We Owe Our Lives to Colliding Neutron Stars

For hundreds of millions of years, a pair of neutron stars – the burned-out cores of once massive stars – spiraled closer and closer together until finally colliding in a tremendous explosion known as a “kilonova.” The force of this massive explosion — many times stronger than a nova yet not quite a supernova — produced rare chemical elements, including a few that are essential for life of Earth.

Astronomers recently spotted those elements, which shine with particular wavelengths of infrared light, in data gathered by the instruments aboard the James Webb Space Telescope (JWST). In March 2023, Radboud University astrophysicist Andrew Levan and his colleagues watched the kilonova, located roughly a billion light years away, as it faded from a bright burst of gamma rays to a steady infrared glow. They then used JWST’s Mid-InfraRed Instrument (MIRI) to measure the spectrum of light coming from the cloud of cosmic debris left in the explosion’s wake. The results of their study were published this week in the journal Nature.

This new study focuses on a particular wavelength of light that’s usually emitted by a heavy element called tellurium, which seemingly formed in the fires of the kilonova — and that tellurium is a clue that the kilonova also forged iodine, a mineral that us mammals can’t live without.

Epic Kaboom

Neutron stars are the burned-out cores left behind when massive stars burn up all their nuclear fuel and blast their outer layers into space during a supernova. What’s left collapses under its own gravity and ends up packed so tightly that atoms are crushed into smaller particles called neutrons. The only thing in the universe denser than a neutron star is a black hole. And as you can image, when a pair of neutron stars collides, the explosion is truly epic.

This particular kilonova was so epic that it blasted gamma rays, the highest-energy radiation in the universe, into space for more than three minutes. A kilonova usually spawns a gamma-ray burst, but it’s usually short: less than two seconds. Longer bursts more often come from giant supernovae.

“In principle, everything that happens in a binary neutron star merger occurs very quickly, so you shouldn’t be able to make a gamma-ray burst that lasts this long,” says Levan. “One possibility is that rather than two neutron stars merging, this is a neutron star combining with a black hole, where things take a bit longer. Alternatively, when the merger happens, we may make a hypermassive neutron star; this neutron star has a very high magnetic field and can pump energy into the burst for a long time.”

A third option, according to University of Birmingham astrophysicist and study co-author Ben Gompertz, is that the energy release that powers a gamma-ray burst may be less efficient than physicists thought, which means it could stretch out longer.

“Between that and the two alternatives [Levan] outlined, something new and very exciting is going on,” Gompertz says.

A Cosmic Death Spiral

Before their dramatic demise, this pair of neutron stars had apparently flung themselves – or each other – out of their home galaxy. Levan says the two doomed neutron stars were once a cosmic power couple: a pair of bright massive stars, orbiting a common center of gravity. Their lives ended in fiery supernovae.

“Each supernova is a very violet event, imparting a ‘kick’ to the binary system. In many cases, this kick forces the two stars in the binary apart, but when it doesn't, the binary gets a substantial velocity, perhaps hundreds of kilometers per second,” says Levan. The researchers traced the neutron stars back to their original home by comparing their chemical fingerprint to that of the host galaxy using JWST data.

Trapped in each other’s inescapable gravity, they danced a slow death spiral as they sailed together 120,000 light years into space, which is about the whole length of the Milky Way. It took hundreds of millions of years for the two stars to finally fall into each other in a violent finale — and that finale forged some rare but crucial elements.

Chemistry ... IN SPACE!

If you want to make tellurium, here’s how. First, form a couple of truly massive stars, and then wait a few billion years for them to go supernova. Next, smash the neutron stars together.

“All the neutrons in the neutron star get thrown out and caught by atoms to make a set of unstable elements,” Levan tells Inverse. “These decay to make the stable elements.” Each of the unstable elements Levan mentioned can decay into several different stable elements, so the same process that makes tellurium also makes iodine.

Chemists had suspected for decades that tellurium formed when atoms captured stray neutrons, but nobody was sure exactly what kind of cosmic event would create the right conditions for that process to happen. The answer? Kilonovae.

“This is the first time we’ve seen a significant amount [of tellurium] being made, and it’s helping to bridge the gap between what we see in the world around us and what we know the universe can provide,” Gompertz tells Inverse.

What’s All the Fuss About?

Without iodine, vertebrates (animals with backbones) here on Earth would be in big trouble. It’s an essential ingredient in thyroid hormone, which controls the body’s metabolism. Knowing where our planet’s supply of iodine came from billions of years ago, is another piece in the story of how life emerged and evolved.

Tellurium, meanwhile, is a delightfully weird metal that’s used in some solar panels, along with various other technologies. We don’t have much tellurium on Earth because it tends to bind with hydrogen to form a compound that evaporates into space very easily during the turbulent process of planet formation. Unlike iodine, tellurium is poisonous in the human body and even makes your breath smell like garlic.

So while its true we’re all made of star stuff, don’t forget the dead star stuff provided by cataclysmic mergers between dense, and ultimately doomed, neutron stars.