Heavy elements form through neutron capture inside stars and explosive events. Two main processes build nuclei beyond iron: the slow process, or s-process, and the rapid process, or r-process.
The s-process takes place in stars during their later burning stages. Neutrons are captured one at a time. The nucleus then has time to decay before the next neutron arrives. As a result, the path stays close to the valley of stability.
This process produces many of the heavier elements found in the solar system. Examples include strontium, barium, and lead. The s-process occurs mainly in asymptotic giant branch stars.
The r-process works under very different conditions. It happens when the neutron flux is extremely high. Nuclei capture many neutrons in a short time, before they can decay. Consequently, they move far to the neutron-rich side of the nuclear chart.
After the neutron flux drops, these nuclei decay toward stability. This path creates the heaviest elements, including gold, platinum, and uranium. Likely sites include neutron star mergers and some types of supernovae.
The two processes therefore complement each other. The s-process explains many stable heavy isotopes. The r-process accounts for the most neutron-rich and heaviest nuclei. Together they shape the abundance pattern of elements heavier than iron.
Observations of stars and gravitational-wave events continue to test these ideas. They help scientists identify where each process occurs and how the heavy elements of the universe were made.