Scientists have made two important nuclear measurements that could help explain what happens inside some of the most powerful explosions in the universe.
In two studies published in Physical Review Letters, researchers examined nuclear reactions involved in supernovae and X-ray bursts.
By measuring these reactions more precisely, the scientists hope to improve computer models of stellar explosions and better understand how chemical elements are created and scattered through space.
The first study focused on supernovae, the enormous explosions that can occur when massive stars reach the end of their lives. Although astronomers have observed supernovae for centuries, many details about exactly how these explosions develop remain uncertain.
One important clue is titanium-44, a radioactive form of titanium created during a supernova. Because titanium-44 remains detectable long after the initial explosion, astronomers can use space telescopes to measure it and compare those observations with predictions from supernova models.
Researchers from the University of Surrey carried out experiments at Argonne National Laboratory in the United States to investigate a nuclear reaction that determines how much titanium-44 survives during an explosion.
For the first time, they obtained experimental data needed to calculate the rate of this reaction. Their results showed that it occurs much more slowly than scientists had previously estimated. As a result, models could predict up to 35% more titanium-44 being produced in supernovae.
This improved estimate should make it easier for astronomers to compare simulations with observations of actual stellar explosions.
The second study examined type I X-ray bursts, which are among the most common stellar explosions in the universe. These events happen in binary star systems when an extremely dense neutron star pulls material from a nearby companion.
As this material accumulates on the neutron star’s surface, temperatures and pressures rise until runaway nuclear reactions trigger a powerful explosion. Unlike a supernova, the process can happen repeatedly as the neutron star continues collecting fresh material.
At the Facility for Rare Isotope Beams in Michigan, researchers measured an important nuclear reaction involved in these bursts with much greater precision than before. The new experiment reduced uncertainty surrounding the reaction by more than tenfold.
The results also helped settle a long-running question about the nickel-copper cycle, a series of nuclear reactions that can temporarily trap material during an X-ray burst. The researchers confirmed that material does enter this cycle, although probably only in relatively small amounts. Even so, the process appears to influence the burst’s light curve—how its brightness changes over time.
Professor Gavin Lotay, a nuclear astrophysicist at Surrey, said scientists have spent decades trying to understand the reactions powering these spectacular events.
Together, the two studies replace some previous theoretical estimates with direct experimental evidence.
That could lead to more realistic models of stellar explosions and help scientists understand not only how stars explode, but also how the elements produced inside them eventually become part of the wider universe.
