In the world of astrophysics, where the boundaries of our understanding of the universe are constantly being pushed, an international team of scientists, led by astrophysicists from Northwestern University, recently recorded a previously unseen type of star explosion, known as a supernova. This extraordinary discovery, named SN2021yfj, gives us a unique insight into the internal mechanisms of a dying star, revealing layers of heavy elements such as silicon, sulfur, and argon, which contradicts common observations.

An incredible discovery: A supernova stripped to its core

When massive stars experience their spectacular explosion, astrophysicists usually expect strong signals of lighter elements, primarily hydrogen and helium. However, supernova SN2021yfj showed a strikingly different chemical signature. Its light was rich in silicon, sulfur, and argon, elements typically found deep within the stellar core. This discovery has caused great excitement in the scientific community, as it provides direct evidence of the long-theorized layered structure of stellar giants and offers an unprecedented look into the deep interior of a massive star—just moments before its explosive death.

Observations of SN2021yfj strongly suggest that the massive star, in some unusual way, lost its outer layers of hydrogen, helium, and carbon before it exploded. This resulted in the exposure of the inner layers rich in silicon and sulfur. The study describing this revolutionary discovery was published on August 20, 2025, in the prestigious journal Nature.

"This is the first time we've seen a star that has literally been stripped to the bone," said Steve Schulze of Northwestern University, who led the study. "This shows us how stars are structured and proves that stars can lose a huge amount of material before they explode. Not only can they lose their outermost layers, but they can be completely stripped down to the core and still produce a brilliant explosion that we can observe from very, very large distances."

"This event literally looks like nothing anyone has ever seen before," added Adam Miller, also of Northwestern University and one of the lead authors of the study. "It was almost so strange that we thought we might not be observing the right object. This star tells us that our ideas and theories about how stars evolve are too narrow. It's not that our textbooks are incorrect, but they obviously do not fully encompass everything that is produced in nature. There must be more exotic paths for a massive star to end its life that we haven't considered."

Schulze, an expert on the most extreme transient objects in astronomy, is a research associate at the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) at Northwestern. Miller is an assistant professor of physics and astronomy at the Weinberg College of Arts and Sciences at Northwestern and a leading member of CIERA and the NSF-Simons AI Institute for the Sky.

The layered structure of massive stars: A cosmic 'onion'

Massive stars, whose mass can be 10 to 100 times greater than that of our Sun, are powered by nuclear fusion. In this process, intense pressure and extreme heat in the stellar core cause lighter elements to fuse, creating heavier elements. Scientists have long theorized that massive stars have a layered structure, similar to an onion. The outermost layers consist predominantly of the lightest elements, such as hydrogen and helium. As the layers move inward, the elements become heavier and heavier, until the innermost iron core is reached.

When the temperature and density in the core increase, fusion also begins in the outer layers. As the star evolves over time, successively heavier elements are fused in the core, while lighter elements are fused in a series of shells surrounding the core. This process continues, eventually leading to a core composed of iron. Iron is the final product of fusion in stars because its fusion does not release energy, but rather consumes it. When the iron core becomes too large and unstable, it collapses under its own gravity, triggering a massive explosion known as a Type II supernova, or leading to the formation of a black hole or a neutron star.

Although massive stars typically shed layers before exploding, SN2021yfj ejected far more material than scientists have ever detected before. Other observations of "stripped stars" have revealed layers of helium or carbon and oxygen—exposed after the loss of the outer hydrogen envelope. But astrophysicists have never peered deeper than that, suggesting that something exceptionally violent and extraordinary must have been at work.

The search for a cosmic anomaly: How SN2021yfj came to light

The discovery of SN2021yfj began in September 2021, when Schulze and his team used the Zwicky Transient Facility (ZTF), a telescope located east of San Diego. The ZTF uses a wide-field camera to scan the entire visible night sky. Since its launch, the ZTF has become the world's primary engine for discovering astronomical transients—short-lived phenomena like supernovae that suddenly flash and then quickly fade.

