The surprisingly high amounts of chlorine and potassium in a supernova remnant could help solve the mystery of where these crucial elements come from

Sara Hashemi - Daily Correspondent

December 12, 2025 11:33 a.m.

Supernova remnant Cassiopeia A, which sits within the Milky Way X-ray: NASA / CXC / SAO; Optical: NASA / ESA / STScI; IR: NASA / ESA / CSA / STScI / Milisavljevic et al., NASA / JPL / CalTech; Image Processing: NASA / CXC / SAO / J. Schmidt and K. Arcand

Within the rubble of an exploded star, astrophysicists have found unexpectedly high amounts of two ingredients essential for life: chlorine and potassium.

The discovery could help explain where these elements come from, a puzzle that has long stumped scientists, according to the study published December 4 in the journal Nature Astronomy. Additionally, the findings may narrow down where extraterrestrial life could be lurking within the Milky Way.

“When we saw the … data for the first time, we detected elements I never expected to see,” says study co-author Toshiki Sato, an astrophysicist at Meiji University in Japan, in a statement from Kyoto University.

Leftover clouds of gas and dust from exploded stars, or supernova remnants, contain lots of elements like oxygen, carbon and neon, which carry even numbers of positively charged particles, or protons. Chlorine and potassium, however, possess an odd number of protons and are therefore called odd-Z elements. These elements are less stable and less likely to be created in stars than elements with even numbers of protons, reports Victoria Corless at New Scientist. Because of that, existing theoretical predictions suggest that stars make only around 10 percent of odd-Z elements found in the universe.

To better understand these mysterious elements, Sato and his colleagues looked to Cassiopeia A, a young supernova remnant around 340 years old that’s located roughly 11,000 light years away from Earth. They relied on NASA and the Japanese Aerospace Exploration Agency’s XRISM spacecraft, which launched in 2023.

Fun fact: The oldest supernova ever discovered

The James Webb Space Telescope recently spotted the oldest known supernova, which exploded when the 13.8-billion-year-old universe was merely 730 million years old.

The craft uncovered light signals emitted by different elements’ atoms within Cassiopeia A. Comparing the elements’ abundances with those predicted by theoretical work revealed the surprisingly high levels of chlorine and potassium, suggesting that more of these elements can be produced deep inside stars than previously thought.

“Although the authors highlight that their observations conflict with previous models, the picture is more nuanced,” Stan Woosley, an astrophysicist at the University of California, Santa Cruz who wasn’t involved in the work, tells New Scientist. The findings don’t mean that all past theoretical work is wrong; some models simply work better than others.

“The main thing is that these observations give astronomers new, concrete information to improve models and better understand what happens when a massive star explodes,” he says.

The study authors suggest that prior to going supernova, the star’s onion-like layers could have been disrupted. “That kind of upheaval might have led to persistent, large-scale churning of material inside the star that created conditions where chlorine and potassium formed in abundance,” writes NASA’s Jeanette Kazmierczak. The scenario may also explain the low amount of neon detected in Cassiopeia A.

Still, researchers don’t know what kinds of stars or circumstances could create large amounts of odd-Z elements. But if more supernova remnants contain these crucial ingredients for life, that may hint that some parts of the Milky Way are better suited to spawning life than others, per New Scientist.

Cassiopeia A might just be an oddball, or it could represent a broader, more generalizable finding, study co-author Kai Matsunaga, an astrophysicist at Kyoto University, tells the outlet. “Future observations of other supernova remnants with XRISM or upcoming instruments will be crucial for addressing this question.”

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