How were the heavy chemical elements beyond iron made in the Universe

Nuclear astrophysics, which is an interdisciplinary branch of nuclear physics (micro scale) and astrophysics (macro scale), addresses some of the most compelling questions in the universe. Research efforts have been devoted to many topics such as the origins of the chemical elements, the unique conditions of earth that makes life possible, and the formation and evolution of the sun, stars, and galaxies. Nuclear processes play an extremely important role in cosmic evolution after the Big Bang, and they are the only known mechanisms that synthesize heavy elements, in addition to providing the energy for stars to resist the force of gravity. Over the past 50 years, scientists have developed a deep understanding of Big-Bang primordial nucleosynthesis and the mechanisms for synthesizing heavy elements. However, the astrophysical models have yet to adequately reproduce the observed solar abundances of those elements beyond iron (referred to as ultra-iron elements). It is widely believed that these ultra-iron elements were primarily synthesized via the slow neutron capture process (s-process) and the fast neutron capture process (r-process). The fundamental s-process component is thought to originate from thermally pulsing low-mass AGB stars, with reaction pathways closed after the stable nuclides. About half of the heavy elements (up to bismuth) were produced via the s-process. The r-process is thought to occur in the explosive burning of core-collapse supernova and/or neutron-star mergers. Although the r-process site remains a mystery, experts believe that more than half of the ultra-iron elements (up to thorium and uranium) were produced via the r-process. In addition, there are 35 neutron-deficient stable isotopes, which are present in significantly less abundance in our solar system. These so-called p-nuclei are likely produced in Type II supernova, through the photo-dissociation (referred to as p-process or r-process) of existing s- or r-process seeds, or through the recently proposed neutrino-proton v p-process. Astrophysical models require a huge amount of nuclear-physics input data. The most essential data include nuclear mass, structure, decay and fission characteristics, and related nuclide cross-sections along the various nucleosynthesis paths. Thus far, the lack of systematic and precise nuclear inputs is one of the main reasons scientists have not been able to reproduce the observed solar abundances of ultra-iron elements. Published by Discover magazine in 2002, the American National Research Council ranked the question, "how were the heavy elements from iron to uranium made?" as one of the 11 Greatest Unanswered Questions of Physics in this Century . More complete and precise nuclear physics inputs are therefore urgently needed to improve astrophysical models and decode the observations. This paper introduces the most important nucleosynthesis processes that are responsible for the ultra-iron elements production in the universe, while also summarizing the relevant key nuclear-physics inputs required in the astrophysical models. Finally, the frontier in this field study includes the origins of ultra-iron elements, and the future of nuclear astrophysics research in China is examined.