The three lightest elements in the universe, hydrogen, helium and lithium, originally formed in the Big Bang. The next two, beryllium and boron, are primarily produced by cosmic ray interactions with the interstellar medium. The bulk of the next two dozen heavier elements come from ordinary stars and supernovae. But where do heavier elements, such as gold, platinum and lead, come from? Most of the heavier elements had to be synthesized in neutron capture processes, either slowly (the s-process) or rapidly (the r-process); each is responsible for about half of all isotopes heavier than iron. The source of s-process elements, such as copper, zinc, tin and lead, are thought to be stellar winds from evolving low- to medium-mass stars (up to about 8 solar masses). The source of the r-process elements, such as gold, platinum and uranium, is controversial. Since the 1950s, the prevailing theory has been supernovae. However, the r-process requires ejected matter that is more neutron-rich than computations indicate. In the mid-1970s, my advisor and I proposed a speculative alternative theory, that they are produced in matter ejected from neutron stars merging with neutron stars or black holes. In the last five years, evidence has been accumulating in favor of the neutron star theory. The evidence comes from observations ranging from spectra of the most metal-poor stars, studies of ultra-faint dwarf galaxies, and afterglows from short gamma-ray bursts (which may be mergers). Theoretical support stems from numerical simulations of both mergers and cosmological studies of galaxy formation. In the near future, mergers involving neutron stars might even be observed by gravitational wave observatories, which could vindicate or refute this theory.
Prof. Lattimer is a Distinguished Professor in the Dept. of Physics & Astronomy at Stony Brook. His interests lie mostly in nuclear astrophysics, especially with theories of dense matter and neutron stars.