Kyoto, Japan -- "Why are we here?" is humanity's most fundamental and persistent question. Tracing the origins of the elements is a direct attempt to answer this at its deepest level. We know many elements are created inside stars and supernovae, which then cast them out into the universe, yet the origins of some key elements has remained a mystery.
Chlorine and potassium, both odd-Z elements -- possessing an odd number of protons -- are essential to life and planet formation. According to current theoretical models, stars produce only about one-tenth the amount of these elements observed in the universe, a discrepancy that has long puzzled astrophysicists.
This inspired a group of researchers at Kyoto University and Meiji University to examine supernova remnants for traces of these elements. Using XRISM -- short for X-Ray Imaging and Spectroscopy Mission , an X-ray satellite launched by JAXA in 2023 -- the team was able to perform high-resolution X-ray spectroscopic observations of the Cassiopeia A supernova remnant within the Milky Way.
The scientists utilized the microcalorimeter Resolve device onboard XRISM, providing high energy resolution an order of magnitude better than previous X-ray detectors, which allowed them to detect faint emission lines from rare elements. They then analyzed the X-ray spectrum from Cassiopeia A and compared the abundances of chlorine and potassium with several supernova nucleosynthetic models.
The team discovered clear X-ray emission lines of both elements at abundances far higher than predicted by standard supernova models. This provided the first observational evidence that a supernova can create sufficient chlorine and potassium. The team suggests that strong mixing inside massive stars caused by fast rotation, binary interaction, or shell-merger events, can significantly enhance 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.
These results reveal that the elements vital for life were produced in harsh, intense environments deep inside stars, far removed from anything resembling the conditions needed for life to emerge. The study also demonstrates the power of high-precision X-ray spectroscopy for probing the origins of elements and physical processes deep inside stars.
"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, the team plans to observe other supernova remnants with XRISM to determine whether the enhanced production of chlorine and potassium is common among massive stars or unique to Cassiopeia A. This will help reveal whether such internal mixing processes are a universal 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.
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The paper "Chlorine and Potassium Enrichment in the Cassiopeia A Supernova Remnant" appeared on 4 December 2025 in Nature Astronomy , with doi: 10.1038/s41550-025-02714-4
About Kyoto University
Kyoto University is one of Japan and Asia's premier research institutions, founded in 1897 and responsible for producing numerous Nobel laureates and winners of other prestigious international prizes. A broad curriculum across the arts and sciences at undergraduate and graduate levels complements several research centers, facilities, and offices around Japan and the world. For more information, please see: http://www.kyoto-u.ac.jp/en
Nature Astronomy
Observational study
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Chlorine and Potassium Enrichment in the Cassiopeia A Supernova Remnant
4-Dec-2025