Niels Bohr Lecture by Alexander Ji, Kavli Institute
Title: On the Origin of the Heaviest Elements
An essential question in astrophysics is understanding the origin of the chemical elements that make up the world around us. Today, we know the answer to this question for nearly the entire periodic table, except for some of the heaviest elements. These elements are synthesized in the rapid neutron-capture process (r-process), but the astrophysical sites where this process occurs have been debated for over sixty years. The leading candidates are rare types of core collapse supernovae and neutron star mergers.
In 2017, the multimessenger detection of a binary neutron star merger provided the first direct evidence of r-process nucleosynthesis in the universe. For a time, this discovery appeared to settle the long-standing mystery of their origin. Recent developments have revived this debate, and it is currently unclear whether such mergers can explain all, or even most, of the r-process elements found in the universe.
In this talk, I will show how the elemental compositions of stars provide a roadmap for understanding the origin of the heaviest elements. Stars form from the ashes of earlier generations of stars, so their compositions preserve a record of when and where different elements were made.
By studying stars of different ages and in different galaxies, we can reconstruct the cosmic history of r-process nucleosynthesis. Stellar abundances thus reveal the relative importance of different r-process sources across cosmic time and make predictions about the properties of future directly observed r-process events. By combining stellar observations with recent advances in multimessenger astronomy, nuclear experiments, and theoretical modeling, we may finally be closing in on the origin of the heaviest elements.
About the speaker
Alexander Ji is an Assistant Professor of Astronomy and Astrophysics and Senior Member of the Kavli Institute for Cosmological Physics at the University of Chicago. His research in near-field cosmology uses observations of nearby stars to study the first stars and galaxies, the origin of the elements, the history of the Milky Way galaxy, and the nature of dark matter.
Alex did his BS in physics and MS in statistics at Stanford University, PhD in physics at MIT, and postdoctoral work as a Hubble Fellow and Carnegie Fellow at Carnegie Observatories before joining the University of Chicago in 2021. He is a 2025 Sloan Research Fellow.