FWIW: Sagan was poetic but the intuition turns out to have been wrong. Models are still not super convincing, but supernovae alone aren't enough to explain the abundance of heavy elements. A big chunk of the rare earth nuclei we apply in industry today seem likely to be tiny remnants of neutron stars that were thrown off in a merger event with another neutron star or black hole.
Thanks for bringing this up! I was always curious and decided to research it. The graphic at the top of this Phys article describes the origins of all elements on the periodic table. It claims that only a small number of elements were remnants of neutron stars, however. https://phys.org/news/2020-09-elements-neutron-stars-contrib...
>Half of all the elements that are heavier than iron—such as thorium and uranium—were thought to be made when neutron stars, the superdense remains of burnt-out suns, crashed into one another. Long theorized, neutron star collisions were not confirmed until 2017. Now, however, fresh analysis by Karakas and fellow astronomers Chiaki Kobayashi and Maria Lugaro reveals that the role of neutron stars may have been considerably overestimated—and that another stellar process altogether is responsible for making most of the heavy elements.
