On 17 August 2017, two neutron stars in a galaxy called NGC 4993, about 130 million light-years from Earth, completed a spiral inward that had taken them millions of years and ended in a collision lasting fractions of a second. The gravitational waves from that collision reached Earth and were detected by the LIGO and Virgo observatories. Within hours, telescopes around the world had identified the afterglow of the event across the electromagnetic spectrum, from gamma rays to radio waves. The event, designated GW170817, was the first confirmed observation of a neutron-star merger. It also resolved one of the longest-standing open questions in astrophysics: where the universe’s gold comes from.
According to the 2017 paper in Nature by Daniel Kasen of UC Berkeley, Brian Metzger of Columbia, and colleagues, the GW170817 event produced and ejected heavy elements totalling approximately 6 percent of a solar mass — roughly 20,000 Earth-masses of material — including about 200 Earth-masses of gold and nearly 500 Earth-masses of platinum, plus comparable quantities of uranium and other elements heavier than iron. The team’s models, developed over the preceding decade in anticipation of exactly this kind of observation, matched the optical and infrared afterglow of the event with sufficient precision to characterise the composition of the ejected material. The colliding neutron stars had assembled, in the violence of their merger, hundreds of times more gold than exists in the entire mass of Earth.
Why ordinary stars cannot make gold
The elements heavier than iron — including silver, gold, platinum, lead, mercury, and uranium — present a problem for the standard theory of stellar nucleosynthesis. Ordinary stars fuse hydrogen into helium, then helium into carbon, and continue fusing progressively heavier elements all the way up to iron. The process releases energy, which is what makes stars shine. But fusion stops working at iron. Combining iron nuclei into heavier elements requires an input of energy rather than producing one, so ordinary stellar fusion cannot proceed past iron. Some heavier elements form slowly in giant stars via a process called slow neutron capture, or s-process, which can build elements up to bismuth. The heaviest elements, including gold, require something else.
The “something else” turned out to be a process called rapid neutron capture, or r-process. In r-process nucleosynthesis, atomic nuclei are bombarded with so many free neutrons in such a short time that they absorb neutrons faster than they can decay, building up to extremely heavy and neutron-rich isotopes which then beta-decay into stable heavy elements. The conditions required — enormously high neutron densities, sustained for fractions of a second — exist almost nowhere in the universe. For decades, the leading candidates were rare types of supernovae and the collisions of compact stellar remnants. The 2017 detection settled at least part of the question. Neutron-star mergers really do produce heavy elements via r-process nucleosynthesis, and they do so in quantities sufficient to enrich entire galaxies.
What a neutron-star merger looks like
A neutron star is what remains when a massive star runs out of fuel, collapses, and explodes as a supernova, leaving behind a dense core. A typical neutron star contains roughly 1.4 times the mass of the Sun, compressed into a sphere about 10 kilometres across. A teaspoon of neutron-star material weighs roughly a billion tonnes. The density is comparable to that of an atomic nucleus, because the star is essentially a single giant nucleus made of densely-packed neutrons. When two neutron stars exist in a binary system, they slowly lose energy through gravitational wave emission and spiral inward over geological timescales. The final phase of the inspiral, when the two stars are within a few kilometres of each other, can complete in fractions of a second. The GW170817 neutron stars were spinning around each other more than 300 times per second in the final moments before merger.
The collision itself is described in the literature as a kilonova — a term coined by Brian Metzger and colleagues in 2010, who calculated that the light from a neutron-star merger would be approximately one thousand times brighter than a typical nova explosion but much fainter than a typical supernova. According to Lawrence Berkeley National Laboratory’s coverage of the GW170817 detection, the radioactive decay of the freshly synthesised heavy elements in the ejected debris is what makes a kilonova glow. The team’s models had predicted that this glow would be “tinged red if heavy elements were produced,” distinctive enough to identify the kilonova by its colour signature. The 2017 observations matched the predictions in detail, providing the first direct spectroscopic evidence of r-process nucleosynthesis as it happens.
What this means for the gold on Earth
The gold in any wedding ring, any coin, any bar of bullion sitting in any bank vault on Earth, was produced in events like GW170817 that occurred long before the Solar System existed. The Sun and its planets formed approximately 4.6 billion years ago, from a cloud of interstellar gas and dust that had been enriched, over previous billions of years, by the ejecta of supernovae and kilonovae from the prior generations of stars in the Milky Way. The heavy elements in that cloud, including all the gold, had been scattered across hundreds of light-years by the violence of their original production. The cloud collapsed under its own gravity, the Sun formed at the centre, and the remaining material accreted into the planets. Earth inherited its share of pre-existing heavy elements from this enriched cloud.
The Earth as a whole contains roughly 1.6 × 10²¹ grams of gold, most of it in the planet’s core, where it sank during the molten phase of Earth’s early history. The gold accessible at Earth’s surface, and therefore the gold that has been mined throughout human history, represents a small fraction of the total — itself the result of a late veneer of asteroid impacts that delivered fresh heavy elements to the crust after the core had finished forming. Every gold atom in human possession spent billions of years in interstellar space before it became part of Earth, and was produced billions of years before that in the collision of two dead stars somewhere in the early Milky Way or one of its progenitor galaxies.
The 2017 confirmation also left an open question, which is still under active investigation. According to a 2024 analysis by the astrophysicist Ethan Siegel, the rate of observed neutron-star mergers may be too low to fully account for the abundance of gold and other heavy elements in the present-day universe. Other mechanisms — including a rare type of supernova called a collapsar, in which a massive star’s core collapses directly to a black hole, and magnetar giant flares, in which the magnetic fields of highly magnetised neutron stars rearrange catastrophically — may contribute additional r-process production. The 2017 event confirmed that neutron-star mergers produce gold. It did not settle whether they produce all of it. What is settled is that most of the gold on Earth was forged in events of cosmic violence whose like has not been seen near our solar system since long before our solar system existed.
