On August 17, 2017, two neutron stars in the galaxy NGC 4993 collided after an inward spiral lasting millions of years. This event, named GW170817, was the first confirmed observation of a neutron star merger, with gravitational waves detected by LIGO and Virgo Observatory. Following the collision, telescopes worldwide observed an afterglow across the electromagnetic spectrum, providing insight into the origins of heavy elements like gold.
The merger produced heavy elements totaling about 6% of the solar mass, including approximately 200 Earth masses of gold. This event confirmed that neutron star mergers create heavy elements through a process called rapid neutron capture, or r-process, which requires conditions that exist temporarily during these catastrophic events. Ordinary stars cannot produce gold because nuclear fusion stops at iron; elements heavier than iron require the high neutron densities found in neutron star collisions.
Neutron stars are extremely dense remnants left after massive stars explode as supernovae, with densities comparable to atomic nuclei. In binary systems, they lose energy through gravitational waves, spiraling inward until they merge, which is characterized as a kilonova—brighter than a nova but dimmer than a supernova.
The gold present on Earth, created in events like GW170817, formed from clouds of interstellar gas enriched by such cosmic collisions long before our solar system was formed. These heavy elements were scattered across vast distances and later condensed to form the Sun and Earth, with much of Earth’s gold residing in its core.
While this discovery clarified some aspects of gold’s origins, questions remain regarding the overall abundance of heavy elements in the universe, with ongoing research exploring additional processes that may also contribute to their production. Ultimately, most of the gold on Earth was forged in violent cosmic events long before the formation of our solar system.
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