James Webb Space Telescope Finds Tellurium in a Kilonova

James Webb Space Telescope detects heavy elements in a kilonova

A kilonova created when two neutron stars collided about a billion light-years away has proved to be a production site for rare, heavy elements.

This is the first occasion on which the James Webb Space Telescope has examined an event of this kind. Following an immense gamma-ray burst detected on 7 March 2023, observations from the telescope found signs of tellurium - a rare metal too heavy to be made by stellar fusion.

The data also hinted at further metals, including tungsten and selenium. According to the researchers, the finding establishes neutron star mergers as a source of heavy elements, helping to explain how the Universe creates matter and disperses 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 force together the atoms of hydrogen, which accounts for most visible matter in the Universe, producing heavier elements: first hydrogen becomes helium, then progressively heavier atoms form, up to iron.

At that point, however, a star's fusion process can no longer continue effectively. Fusing iron into still heavier elements consumes more energy than it produces, placing the star on course to explode under its own gravitational weight.

That powerful explosion can nevertheless trigger nuclear reactions in which atomic nuclei strike free neutrons, creating even heavier elements.

These reactions must proceed rapidly enough that radioactive decay cannot take place before extra neutrons join the nucleus. They therefore require an environment containing many free neutrons, such as a supernova or a kilonova. This form of nucleosynthesis is called the rapid neutron capture process, or r-process.

When astronomers first witnessed two neutron stars merge in 2017, the resulting observations confirmed that kilonovae create r-process elements. They identified strontium, the 38th element in the periodic table.

GRB230307A and the tellurium discovery

After a gamma-ray burst designated GRB230307A was seen flaring in March this year, scientists promptly began closer observations. GRB230307A was exceptionally striking: it ranked among the brightest gamma-ray bursts ever observed, being 1,000 times brighter than usual and more than a million times brighter than the entire Milky Way Galaxy.

Its duration was also unusually long, lasting roughly 200 seconds. Such a lengthy burst is considered a signature of a supernova, whereas gamma-ray bursts from kilonovae are far shorter. However, multi-wavelength observations showed that the burst's aftermath matched the profile expected from a kilonova.

Because kilonovae are an established source of r-process elements, astronomers asked to observe the explosion's origin with the infrared JWST.

They aimed the telescope at the glow on 5 April, when it already had a substantial infrared component, and obtained spectra.

The resulting data showed tellurium, the 52nd element in the periodic table. This is a notably heavy element. Its presence indicates that the expanding material ejected by the neutron star collision probably contains other r-process elements too, although additional observations will be required to verify this.

An intergalactic neutron star collision

The explosion also 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 as ordinary massive stars. When the stars each underwent supernova explosions in the past, one after the other, the blasts gave them enough of a kick to yeet them out of the galaxy.

The researchers say this intriguing event still has much more to reveal.

"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.