Gold is forged when dead stars collide

The collision of two neutron stars, watched in 2017, produced a cloud of the heavy elements. Gold is in that family. How much of Earth's gold came that way is still a share, not a census.

Building a nucleus past iron takes neutrons, added quickly, before the nucleus can decay. That r-process needs a place denser in neutrons than an ordinary stellar furnace. Neutron stars are one such place, and when two of them merge they throw a fraction of themselves out at a large fraction of the speed of light. In August 2017 detectors caught the gravitational waves of such a merger, and telescopes caught the flare. The light changed over days in the way models predict if the cloud is laced with lanthanides and other r-process nuclei. Pian, Kasen and others read the spectrum as the first time that forge had been seen working.

Seen working is not the same as a ledger of the jewellery on Earth. Some gold may come from rare supernovae, or from the collapse of a single neutron star. The 2017 event showed that mergers make the heavy elements in quantity, enough that they are a leading source, and perhaps the main one. It did not weigh the ring on a hand and assign each atom a birthplace. The colour of gold, elsewhere in this library, is a fact about electrons and relativity. This is a fact about where the nucleus came from. They are the same metal and different sentences.