s-process
/ ESS-process /
Fusion can build elements up to iron, but it stalls there — heavier atoms like silver, barium, and lead cannot be made by simply fusing nuclei together, because beyond iron, fusion costs energy rather than releasing it. So how does nature build them? One of the two main answers is the s-process: a patient, step-by-step assembly line in which a nucleus captures a stray neutron, then another, then another, slowly climbing the ladder of heavier elements. The 's' stands for slow, and slow is exactly the point.
Here is the trick. A free neutron has no electric charge, so it can drift into a nucleus without fighting the repulsion that blocks charged particles. When a nucleus catches a neutron it grows heavier; if that new nucleus is unstable, it will eventually undergo radioactive decay, turning a neutron into a proton and nudging the atom up to the next element. In the s-process the neutrons arrive so slowly — perhaps one every few years to thousands of years — that any unstable nucleus has plenty of time to decay before the next neutron arrives. So the process tiptoes along a stable, well-trodden path through the chart of nuclei. This happens mainly in the gentle, churning interiors of AGB stars.
The s-process is responsible for roughly half of all the elements heavier than iron in the universe — much of the strontium, barium, lead, and other heavy atoms found on Earth and in the stars. Astronomers can even watch it in action: the element technetium, which has no stable form and decays away in a few million years, has been detected in the atmospheres of AGB stars, proving it was made there only recently. Its partner is the r-process (rapid neutron capture), which makes the rest under far more violent conditions; together they account for the heaviest elements.
Barium stars are red giants oddly rich in barium and other s-process elements — but they are not making it themselves. They were once orbited by an AGB companion that produced these elements, spilled them across, and has since faded into a white dwarf, leaving its partner chemically marked for life.
The s-process builds about half the elements heavier than iron, one slow neutron at a time.
The s-process makes heavy elements by neutron capture, not by fusion — and it cannot create the very heaviest, most neutron-rich nuclei (like gold and uranium); those need the violent r-process. The two pathways make different, overlapping sets of elements.