the environment of neutron radiation, these eaters may " 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 its nucleus will decay into a proton and release an electron and an antineutrino, and its atomic number will be increased by 1, and then it will become a cobalt-59 nucleus.

Compared to the iron-56 nucleus, the cobalt-59 nucleus is much less "digestible", and after it "eats" a neutron, it becomes the unstable cobalt-60, so it also undergoes beta decay, and its atomic number is increased 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, in all the known elements in the universe heavier than iron, the contribution of "slow neutron capture" is not very large, and it is the "fast neutron capture" that produces a large number of such 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 centimetre 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 undergo various decays, and when everything calms down, a lot of elements heavier than iron appear in the universe.