All 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 10^20 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.

In 2016 a tiny, faint galaxy, a satellite of the Milky Way called Reticulum II (Ret II), provided evidence that the supernova-explosion scenario that had long been favored could not be the main mechanism for the production of the heaviest elements. Instead, the chemical composition of the stars in Ret II strongly suggests that neutron-star mergers are the universe’s way to make elements such as gold and platinum. The neutron-star formation scenario is supported by striking observations reported in October of last year: the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo interferometer measurements of gravitational waves from the merger of a pair of mutually orbiting neutron stars1 and associated weeks-long outbursts of electromagnetic radiation pointing to a kilonova event (see references 2 and 3 and Physics Today, December 2017, page 19).

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