Stellar explosions come into focus with new nuclear reaction measurements

In two separate papers published in Physical Review Letters, researchers investigated the nuclear reactions that take place inside supernovae and X-ray bursts—two of the universe’s most powerful explosions. Together, the findings will help scientists build more accurate models of how stars explode and how newly formed elements are produced.

The first study focused on supernovae—bright, powerful explosions that mark the death of massive stars. Despite centuries of observations, scientists still do not fully understand how these explosions unfold. One of the best clues comes from radioactive titanium-44, which is produced during a supernova and can still be detected by space telescopes long after the explosion.

At Argonne National Laboratory in the United States, Surrey researchers obtained the first experimental data needed to determine the rate of a nuclear reaction that controls how much titanium-44 is produced during a supernova. They found the reaction happens much more slowly than previously thought, increasing predicted titanium-44 production by up to 35%.

This will allow astronomers to compare computer models more closely with real observations, bringing them a step closer to understanding how supernovae occur.

“It’s exciting to see just how far the field has come. A measurement like this would have been considered out of reach only a couple of decades ago, but it now gives us new insight into one of the biggest unanswered questions in astrophysics,” said Dr. Christopher Cousins, a postdoctoral researcher at the University of Surrey’s Nuclear Physics Group.

Tighter view of X-ray bursts

The second study investigated X-ray bursts (type I)—the most frequent stellar explosions in the universe—which occur when a dense neutron star pulls material from a nearby companion star, triggering repeated thermonuclear reactions that build heavier elements and release enormous amounts of energy.

Working at the new Facility for Rare Isotope Beams (FRIB) in Michigan, the team measured a nuclear reaction that powers X-ray bursts with far greater precision than previously possible, reducing uncertainty about how the reaction behaves by more than tenfold. The results settle a long-running question over the role of the nickel-copper cycle—a process that can temporarily trap nuclear material during an explosion—showing it influences X-ray burst light curves.

“One of the biggest unknowns was whether material becomes trapped in the nickel-copper cycle during an X-ray burst. We’ve shown that it does, but likely only a small proportion, giving us a much more realistic picture of these explosions,” said Dr. Connor O’Shea, a postdoctoral research fellow in nuclear astrophysics.

Clearer models of stellar explosions

“Despite decades of research, we still don’t fully understand the nuclear reactions that power some of the universe’s most spectacular stellar explosions. These two studies answer important questions about what happens inside both X-ray bursts and supernovae, providing experimental evidence where scientists previously had to rely on theory and estimates.

Together, they give us a much clearer picture of how these explosions happen, allowing us to compare our models more closely with astronomical observations and bringing us closer to understanding how the chemical elements are created and spread throughout the universe,” said Gavin Lotay, professor of nuclear astrophysics and Research Excellence Framework lead for physics.

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