There are two main astrophysical processes that produce heavy nuclides, the s-process and the r-process. Both involve neutron capture onto stable nuclides followed by beta decay, but they differ greatly in terms of timescale. The s-process (s for slow) occurs in environments where there is a low but stable neutron flux, such that nuclei have a good chance of decaying before they capture another neutron. The r-process (r for rapid), on the other hand, happens when the neutron flux is so high that beta decay is slow compared to neutron capture. Consequently, nuclei get stuffed with as many neutrons as they can physically hold, until they undergo beta decay and can accept more neutrons.

The s-process is limited to the heaviest stable element, lead, because further neutron captures eventually produce polonium, the most stable isotope of which has a half-life of just over 125 years. Nuclei typically go several thousand years between neutron captures, so the s-process runs into a wall at polonium. The r-process generally produces the heaviest elements including the transuranics, and also the most neutron-rich isotopes of lighter post-iron elements.

The s-process occurs mostly in dying stars, where nuclei can hang around for a relatively long time in the stellar envelope before being lost through stellar winds, or shed as planetary nebulae. The r-process was originally thought to occur in core-collapse supernovae (ccSNe) but modelling suggests that it is unlikely to account for more than a small fraction of the r-process nuclides we observe. Instead, binary neutron star mergers are now the leading candidate for hosting the r-process.

Iron can fuse but the fusion absorbs energy instead of releasing it. So it only accelerates the collapse of the core.