“Why are we here?” remains one of the most enduring questions humans have posed. One way scientists approach this idea is by tracing where the elements around us first formed. Many elements are created inside stars and in the explosive debris of supernovae, which scatter this material across space, but the origins of several important elements have been difficult to explain.
Chlorine and potassium fall into this category. They are classified as odd-Z elements — possessing an odd number of protons — and are crucial for both life and the development of planets. Current models, however, indicate that stars should produce only about one-tenth of the chlorine and potassium that astronomers actually observe in the universe, leading to a long-standing scientific puzzle.
XRISM Offers a New Way to Study Supernova Debris
This gap in understanding led researchers at Kyoto University and Meiji University to investigate whether supernova remnants might hold the missing clues. They used XRISM — short for X-Ray Imaging and Spectroscopy Mission, an X-ray satellite launched by JAXA in 2023 — to gather high-resolution X-ray spectroscopic data from the Cassiopeia A supernova remnant in the Milky Way.
To accomplish this, the team relied on the microcalorimeter Resolve instrument on XRISM. The device provides energy resolution roughly ten times sharper than earlier X-ray detectors, which allowed the researchers to pick up faint emission lines associated with rare elements. After collecting the data from Cassiopeia A, they compared the measured amounts of chlorine and potassium with several theoretical models of how supernovae create elements.
Evidence That Supernovae Produce Life-Related Elements
The results showed clear X-ray emission lines of both chlorine and potassium at levels far higher than expected from standard models. This marks the first observational confirmation that a single supernova can generate enough of these elements to match what astronomers see in the cosmos. The researchers believe that strong internal mixing inside massive stars, possibly driven by rapid rotation, binary interactions, or shell-merger events, can greatly increase the production of these elements.
“When we saw the Resolve data for the first time, we detected elements I never expected to see before the launch. Making such a discovery with a satellite we developed is a true joy as a researcher,” says corresponding author Toshiki Sato.
Insights Into How Stars Shape the Building Blocks of Life
These findings show that the chemical ingredients essential for life formed under extreme conditions deep within stars, far removed from anything resembling the environments where life later emerged. The work also demonstrates how powerful high-precision X-ray spectroscopy has become in uncovering the processes at work inside stellar interiors.
“I am delighted that we have been able, even if only slightly, to begin to understand what is happening inside exploding stars,” says corresponding author Hiroyuki Uchida.
Next Steps for Understanding Stellar Evolution
The team plans to continue studying additional supernova remnants with XRISM to determine whether the elevated levels of chlorine and potassium found in Cassiopeia A are typical of massive stars or unique to this particular remnant. This will help reveal whether the internal mixing processes identified here are a widespread feature of stellar evolution.
“How Earth and life came into existence is an eternal question that everyone has pondered at least once. Our study reveals only a small part of that vast story, but I feel truly honored to have contributed to it,” says corresponding author Kai Matsunaga.
