On August 17, 2017, a remarkable event unfolded 130 million light-years away in the galaxy NGC 4993. Two neutron stars spiraled in toward each other and collided. This brief but powerful event generated gravitational waves that reached Earth, detected by the LIGO and Virgo observatories. This discovery, known as GW170817, was groundbreaking: it not only confirmed a neutron-star merger but also helped answer a long-standing question in astrophysics: where does gold come from?
A study published in Nature by researchers from UC Berkeley and Columbia University revealed that this collision produced about 6% of a solar mass in heavy elements. This translates to roughly 20,000 Earth masses of material, including around 200 Earth masses of gold and nearly 500 of platinum. The findings showed that neutron-star mergers can create vast amounts of gold—hundreds of times more than what exists on our planet.
Ordinary stars can’t make gold because their fusion processes stop at iron. They convert hydrogen into helium, and then into heavier elements up to iron. Beyond iron, fusion requires energy rather than releasing it. Some heavier elements form over time through a process known as the slow neutron capture, or s-process, but gold and other heavy elements need something different.
This “something different” is called rapid neutron capture, or r-process. In r-process nucleosynthesis, atomic nuclei absorb neutrons incredibly quickly, enabling the formation of heavy isotopes that eventually decay into stable heavy elements. Conditions for this process—high neutron densities sustained for brief moments—are rare in the universe. For many years, scientists speculated that neutron-star mergers were among the best candidates for generating these elements. GW170817 confirmed this, showing that these collisions produce heavy elements in significant quantities.
So, what does a neutron-star merger look like? A neutron star is the dense core left after a massive star explodes as a supernova. To give you an idea, just a teaspoon of neutron-star material weighs about a billion tons. When two neutron stars spiral toward each other, they lose energy through gravitational waves. In the final moments before collision, they can spin around each other more than 300 times per second, leading to what’s called a kilonova—a spectacular astronomical event.
A kilonova shines brightly, outshining typical novae but not as dramatically as supernovae. The light from these events comes from the radioactive decay of heavy elements ejected in the explosion. Researchers were able to observe this glow and confirm the presence of the r-process in action.
Now, let’s talk about gold. The gold in wedding rings or bank vaults on Earth was created long before our Solar System formed. Approximately 4.6 billion years ago, our Solar System developed from gas and dust enriched by ancient supernovae and kilonovae, including GW170817. The heavy elements, scattered throughout space from these explosive events, eventually coalesced into the Sun and planets.
Earth itself holds around 1.6 × 10²¹ grams of gold, primarily located in the core. Most gold accessible to us today comes from asteroid impacts that deposited fresh heavy elements after the Earth’s core had formed. Every gold atom has a rich history, spent billions of years drifting through space before becoming part of our world.
However, while GW170817 confirmed that neutron-star mergers form gold, it also raised more questions. Recent research, including a 2024 analysis by astrophysicist Ethan Siegel, suggests that neutron-star mergers may not account for all gold in the universe. Other events, like certain types of supernovae and magnetar flares, might also contribute to the creation of heavy elements.
In short, we now know that much of the gold on Earth was forged in ancient cosmic collisions. Yet, there’s still more to discover about the various processes that give rise to these precious elements. The story of gold is not just a tale of minerals; it’s a saga of the universe itself.
