An international group of scientists led by Northwestern University has identified a new type of supernova, marked by high concentrations of silicon, sulfur, and argon. The event, named SN2021yfj, challenges existing ideas about the life cycles of massive stars.
Supernovae are typically observed with signatures of lighter elements such as hydrogen and helium. However, this newly discovered explosion lacked these outer layers. Instead, it exposed inner shells rich in heavier elements, providing direct evidence for the theorized layered structure inside large stars.
The research team published their findings in Nature on August 20. Steve Schulze from Northwestern University led the study. “This is the first time we have seen a star that was essentially stripped to the bone,” said Schulze. “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.”
Adam Miller, also at Northwestern and senior author on the study, added: “This event quite literally looks like nothing anyone has ever seen before. It was almost so weird that we thought maybe we didn’t observe the correct object. 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.”
Massive stars—ranging from 10 to 100 times heavier than the sun—are powered by nuclear fusion which creates increasingly heavy elements in layers around an iron core over time. When this core collapses under gravity, it triggers either a supernova or forms a black hole.
Previous observations had only detected stripped supernovae exposing helium or carbon-oxygen layers after losing hydrogen envelopes; seeing deeper layers like those found in SN2021yfj is unprecedented.
Schulze’s team discovered SN2021yfj using data from the Zwicky Transient Facility (ZTF), which scans the night sky for transient astronomical events such as supernovae. The unusual brightness drew attention to an object located about 2.2 billion light-years away.
To determine its composition, researchers needed spectral data showing what elements were present during the explosion. Although initial attempts were unsuccessful due to poor telescope availability and weather conditions, help came unexpectedly from UC Berkeley colleagues who obtained crucial spectra using Hawaii's W.M. Keck Observatory.
“We thought we had fully lost our opportunity to obtain these observations,” said Miller. “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.”
Instead of common elements found in other stripped supernovae—such as helium or oxygen—the spectra revealed strong signals from silicon, sulfur and argon formed during advanced stages of nuclear burning inside massive stars.
“This star lost most of the material that it produced throughout its lifetime,” Schulze said.“So,we could only see thematerial formed duringthe months right before its explosion.Something very violent must have happened to cause that.”
The exact cause remains unclear,but researchers propose possibilities including interaction with another star,a major eruption before detonation or powerful stellar winds.The leading hypothesis suggests repeated internal instability caused explosive pulses shedding successive shells until only deep interior material remained visible at death.
“One of the most recent shell ejections collided witha pre-existing shell ,which producedthe brilliant emissionthatwe sawas SN2021yfj,”Schulze said.
“While we havea theoryfor how nature createdthis particular explosion,” Miller said,“I wouldn’t bet my life thatit’s correct,because westill onlyhave one discovered example.Thisstar really underscores theneedto uncovermoreoftheserare supernovaetobetter understandtheirnatureandhowtheyform.”
Funding came fromthe National Science Foundationand Northwestern's Center for Interdisciplinary Explorationand Researchin Astrophysics (CIERA),which also enabled access tothe ZTF telescope data.