The formation of the heaviest elements

A ll of the hydrogen and most of the helium in the universe emerged 13.8 billion years ago from the Big Bang. The remainder of the chemical elements, except for a tiny amount of lithium, were forged in stellar interiors, supernova explosions, and neutron-star mergers. Elements up to and including iron are made in the hot cores of short-lived massive stars. There, nuclear fusion creates ever-heavier elements as it powers the star and causes it to shine. Elements heavier than iron—the majority of the periodic table—are primarily made in environments with free-neutron densities in excess of a million particles per cubic centimeter. The free neutrons, if captured onto a seed nucleus, result in a heavier, radioactive nucleus that subsequently decays into a stable heavy species. The so-called slow neutron-capture process, or s-process, mostly occurs during the late stages in the evolution of stars of 1–10 solar masses (M⊙). But the s-process accounts for the formation of only about half of the isotopes beyond iron. Creating the other half requires a rapid capture sequence, the r-process, and a density of greater than 1020 neutrons/cm3 that can bombard seed nuclei. The requisite neutron fluxes can be provided by supernova explosions (see the article by John Cowan and Friedrich-Karl Thielemann, PHYSICS TODAY, October 2004, page 47) or by the mergers of binary neutron-star systems. The rapid neutron-capture process needed