A companion star may have stripped silicon layers. This provided a new window into stellar death.
Astronomers have observed the inner layers of a dying star through a rare type of stellar explosion known as an “extremely stripped supernova.”
In a study published on August 20, 2025, in Nature, Steve Schulze of Northwestern University in the United States and his collaborators reported on supernova 2021yfj and the thick shell of gas surrounding it.
Their results reinforce long-standing theories about what occurs inside massive stars at the end of their lives, as well as how these processes contribute to the formation of the universe’s fundamental elements.
How stars make the elements
The energy of stars comes from nuclear fusion, a reaction in which lighter atoms are pressed together to form heavier ones, releasing energy in the process.
Fusion unfolds in a sequence of stages over the lifetime of a star. Hydrogen (the lightest element) fuses into helium first, followed by progressively heavier elements such as carbon. The largest stars advance further, producing neon, oxygen, silicon, and eventually iron.
Each successive burning stage happens more quickly than the last. While hydrogen fusion can persist for millions of years, silicon fusion may last only a few days.
As the core of a massive star keeps burning, the gas outside the core acquires a layered structure, where successive layers record the composition of the progression of burning cycles.
While all this is playing out in the star’s core, the star is also shedding gas from its surface, carried out into space by the stellar wind. Each fusion cycle creates an expanding shell of gas containing a different mix of elements.
Core collapse
What happens to a massive star when its core is full of iron? The great pressure and temperature will make the iron fuse, but unlike the fusion of lighter elements, this process absorbs energy instead of releasing it.
The release of energy from fusion is what has been holding the star up against the force of gravity – so now the iron core will collapse. Depending on how big it is to start with, the collapsed core will become a neutron star or a black hole.
The process of collapse creates a “bounce,” which sends energy and matter flying outwards. This is called a core-collapse supernova explosion.
The explosion lights up the layers of gas shed from the star earlier, allowing us to see what they are made of. In all known supernovae until now, this material was either the hydrogen, the helium or the carbon layer, produced in the first two nuclear burning cycles.
The inner layers (the neon, oxygen, and silicon layers) are all produced in a mere few hundred years before the star explodes, which means they don’t have time to travel out far from the star.
An explosive mystery
But that’s what makes the new supernova SN2021yfj so interesting. Schulze and colleagues found the material outside the star came from the silicon layer, the last layer just above the iron core, which forms on a timescale of a few months.
The stellar wind must have expelled all the layers right down to the silicon one before the explosion occurred. Astronomers don’t understand how a stellar wind could be powerful enough to do this.
The most plausible scenario is a second star was involved. If another star were orbiting the one that exploded, its gravity might have rapidly pulled out the deep silicon layer.
Exploding stars made the universe what it is today
Whatever the explanation, this view deep inside the star has confirmed our theories of the cycles of nuclear fusion inside massive stars.
Why is this important? Because stars are where all the elements come from.
Carbon and nitrogen are manufactured primarily by lower mass stars, similar to our own Sun. Some heavy elements such as gold are manufactured in the exotic environments of colliding and merging neutron stars.
However, oxygen and other elements such as neon, magnesium, and sulfur mainly come from core-collapse supernovae.
We are what we are because of the inner workings of stars. The constant production of elements in stars causes the universe to change continuously. Stars and planets formed later are very different from those formed in earlier times.
When the universe was younger it had much less in the way of “interesting” elements. Everything worked somewhat differently: stars burned hotter and faster and planets may have formed less, differently, or not at all.
How much supernovae explode and just what they eject into interstellar space is a critical question in figuring out why our Universe and our world are the way they are.
Reference: “Extremely stripped supernova reveals a silicon and sulfur formation site” by Steve Schulze, Avishay Gal-Yam, Luc Dessart, Adam A. Miller, Stan E. Woosley, Yi Yang (杨轶), Mattia Bulla, Ofer Yaron, Jesper Sollerman, Alexei V. Filippenko, K-Ryan Hinds, Daniel A. Perley, Daichi Tsuna, Ragnhild Lunnan, Nikhil Sarin, Seán J. Brennan, Thomas G. Brink, Rachel J. Bruch, Ping Chen, Kaustav K. Das, Suhail Dhawan, Claes Fransson, Christoffer Fremling, Anjasha Gangopadhyay, Ido Irani, Anders Jerkstrand, Nikola Knežević, Doron Kushnir, Keiichi Maeda, Kate Maguire, Eran Ofek, Conor M. B. Omand, Yu-Jing Qin, Yashvi Sharma, Tawny Sit, Gokul P. Srinivasaragavan, Nora L. Strothjohann, Yuki Takei, Eli Waxman, Lin Yan, Yuhan Yao, WeiKang Zheng, Erez A. Zimmerman, Eric C. Bellm, Michael W. Coughlin, Frank J. Masci, Josiah Purdum, Mickaël Rigault, Avery Wold and Shrinivas R. Kulkarni, 20 August 2025, Nature.
DOI: 10.1038/s41586-025-09375-3
Adapted from an article originally published in The Conversation.
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