Astronomers using the XRISM satellite have discovered unusually high amounts of chlorine and potassium in the Cassiopeia A supernova remnant, challenging existing models of element production in stars. These odd-Z elements are vital for planet formation and life, suggesting supernovae play a larger role in distributing life's building blocks. The findings could reshape understanding of chemical distribution in the Milky Way.
The Cassiopeia A supernova remnant, the youngest known exploded star in our galaxy, has yielded surprising insights into the origins of elements essential for life. Researchers from Kyoto University and Meiji University analyzed high-resolution X-ray data collected by the X-Ray Imaging and Spectroscopy Mission (XRISM), launched by JAXA in September 2023. XRISM observed Cassiopeia A twice in December 2023 using its Resolve microcalorimeter, which offers energy resolution ten times sharper than previous detectors.
The data revealed strong X-ray emission lines for chlorine and potassium—odd-Z elements with an odd number of protons—at levels far exceeding predictions from standard stellar evolution models. These models had suggested stars produce only about one-tenth of the observed cosmic abundances of such elements. By comparing faint odd-Z signals with stronger even-Z references like sulfur and argon, the team confirmed that a single supernova can generate sufficient quantities to account for galactic levels.
The elevated production is attributed to powerful internal mixing in massive stars, possibly from rapid rotation, binary interactions, or shell-merger events. "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," said corresponding author Toshiki Sato.
This discovery addresses a long-standing puzzle about odd-Z element origins, which are crucial for oceans, biology, and planetary development. "I am delighted that we have been able, even if only slightly, to begin to understand what is happening inside exploding stars," added Hiroyuki Uchida. While some models align reasonably, others require refinement, as noted by astrophysicist Stan Woosley.
The implications extend to astrobiology: uneven distribution of these elements might mean some Milky Way regions are more habitable than others. However, Kai Matsunaga cautions that further XRISM observations of other remnants are needed to determine if Cassiopeia A is typical. "How Earth and life came into existence is an eternal question... Our study reveals only a small part of that vast story," Matsunaga reflected.
Published in Nature Astronomy in 2025, the study underscores XRISM's power in probing stellar interiors and chemical evolution.