The first detection of a neutron-star collision took 60 years after Margaret Burbidge and colleagues realized that the heaviest elements in the universe must be created in violent cosmic explosions. Gravitational waves observed from the collision by LIGO and Virgo in 2017 led to a worldwide hunt for the cosmic flash produced by the neutron star debris. The findings elevated neutron stars to be leading candidates to create Burbidge’s elements.
Some of the elements from Burbidge’s process are radioactive; they steadily decay after the initial explosion that created them. These elements act as omnipresent cosmic clocks that carry information about their origin. We found that such clocks, present in the solar system, point to a neutron star collision that occurred a mere 1,000 light-years from what is today the solar system, a distance that is 1 percent the size of the Milky Way. This single event is responsible for the creation of 0.3 percent of some of the heaviest elements found on Earth today, including gold, uranium and iodine.
Matter deposited by this single event is ubiquitous. Every human has an eyelash worth of matter in them from this collision, mostly in the form of iodine. A gold wedding ring contains 10 milligrams. A Tesla Model 3 has five grams. A nuclear reactor has 200 kilograms. Of the gold ever refined by humanity, 600 tons are from this single collision.
Neutron stars are ultracompact dead stars, formed when massive stars collapse under their own gravitational pull. They weigh as much as our sun, but their size is that of New York City. They are in effect a gigantic atomic nucleus, made up primarily of neutrons, hence their name.
Scientists long suspected that neutron stars occasionally collide with each other in the universe, even though definite proof was elusive. Freeman Dyson suggested in the early 1960s that two neutron stars may be closely orbiting each other; he was interested in the extraction of energy from such a system in humanity’s distant future. Russell Hulse and Joe Taylor were the first to detect two close-by neutron stars, in the 1970s. These neutron stars are set to collide in 300 million years; their distance is continuously decreasing as a result of the emission of gravitational waves.
The first hint of neutron-star collisions came as a serendipitous side effect of the cold war. The United States launched a network of military satellites, called Vela, to monitor the Soviet Union’s compliance with a nuclear test ban treaty during the 1960s. Instead of rogue detonations on Earth, the satellites detected mysterious signals from outer space. Some of these cosmic flashes of energetic photons, called gamma rays, later turned out to be telltale signs of colliding neutron stars billions of light-years away.
In 2017, the LIGO detectors in the U.S. and the Virgo detector in Europe observed a gravitational wave signal that was identified as a neutron star collision 130 million light-years from Earth, proving that such collisions do happen in the universe. This was a distant event, but the fact that LIGO and Virgo detected it after monitoring the sky for only about four months meant that neutron stars regularly collide. In our Milky Way galaxy, there is one collision every 100,000 years. If we had millions of years to wait, we would eventually encounter a collision by chance not too far from our solar system.
When and where neutron stars collide has particular importance for the birth of the solar system. Before the sun and the planets were formed, matter that eventually made them was in the form of a primordial cloud made of gas and dust. This cloud was many times greater than the solar system today, casting a giant cosmic net that collected atoms traveling between stars in the so-called interstellar medium. Some of the matter accumulated in this presolar cloud came from neutron star collisions, including most of the gold, platinum, uranium and other heavy elements we find on Earth today.
To reconstruct what happened, we can turn to radioactive isotopes that were produced by neutron stars and were deposited in the presolar cloud. While many of these isotopes have long since decayed, their presence in the early solar system is recorded in meteorites that were formed soon after the presolar cloud collapsed and began to form the sun and the planets.
The information encoded in radioactive isotopes is revealing. We found that the relative quantities of several short-lived isotopes with half-lives less than 100 million years is typical if they originate from neutron stars, but they are inconsistent with another kind of cosmic explosion: a supernova, produced by massive dying stars. Supernova explosions are 1,000 times more frequent in the Milky Way than neutron star collisions.
This means that the shortest-lived isotopes would be regularly replenished if produced by supernovae, making them certain to be present at the time of the solar system’s formation. For the rarer neutron star collisions, the shortest-lived isotopes are depleted soon after a merger, and stay depleted until the next. This means that it is probable that at the time of the solar system’s formation, this isotope will be depleted. The observed abundances of the short-lived curium-247 and iodine-129 isotopes in the early solar system show this depletion, ruling out supernovae.
At the time of Burbidge’s original work and in the decades that followed, supernovae were considered to be the main candidates for sources of the heaviest elements. This paradigm has been challenged by multiple lines of evidence, including the optical signal observed from the neutron star collision detected by LIGO and Virgo, and our work.
Radioactive isotopes also point to a collision near the presolar cloud. Comparing the measured quantities to numerical simulations of the Milky Way, we found that a single collision likely produced a substantial fraction of the short-lived elements of the early solar system. For example, curium, whose longest-lived isotope has a half-life of 15.6 million years, was mainly produced by a single collision.
Elements that come from neutron stars play important roles in our lives. They include valuable metals, such as gold and platinum, or elements essential to making modern electronic devices. Some of them, such as iodine, are critical to life itself. Had the rate of collisions been different, or had Earth been at a different part of the Milky Way, the abundance of neutron-star matter on Earth, and with it our environment, may have been very different.
With the advent of new types of observatories, such as those capable of detecting gravitational waves or neutrinos, our exploration of the universe increasingly relies on combining information from different cosmic messengers. In this new field called multimessenger astrophysics, radioactive elements found in the solar system are a valuable addition. There is much to learn about the most extreme cosmic events and how they shaped our environment here on Earth.