For the first time in history, researchers observed a dying star stripped nearly to its bare core before it exploded. This newly discovered supernova, designated SN 2021yfj, offers an unprecedented glimpse into the inner workings of stellar deaths. The discovery was made by researchers from Northwestern University. It was first detected in 2021 by the Zwicky Transient Facility, and later studied in detail using the W.M. Keck Observatory in Hawaii.
Unlike typical supernovae, SN 2021yfj revealed spectral signatures dominated by silicon, sulfur, and argon. These elements are typically found deep inside massive stars, while their outer layer is composed of hydrogen and helium. That means that this particular star was stripped of its outermost layer, and its core remained exposed before its final explosion.
The study was published in the August 2025 edition of Nature by astronomer Steve Schulze and his team. In this study, they challenge long-held models of stellar mass loss and reveal a new class of stellar explosion. This rare discovery confirms the textbook theories about the onion-like structure of massive stars. However, it also opens new questions about how and how much matter stars can lose before their explosion.
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A Rare Glimpse Beneath The Surface
Image of a sun-like star – Nazarii_Neshcherenskyi/Shutterstock
It’s challenging to learn what stars look like on the inside. For decades, astronomers relied on indirect models and late-stage supernovae to conclude what lies beneath a star’s glowing surface. The newly discovered SN 2021yfj finally allowed the scientists to directly observe a star’s bare core.
The first clue was the unusual chemical fingerprint this star left. Most supernovae showcase lighter elements like hydrogen, helium, or carbon. SN 2021yfj’s spectrum was dominated by heavier elements such as silicon and sulfur. These are hallmarks of the star’s deeper inner layers. But the most intriguing was the presence of argon, an element rarely observed in such abundance outside of nuclear burning zones.
This means that the star didn’t lose just its outermost shell, but several deeper layers too. Although scientists don’t know how, there are several possible explanations. It could have been powerful stellar winds or perhaps a violent interaction with a binary companion. Unfortunately, we will never know how it happened as the star already exploded. The Northwest University team of astronomers that made the discovery believes this supernova challenges the assumption that stars must retain at least some of their outer layers to explode.
Rethinking How Stars Die
Image of a supernova – overlays-textures/Shutterstock
The discovery of SN 2021yfj has sent ripples through the astrophysicists’ community, prompting reevaluation of how stars evolve and how they end their lives. Typically, the outer layers of stars remain intact before they explode. SN 2021yfj defies this idea, challenging all the assumptions made about the late-stage life cycle of massive stars. It proves that the models of stars collapsing must change to include more extreme mass loss and core exposure.
Due to the unique chemical makeup of SN 2021yfj, astronomers proposed a new category for this kind of explosion: Type Ien Supernova (pronounced One-en). The letters “en” stand for two key features: envelope stripping and interaction with the material that surrounds a star. It also reflects the presence of heavy metals usually found deep inside the core of a star. This new category could account for a small but critical population of explosive events that were previously misclassified or misunderstood. The SN 2021yfj isn’t just an anomaly. It’s a missing piece of a stellar death puzzle, one that could redefine how we understand the balance between mass, structure, and explosive forces that occur at the end of the star’s life.
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