Stars begin their lives by fusing hydrogen atoms into helium, releasing energy that keeps them shining. As they age, the helium produced becomes fuel for the next stage of fusion, resulting in the creation of heavier elements. This process builds up layers inside the star, with hydrogen on the outside and increasingly heavier elements deeper within, such as helium, carbon, oxygen, as well as neon, magnesium, and silicon-sulfur.
Eventually, the star starts fusing silicon and sulfur into iron. But iron doesn’t produce energy through fusion, so when it builds up in the core, the star can no longer support itself. This leads to the collapse of the core, resulting in either a supernova explosion or the formation of a black hole.
Some stars lose their outer hydrogen layers before they explode. These “stripped stars” reveal their inner layers, including helium, carbon, and oxygen, providing scientists with clues about how stars are structured and how they produce different elements.
Supernovae from these stripped stars, especially when surrounded by material they shed earlier, support this layered model. However, direct evidence of the deepest layers, where elements heavier than oxygen are made, has been hard to find.
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In a remarkable new study, astronomers from Stockholm University have helped uncover a cosmic rarity: a supernova that exploded without the usual ingredients. Named SN 2021yfj, this stellar event was born from a star that had shed its outer layers, leaving behind only its oxygen-, silicon-, and sulfur-rich core—a configuration never observed so clearly before.
The explosion lit up the constellation Cetus in September 2021, and despite its distance of 2 billion light-years, it was bright enough to catch the attention of telescopes around the world.
What makes SN 2021yfj so extraordinary is that it lacked hydrogen, helium, carbon, and oxygen, the most common elements in the Universe. Instead, it exposed the deepest fusion layers of a massive star, offering direct evidence of the shell-like structure scientists have long theorized.
Steve Schulze, who led the study, said, “This is the first time we have seen a star that was essentially stripped to the bone. It shows us how stars are structured and proves that stars can lose a lot of material before they explode.”
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Jesper Sollerman, professor at the Department of Astronomy at Stockholm University, is one of the co-investigators of the study:
“Most of the Universe is made up of hydrogen and helium, but we have previously found exploding stars that are stripped of these elements and instead contain mostly carbon, oxygen, and nitrogen. However, SN 2021yfj contains instead the even rarer elements: silicon and sulphur. These are known to be produced in the very innermost cores of massive stars, but we have never before seen the evidence of this in such a clear way.”
Astronomers observed a thick shell of silicon and sulfur that was thrown off by a dying star just before it exploded as a supernova. Seeing this deep layer is extremely rare; it means the star has unusually lost its outer layers. This event revealed advanced stages of stellar evolution, where heavier elements like silicon, sulfur, and argon are formed. These elements are typically hidden deep within and are rarely visible on the surface of massive stars.
To study this rare explosion, scientists around the world used some of the biggest telescopes, including Keck in Hawai‘i, the Very Large Telescope in Chile, and the Nordic Optical Telescope in La Palma, collecting data for nearly two months.
Journal Reference:
- Schulze, S., Gal-Yam, A., Dessart, L. et al. Extremely stripped supernova reveals a silicon and sulfur formation site. Nature 644, 634–639 (2025). DOI: 10.1038/s41586-025-09375-3