Neutron stars can be used to study the origins of heavy elements because the expansion of material after a kilonova allows neutrons to decay and combine into ordinary matter.

“Once the kilanova has exploded, maybe some of it fallen into a black hole, but some of it's been blown off, that expanding matter starts going down in density. And so the free neutrons inside the neutron star start combining with each other. Some of them start decaying into protons.”