Where Did Earth's Gold Come From? What the 2017 Neutron-Star Collision Showed
What is new: on 17 August 2017 at 12:41:04 UTC, the LIGO and Virgo detectors recorded gravitational waves from two neutron stars spiralling together, an event named GW170817. About 1.7 seconds later a satellite saw a burst of gamma rays from the same patch of sky, and within 11 hours telescopes found a new glowing source in a galaxy about 130 million light years away. Over the following days that glow behaved the way theory predicted for a cloud of freshly made heavy elements, and in 2019 a team identified one of them, strontium, in its light. It was the first time anyone had watched heavy elements being made, and it is the main evidence behind the answer to "where does gold come from?"
This page explains the question, what the 2017 observations measured, how strong the link to gold actually is, and what scientists still argue about. Every number comes from the papers listed at the end.
Why gold needs a special explanation
Stars make most of the elements up to iron by fusing lighter nuclei in their cores. Beyond iron, fusion stops paying off, so heavier elements are built a different way: by adding neutrons to existing nuclei, which then turn some of those neutrons into protons through radioactive decay and become new elements.
There are two main versions of this. In the slow process, nuclei capture a neutron now and then, inside ageing stars. In the rapid process, called the r-process, a flood of neutrons hits nuclei so fast that they swell far beyond their stable form before decaying. Gold and platinum are mostly made the rapid way.
The 2019 strontium paper puts the scale plainly: half of all the elements in the universe heavier than iron were created by rapid neutron capture. The theory was worked out about six decades earlier, but it needs an enormous flux of neutrons, and where that happens was, in the paper's words, "still debated". Two candidate sites had been argued over for years: certain rare supernovae, and collisions of neutron stars.
What was detected in 2017
The LIGO and Virgo collaborations reported the event in Physical Review Letters. Their headline measurements:
| Measurement | Reported value |
|---|---|
| Time of merger | 17 August 2017, 12:41:04 UTC |
| Signal strength | Combined signal-to-noise ratio 32.4; false-alarm estimate less than once per 80,000 years |
| Masses of the two objects | 0.86 to 2.26 solar masses each (1.17 to 1.60 if spins are like known neutron-star pairs), total about 2.74 solar masses |
| Distance | About 40 megaparsecs (roughly 130 million light years), with an uncertainty of +8 / −14 Mpc |
| Sky area | 28 square degrees (90% probability), the closest and best-located gravitational-wave source at the time |
| Gamma rays | Fermi detected GRB 170817A about 1.7 seconds after the merger |
Sources: LIGO Scientific Collaboration and Virgo Collaboration, Phys. Rev. Lett. 119, 161101 (2017); companion papers in Astrophysical Journal Letters 848.
The masses matter because they match known neutron stars, not black holes. A companion paper calculated that the chance of the gamma-ray burst lining up in time and position with the gravitational waves by coincidence was 5.0 × 10−8, which confirmed neutron-star mergers as a source of short gamma-ray bursts. The same 1.74-second delay let physicists show that gravity and light travel at the same speed to within about one part in 1015.
The glow that pointed to heavy elements
The multi-messenger paper, a joint report by the gravitational-wave collaborations and the many observing teams that followed up, describes how the light was found. A bright optical source, later catalogued as AT 2017gfo, was discovered in the galaxy NGC 4993 less than 11 hours after the merger by the One-Meter, Two Hemisphere team using the 1-metre Swope Telescope, and several other teams found it independently within an hour. Early ultraviolet observations showed a blue source that faded within 48 hours. In optical and infrared light it then reddened over about ten days. X-rays and radio waves appeared at the same spot about 9 and 16 days after the merger.
That sequence is what a kilonova was predicted to look like: heat from the radioactive decay of newly made heavy nuclei, shining first in blue and then in red as heavier, more opaque material dominates.
A modelling study published in Nature by Daniel Kasen and colleagues compared detailed kilonova models with the observed light. It argued that the source was indeed a kilonova and inferred two separate components of ejected matter: one made mainly of lighter r-process elements (atomic mass below 140) and one of heavier ones (above 140). Gold, at mass 197, belongs to the heavy group. From the ejected mass and an estimated merger rate, the authors concluded that such mergers are "a dominant mode of r-process production in the Universe".
The first named element: strontium
Models that fit a light curve are not the same as reading an element's fingerprint in a spectrum. In 2019, Darach Watson and colleagues re-analysed the kilonova's spectra and reported in Nature the identification of strontium, an element made by neutron capture. They note that when the spectra were first published there was "no robust identification of any element", only a broad match with radioactive heavy elements.
