Based on accepted cosmological models, hydrogen and helium were the only elements in the early Universe.
These coalesced to form the first stars and galaxies, which fused hydrogen and helium to create heavier elements like carbon, silica, and iron.
These, in turn, were distributed throughout the Universe once these stars experienced gravitational collapse and went supernova.
This process, known as “stellar nucleosynthesis,” is how the elements that gave rise to the planets and life formed.
However, the origin of the heaviest elements on the periodic table is one of the most challenging questions in physics.
These include thorium, uranium, plutonium, and other elements that only form under extreme conditions.
To explore this mystery, a team of researchers led by the Los Alamos National Laboratory considered the jets emanating from collapsed stars.
Their findings indicate that gamma-ray bursts produced deep in these jets could dissolve the outer layers of a star into neutrons, creating the conditions that result in heavy elements.
The study was conducted by researchers from the Theoretical Division and the Center for Theoretical Astrophysics at the Los Alamos National Laboratory (LANL) and the Physics Division at the Argonne National Laboratory.
The paper detailing their findings, “Let There Be Neutrons! Hadronic Photoproduction from a Large Flux of High-energy Photons,” appeared in The Astrophysical Journal on March 25th, 2025. As the team indicated, the formation of the heaviest elements on the periodic table comes down to the neutron-capture process (“r process”).
This describes nuclear reactions where atomic nuclei absorb a free neutron and emit a discrete amount of gamma-ray photons.
However, free neutrons have a short half-life of about 15 minutes, making them rare and limiting the scenarios in which they would be common enough to form heavy elements.
Matthew R. Mumpower, a researcher at the Los Alamos National Laboratory and the lead author on the paper, explained in a LANL press release:
“The creation of heavy elements such as uranium and plutonium necessitates extreme conditions. There are only a few viable yet rare scenarios in the cosmos where these elements can form, and all such locations need a copious amount of neutrons. We propose a new phenomenon where those neutrons don’t pre-exist but are produced dynamically in the star.”
Mumpower and colleagues propose a scenario that begins when a sufficiently massive star undergoes gravitational collapse, forming a black hole at its center.
If the black hole is spinning fast enough, frame-dragging spins up the magnetic field, leading to a powerful jet.
This jet then ploughs through the star’s outer envelope, creating a cocoon of hot material and high-energy photons interacting with atomic nuclei. Meanwhile, protons are trapped in the jet by a strong magnetic field, and neutrons are pushed into the cocoon.
According to their calculations, these interactions can produce free neutrons in nanoseconds. These neutrons are accelerated to relativistic speeds, causing collisions that could trigger the r process, forging heavy elements and isotopes expelled into space as the star is ripped apart.
This proposed framework may also help explain the origin of kilonova, the optical and infrared radiation associated with long-duration gamma-ray bursts. These events have been attributed to the merger of neutron stars or a neutron star and a black hole.
Their research is supported by recent findings that revealed iron and plutonium in samples of deep-sea sediment. These deposits were confirmed to be extraterrestrial in origin, with some indications that they were distributed to Earth by a nearby supernova. This scenario not only provides a possible explanation regarding the origins of heavy elements but also addresses key questions in physics and cosmological hazards.
Many challenges remain since none of the heavy isotopes created through the r-process have ever been made on Earth.
Mumpower and his team hope to run more complex simulations on their models to investigate their proposed framework further.
Written by Matthew Williams/Universe Today.
