How are gold, silver and bronze created in the universe?
Copper, which is the primary component of bronze, is created during supernova explosions at the end of the lives of massive stars, using existing nitrogen as a seed. In astronomy, elements heavier than helium are called ‘metals’, including nitrogen. It becomes harder to form copper in ‘metal-poor’ stars that were born much earlier than the Solar System.
Silver is heavier than copper, and contains more neutrons relative to the number of protons in its nucleus. Therefore silver should be created by an extra process, called ‘neutron capture’, during supernova explosions. Gold atoms are much heavier and harder to make than silver and copper, and can be made through two different astronomical events. The first one is a neutron star merger; a neutron star is a high-density star made of neutrons. If two such stars collide with each other, it is possible to form very neutronrich elements such as gold. The neutron star merger was observed with gravitational waves in 2017.
In our recent study, we found that this mechanism alone is not enough to account for all the gold seen in the universe. Another production site of gold is rare spinning massive stars which cause jet explosions powered by magnetic fields. We also found, however, that even including both these production sites it is still not possible to explain the observed amount of gold. To solve this mystery, we need more nuclear experiments, astronomical observations of metal-poor stars and computer simulations of stars and supernovae.