Ancient neutron stars make elements not yet known to humans

Move over uranium, the Milky Way’s oldest stars have bigger and better elements to make.

A group of researchers from across the United States, Canada and Sweden have discovered ancient neutron stars might have created elements with atomic mass greater than 260.

With an atomic mass of 238, uranium is the heaviest naturally occurring element known on Earth, though others like plutonium have been found in trace amounts due to reactions in uranium deposits.

Most heavier elements with masses ranging from 237 (Neptunium) up to 266 (Lawrencium) are the result of human processes or experimentation.

But neutron stars have long been explored as veritable heavy element factories. Elements with masses greater than 260, are forged via rapid neutron capture, or ‘r-process’, which produces most of the elements that are heavier than iron (atomic mass 55.8), including uranium, platinum, gold and silver.

This process unfolds during neutron star events. These stars, which often form as pairs in the aftermath of a supernova – can eventually collide with one another.

Such collisions are crucial to the formation of heavy elements. In these dense, superhot environments, the r-process sees neutrons bombard and stick to atoms, which are spat out into space.

According to a study published in the journal Science today, while these processes are known sites for the creation of heavy periodic metals, more massive ones than those known to Earthlings are possible.

“The r-process is necessary if you want to make elements that are heavier than, say, lead and bismuth,” says the lead author of the study Dr Ian Roederer, a physicist at North Carolina State University.

“You have to add many neutrons very quickly, but the catch is that you need a lot of energy and a lot of neutrons to do so. And the best place to find both are at the birth or death of a neutron star, or when neutron stars collide and produce the raw ingredients for the process.”

The many unknown factors about how the phenomenon plays out in the universe were a motivation for Roederer’s team to investigate 42 ancient stars in the Milky Way to improve understanding of element formation.

They found new patterns in the heavy elements present in these stars, which suggested they were leftovers from the atomic splitting (fission) of heavier elements. Like uranium and its radioactive companions, these heavier elements are unstable and decay over large timescales into stable ones.

“The conditions of the process are quite extreme,” he says.

“We don’t have a good sense of how many different kinds of sites in the universe can generate the r-process, we don’t know how the r-process ends, and we can’t answer questions like, how many neutrons can you add? Or, how heavy can an element be?”

Among the elements studied were stable, mid-periodic table materials like silver and rhodium. The signals identified suggest they were the products of decaying metals that originally held a mass of at least 260, before splitting apart.

“We haven’t previously detected anything that heavy in space or naturally on Earth, even in nuclear weapon tests,” Roederer says.

“But seeing them in space gives us guidance for how to think about models and fission – and could give us insight into how the rich diversity of elements came to be.”