![]() ![]() ![]() ![]() The plots also show estimates for the elemental yields in neutron-star mergers and supernovae. įigure 5 displays the abundances of the r-process elements europium and barium in Milky Way halo stars and dwarf galaxy stars, including those in Ret II. Those surveys expanded the number of recognized stars with iron abundances less than 1% of the Sun’s nowadays, many tens of thousands of such stars are known. But beginning in the 1980s, large-scale surveys began to identify the small population of iron-deficient stars that likely formed in the first billion years following the Big Bang. That long-term accumulation makes it difficult to extrapolate details of the individual nucleosynthesis events that contributed at various times. Unfortunately, the material from which the solar system formed included the products of some 8 billion years of chemical-element enrichment from previous generations of stars. ![]() the abundances of r-process elements heavier than iron had to be inferred from solar-system material, specifically meteorites and the solar atmosphere. When the r-process was first hypothesized six decades ago, 5 5. and Physics Today, December 2017, page 19). and associated weeks-long outbursts of electromagnetic radiation pointing to a kilonova event (see references 2 2. Abbott (LIGO Scientific Collaboration, Virgo collaboration), Astrophys. 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 stars 1 1. 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. 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. 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. Creating the other half requires a rapid capture sequence, the r-process, and a density of greater than 10 20 neutrons/cm 3 that can bombard seed nuclei. But the s-process accounts for the formation of only about half of the isotopes beyond iron. 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 ⊙). The free neutrons, if captured onto a seed nucleus, result in a heavier, radioactive nucleus that subsequently decays into a stable heavy species. 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. There, nuclear fusion creates ever-heavier elements as it powers the star and causes it to shine. Elements up to and including iron are made in the hot cores of short-lived massive stars. The remainder of the chemical elements, except for a tiny amount of lithium, were forged in stellar interiors, supernova explosions, and neutron-star mergers. All of the hydrogen and most of the helium in the universe emerged 13.8 billion years ago from the Big Bang. ![]()
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