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Creation of rare heavy elements witnessed in neutron-star collision

Astronomers have witnessed the creation of rare heavy elements in the aftermath of a violent collision between two neutron stars that were booted out of their home galaxy about 1bn light years away.

The cataclysmic explosion unleashed a burst of gamma rays more than 1m times brighter than the Milky Way and blasted material into space that formed the rare element tellurium and others known as actinides and lanthanides. The more common elements iodine and thorium are also thought to have been forged in the event.

It is the first time such a cosmic spectacle, known as a kilonova, has been observed with the James Webb space telescope, which enabled astronomers to identify the elements produced in the collision by their infrared signatures.

The work, published in Nature, shows that while many elements are produced by the fusion of lighter elements in the cores of stars, or in stellar explosions, some heavier elements are born in the more energetic environment of neutron stars slamming into one another.

“For the first time we have evidence of these particular kinds of elements being formed in these mergers,” said Andrew Levan, professor of astrophysics at Radboud University in the Netherlands.

“It’s 150 years since we’ve had the [periodic table] and we still don’t know where a good number of elements come from. One of the things we’re trying to do is fill in those gaps.”

Neutron stars are incredibly dense and compact objects, as massive as the sun but as small as a city. Astronomers were alerted to the potential neutron-star collision in March when they detected an intense burst of gamma rays from deep space, the second brightest recorded in the past 50 years.

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Drawing on an array of ground- and space-based detectors and telescopes, researchers first located the source of the 200-second radiation burst and then trained the James Webb space telescope on the aftermath.

Telescope image showing distance between star collision and galaxy they came fromAn image taken by the James Webb space telescope showing the distance between the site of the stars’ collision and the galaxy they came from. Photograph: NASA/PA

Over a period of days, the light from the collision changed from blue to red, a hallmark of a kilonova. The neutron stars appeared to have been kicked out of a bright galaxy spotted nearby before merging 120,000 light years away – the width of the Milky Way – several hundred million years later.

The collision is likely to have created a new black hole, but in the merger, vast amounts of neutrons and other material were propelled into space. These produced the heavier elements through a process called rapid neutron capture. Atomic nuclei that are bombarded with neutrons can become unstable and undergo radioactive decay that transforms them into heavier elements.

Kilonovae are extremely rare; scientists have witnessed only one other such event in enough detail to infer elements potentially made in the explosion. While elements such as iron and nickel are made in exploding stars, or supernovae, more violent neutron-star collisions appear ripe for making heavier elements.

“About half of the elements heavier than iron are probably made in these events,” said Levan, who worked on the observations with an international team of astronomers. “We hoped to see this, but you never quite know what you’re going to get.”

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