In the aftermath of a dying star’s supernova explosion, astrophysicists have uncovered a highly unusual chemical signature. Instead of the expected light elements such as hydrogen and helium, the debris contained silicon, sulfur, and argon—evidence of a star burning unusually close to its core before its death.
The discovery, led by an international team based at Northwestern University, was recently published in Nature. Astrophysicists have long theorized that massive stars are layered like onions, with the lightest elements on the outside and the heaviest packed tightly near the core.
SN2021yfj
“We saw an interesting explosion, but we had no idea what it was,” said Steve Schulze, of Northwestern, who led the study. “Almost instantly, we realized it was something we had never seen before, so we needed to study it with all available resources.”
Schulze’s team identified the supernova—designated SN2021yfj—using the Zwicky Transient Facility (ZTF) to study a bright object 2.2 billion light-years away in a star-forming region. The progenitor star, between 10 and 100 times the mass of our Sun, was powered by nuclear fusion. As fusion created heavier elements, the star’s core grew hotter and denser, pushing the burning outward into ordered layers, each fusing progressively heavier elements until an iron core formed. The core’s final collapse triggered the catastrophic explosion that followed.
Using spectroscopic analysis, the team set out to determine the elements present in the blast. Their efforts initially stalled due to the unavailability of telescopes and low-quality data, until colleagues at Hawaii’s W.M. Keck Observatory finally secured the critical spectrum.
“We thought we had fully lost our opportunity to obtain these observations,” said co-author Adam Miller, also of Northwestern. “So, we went to bed disappointed. But the next morning, a colleague at UC Berkeley unexpectedly provided a spectrum. Without that spectrum, we may have never realized that this was a strange and unusual explosion.”
Exploring a Strange Supernova
Analysis revealed that SN2021yfj had shed its outer layers of hydrogen, helium, and carbon in an earlier event, leaving only heavier elements behind. This stripped-down state allowed astronomers to observe a supernova burning through silicon and sulfur prior to detonation—direct evidence supporting the layered-star model.
“This is the first time we have seen a star that was essentially stripped to the bone,” Schulze said. “It shows us how stars are structured and proves that stars can lose a lot of material before they explode. Not only can they lose their outermost layers, but they can be completely stripped all the way down and still produce a brilliant explosion that we can observe from very, very far distances.”
“This event quite literally looks like nothing anyone has ever seen before,” added Miller. “It was almost so weird that we thought maybe we didn’t observe the correct object.”
While scientists have previously observed stars shedding some layers before their explosion, nothing has come close to the depth at which SN2021yfj has removed layers. This led the team to suspect that a highly violent process must be at play.
“This star lost most of the material that it produced throughout its lifetime,” Schulze said. “So, we could only see the material formed during the months right before its explosion. Something very violent must have happened to cause that.”
Stripping a Star
“While we have a theory for how nature created this particular explosion,” Miller said, “I wouldn’t bet my life that it’s correct, because we still only have one discovered example. This star really underscores the need to uncover more of these rare supernovae to better understand their nature and how they form.”
The team ruled out several possible explanations, including interactions with a companion star, powerful stellar winds, or an early eruption. Instead, they propose that as the star contracted under the immense pull of its own gravity, the core became so hot and dense that nuclear fusion reignited, releasing a burst of energy strong enough to blow away the outer layers.
“This star is telling us that our ideas and theories for how stars evolve are too narrow. It’s not that our textbooks are incorrect, but they clearly do not fully capture everything produced in nature. There must be more exotic pathways for a massive star to end its life that we hadn’t considered,” Miller said.
The paper, “Extremely Stripped Supernova Reveals a Silicon and Sulfur Formation Site,” appeared in Nature on August 20, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.