Their conclusion is direct: detecting a neutron-capture element from the collision of two neutron stars "establishes the origin of r-process elements in neutron star mergers", and shows that neutron stars contain neutron-rich matter.
So, is the gold on Earth from colliding neutron stars?
The careful answer has three parts.
- Gold comes from the r-process. This is not in dispute among the papers cited here.
- Neutron-star mergers definitely run the r-process. GW170817 showed it happening: a merger, ejected matter whose light matches heavy-element models, and an identified neutron-capture element.
- Whether mergers made most of the gold is still argued. Kasen's team concluded mergers are a dominant source. A 2019 Nature paper by Daniel Siegel, Jennifer Barnes and Brian Metzger argued that collapsars, the collapse of rapidly rotating massive stars, form similar accretion disks and could supply more than 80 per cent of the universe's r-process material. They point to stars rich in these elements in the dwarf galaxy Reticulum II, and to how europium built up relative to iron in our galaxy, as fitting rare supernovae better than mergers.
Because the Sun and planets formed from gas that earlier generations of stars had already enriched, the gold in Earth's rocks was made before the solar system existed, by one or more of these events. Which kind of event made most of it is the open question.
What the observations did not show
- Gold was not individually identified. Its presence is inferred from models of the heavy component of the ejecta. Strontium, a lighter element, is the one named in the spectra.
- One event is one event. Rates and total yields are estimated from a single well-observed merger. More detections are needed to pin down how often mergers happen and how much heavy material each one throws out.
- The amount of gold per merger is model-dependent. The papers infer ejected mass and composition by fitting models to the light, so the answer depends on the physics inside those models.
Common questions
How far away was the collision?
About 40 megaparsecs, roughly 130 million light years, in the galaxy NGC 4993. The light and gravitational waves that arrived in 2017 left it that long ago.
Did the merger make a black hole?
The Siegel paper describes the heavy r-process material from GW170817 as coming mainly from delayed outflows from an accretion disk around a newly born black hole, based on kilonova analysis. The LIGO detection paper itself reports the masses and calls the source a binary neutron star inspiral.
Why does the colour change from blue to red?
Matter rich in the heaviest elements absorbs blue light strongly, so it glows redder and for longer. The early blue light and later red light are why Kasen's team inferred two components, one lighter and one heavier.
Was the gamma-ray burst unusual?
The companion paper notes GRB 170817A was the closest short gamma-ray burst with a known distance, but between 2 and 6 orders of magnitude less energetic than other bursts with measured distances.
Have there been more events like it?
The 2017 papers describe GW170817 as the first observed binary neutron star inspiral. This article covers what those papers and the follow-up studies cited here report; it does not track later detections.
Sources and how to check
- LIGO Scientific Collaboration and Virgo Collaboration. "GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral." Physical Review Letters 119, 161101 (2017). DOI: 10.1103/PhysRevLett.119.161101 · arXiv: 1710.05832
- LIGO Scientific Collaboration, Virgo Collaboration and partner teams. "Multi-messenger Observations of a Binary Neutron Star Merger." Astrophysical Journal Letters 848, L12 (2017). DOI: 10.3847/2041-8213/aa91c9 · arXiv: 1710.05833
- LIGO Scientific Collaboration, Virgo Collaboration, Fermi GBM and INTEGRAL. "Gravitational Waves and Gamma-rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A." Astrophysical Journal Letters 848, L13 (2017). DOI: 10.3847/2041-8213/aa920c · arXiv: 1710.05834
- Kasen D. et al. "Origin of the heavy elements in binary neutron-star mergers from a gravitational-wave event." Nature 551, 80 (2017). DOI: 10.1038/nature24453 · arXiv: 1710.05463
- Watson D. et al. "Identification of strontium in the merger of two neutron stars." Nature 574, 497 (2019). DOI: 10.1038/s41586-019-1676-3 · arXiv: 1910.10510
- Siegel D. M., Barnes J., Metzger B. D. "Collapsars as a major source of r-process elements." Nature 569, 241 (2019). DOI: 10.1038/s41586-019-1136-0 · arXiv: 1810.00098
- Abstracts and bibliographic details checked on arXiv and the DOI records on 22 October 2026. The light-year figure is our conversion of 40 megaparsecs (1 Mpc ≈ 3.26 million light years).