James Webb Space Telescope Finds Tellurium in a Kilonova from Colliding Neutron Stars

The kilonova blast produced when two neutron stars collided a billion light-years from Earth appears to have acted as a production site for rare heavy elements.

For the first time, the James Webb Space Telescope has examined an event of this kind. Following an enormous gamma-ray burst detected on 7 March 2023, its observations found signs of tellurium – a scarce metal that is too heavy to be created by stellar fusion.

The data also hinted at further metals, including tungsten and selenium. According to the researchers, the finding verifies neutron star mergers as a source of heavy elements, helping to explain how the Universe produces matter and distributes it through space.

"There are only a mere handful of known kilonovas, and this is the first time we have been able to look at the aftermath of a kilonova with the James Webb Space Telescope," says astrophysicist Andrew Levan of Radboud University, who led the analysis.

He adds, "Just over 150 years since Dmitri Mendeleev wrote down the periodic table of elements, we are now finally in a position to start filling in those last blanks of understanding where everything was made."

How stars and kilonovae create heavy elements

Stars are remarkable objects. They repeatedly fuse together the hydrogen that accounts for most of the Universe's visible matter, producing heavier elements: hydrogen becomes helium, before those heavier atoms are combined into progressively heavier ones, up to iron.

At that point, however, a star's fusion machinery can go no further. Fusing iron into heavier elements consumes more energy than it produces, putting the star on course to explode beneath its own gravitational weight.

That powerful explosion can nevertheless trigger a chain of nuclear reactions, in which atomic nuclei strike free neutrons and form still heavier elements.

These reactions must occur rapidly enough that radioactive decay cannot take place before additional neutrons join the nucleus. They therefore require environments containing large numbers of free neutrons, such as supernovae or kilonovae. This form of nucleosynthesis is called the rapid neutron-capture process, or r-process.

When astronomers first saw two neutron stars collide in 2017, the aftermath established that kilonovae make r-process elements. They identified strontium, the 38th element in the periodic table.

James Webb Space Telescope detects tellurium in GRB230307A

When the gamma-ray burst GRB230307A was seen flaring in March of this year, scientists promptly began examining it in greater detail. GRB230307A was extraordinarily impressive: among the brightest gamma-ray bursts ever recorded, it was 1,000 times brighter than usual and more than a million times brighter than the entire Milky Way Galaxy.

It also lasted an unusually long time, at roughly 200 seconds. Such a duration is generally considered a supernova signature, whereas gamma-ray bursts from kilonovae are far shorter. However, observations across multiple wavelengths showed that the burst's aftermath matched the profile expected from a kilonova.

Because kilonovae are established producers of r-process elements, astronomers sought infrared observations of the explosion source using the James Webb Space Telescope.

On 5 April, they pointed the telescope at the fading glow, which had developed a substantial infrared component by then, and obtained spectra.

Those data showed tellurium, the 52nd element in the periodic table. That is a notably heavy element. It suggests that other r-process elements may be present in the expanding material ejected by the neutron star collision, although further observations will be required to verify this.

An intergalactic neutron star collision

It is also notable that the explosion occurred in a highly unusual location: intergalactic space, 120,000 light-years from the closest galaxy. The researchers concluded that this galaxy was probably the birthplace of the two neutron stars, which began life as ordinary massive stars. After each star went supernova in the past, one after the other, the force of the blasts gave them sufficient momentum to send them out of the galaxy.

The researchers say this compelling event still has much to teach us.

"Until recently, we didn't think mergers could power gamma-ray bursts for more than two seconds," says astronomer Ben Gompertz of the University of Birmingham in the UK.

"Our next job is to find more of these long-lived mergers and develop a better understanding of what drives them – and whether even heavier elements are being created. This discovery has opened the door to a transformative understanding of our universe and how it works."

The research has been published in Nature.