by adrian_b | 2025-12-16 18:16:00+00:00
In a star, a huge number of reactions take simultaneously due to collisions between nuclei. Some collisions result in the fusion of lighter nuclei into a heavier nucleus, other collisions result in the fission of a heavy nucleus into lighter nuclei.

At iron 56 there is a peak in binding energy, both for lighter and heavier nuclei the binding energy is lower.

It is possible for nuclei with lower binding energy to form after a collision, but the probability for this to happen becomes lower and lower with decreasing binding energy.

Thus if one computes the probabilities of the reactions that happen during collisions one can compute the abundances of chemical elements that are reached when there is an equilibrium between the rates at which a certain chemical element is created and destroyed.

At this equilibrium, there is a maximum abundance for iron 56 and the heavier nuclei have abundances that decrease very quickly with the atomic number. For example, zinc may be 600 to 700 times less abundant than iron and germanium may be 7000 to 8000 times less abundant than iron.

Therefore, in an old star, which reaches equilibrium concentrations of elements, there are elements heavier than iron, but in extremely small concentrations, which become negligible for the elements much heavier than germanium.

Significant quantities of heavy elements cannot be produced by collisions between nuclei in a star, because they are destroyed in later collisions faster than they are produced.

So most of the elements heavier than germanium are produced by a different mechanism, i.e. by neutron capture, followed by beta decay. A small number of the heavy nuclei produced by neutron capture also capture protons after their formation, producing thus also some isotopes that are richer in protons.

In normal stars, the number of neutrons is negligible so neutron capture reactions do not happen often. On the other hand, some catastrophic events, like a supernova explosion or the collision between two neutron stars, can produce huge amounts of neutrons. In this case a lot of neutron capture reactions happens, exactly like on Earth during the explosion of a nuclear fission or fusion bomb.

These neutron capture reactions can produce all the chemical elements until fermium (Z=100), i.e. well beyond uranium. Heavier elements than that are not produced, because they fission spontaneously too quickly, before being able to capture other neutrons.

Of the trans-uranium elements, most decay very quickly, but plutonium 244 has a half-life long enough to reach other stellar systems, together with uranium, thorium, bismuth and all elements lighter than bismuth, except technetium and promethium (the latter 2 elements decay quickly, but technetium can survive for a few tens of millions of years, so small quantities of it may reach a nearby star, but they will disappear very soon after that; the elements between bismuth and thorium, and also protactinium, decay quickly and those that exist on Earth are recently created, through the decay of Th and U). The other primordial elements can survive many billions of years, but the amount of primordial plutonium becomes negligible after a few billions of years.