The smoking gun of the r-process: first spectroscopic identification of the cosmic production site of r-process neutron capture

The rapid (r) neutron capture process that creates half of all the elements heavier than iron was first postulated six decades ago. But the event or events in which the r-process creates the heavy elements has only recently been confirmed spectroscopically. I present here the identification of the neutron-capture element strontium in the spectral series of the kilonova, AT2017gfo, emerging from the gravitational-wave–discovered neutron star merger, GW170817, the only kilonova where detailed spectra have so far been recorded. This identification of a neutron-capture element associated with the collision of two extreme-density stars firmly establishes the origin of r-process elements.