Searching through ZTF data, Schulze noticed an extremely bright object in a star-forming region, 2.2 billion light-years from Earth. To get more information about the mysterious object, the team wanted to obtain its spectrum, which breaks down the scattered light into its constituent colors. Each color represents a different element. Therefore, by analyzing the supernova's spectrum, scientists can determine which elements are present in the explosion.

Although Schulze immediately sprang into action, the search for the spectrum encountered numerous obstacles. Telescopes around the world were either unavailable or could not penetrate the clouds to get a clear image. Fortunately, the team received a surprise from a fellow astronomer, who had collected a spectrum using instruments at the W.M. Keck Observatory in Hawaii.

"We thought we had completely lost the opportunity to get these observations," said Miller. "So we went to bed very disappointed. But the next morning, a colleague from UC Berkeley unexpectedly delivered the spectrum to us. Without that spectrum, we might never have realized that this was a strange and unusual explosion."

"We saw an interesting explosion, but we had no idea what it was," said Schulze about SN2021yfj. "We realized almost immediately that it was something we had never seen before, so we had to study it with all available resources."

The mystery of the stripped star: What caused the extreme peeling?

Instead of typical elements like helium, carbon, nitrogen, and oxygen—found in other stripped supernovae—the spectrum of SN2021yfj was dominated by strong signals of silicon, sulfur, and argon. These heavier elements are formed by nuclear fusion deep within a massive star during its final stages of life. This means that the star must have lost almost all of its outer layers, revealing its interior just before the explosion.

"This star lost most of the material it produced during its lifetime," explained Schulze. "So we could only see the material formed in the months just before its explosion. Something very violent must have happened to cause this."

Although the precise cause of this phenomenon remains an open question, Schulze and Miller suggest that a rare and powerful process was at work. They are investigating multiple scenarios, including interactions with a potential companion star, a massive pre-supernova eruption, or even unusually strong stellar winds. Each of these mechanisms could explain the loss of the outer layers, but the scale of the stripping of SN2021yfj points to something more extreme.

One of the most likely scenarios, according to the team, is that this mysterious supernova is the result of a massive star literally tearing itself apart. As the star's core contracts under its own gravity, it becomes even hotter and denser. The extreme heat and density then re-ignite nuclear fusion with such incredible intensity that it causes a powerful burst of energy that blows off the star's outer layers. This process, known as pair-instability, can lead to pulsating eruptions that eject enormous amounts of material. Each time the star goes through a new episode of pair-instability, the corresponding pulse ejects more material.

"One of the most recent shell ejections collided with a pre-existing shell, which produced the brilliant emission we saw as SN2021yfj," said Schulze, explaining how this collision created the exceptionally bright flash that was visible from Earth.

"Although we have a theory about how nature created this specific explosion," said Miller, "I wouldn't bet my life that it's correct, because we still have only one discovered example. This star really highlights the need to discover more of these rare supernovae to better understand their nature and how they are formed."

Implications for astrophysics: A new look into the heart of dying giants

The discovery of SN2021yfj represents a significant step forward in astrophysics. It provides direct empirical evidence for theoretical models of stellar evolution that predict the layered structure of massive stars. Until now, such models were based on indirect observations and computer simulations. Now, with SN2021yfj, scientists have the opportunity to directly observe the deep inner layers of a star just before its death, which is invaluable for calibrating and refining existing theories.

This supernova also opens up new questions about the mechanisms of mass loss in massive stars. If stars can lose so much material, even to the point of being stripped down to the silicon core, this could have significant implications for our understanding of the formation of black holes, neutron stars, and the enrichment of the universe with heavy elements. Extreme mass loss before an explosion can affect the final mass and type of the stellar remnant.

Future research will focus on searching for similar objects to either confirm the uniqueness of SN2021yfj or discover that it is a new class of supernovae. The development of more advanced telescopes and observation techniques, as well as more sophisticated computer models, will be key to unraveling these cosmic mysteries. Each new supernova discovered, like SN2021yfj, brings us closer to understanding the most spectacular events in the universe and the very evolution of the stars that are the building blocks of our cosmic home.

The study was supported by the National Science Foundation, and support from CIERA enabled access to data from the ZTF telescope.

Source: Northwestern University