Rare stellar blast reveals a heavy-element core, challenging theories of how massive stars live and die
Media Contact: Meagan O’Shea
Maunakea, Hawaiʻi – For the first time, scientists have confirmed a new subclass of exploding star, made possible by spectral data captured by the W. M. Keck Observatory atop Maunakea, Hawaiʻi Island. The discovery challenges existing models of stellar evolution, revealing new pathways for how massive stars end their lives.
The study is published in the journal Nature.
“This is the first time we have seen a star that was essentially stripped to the bone,” said Steve Schulze, lead author and research associate at Northwestern University’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). “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 to the core and still produce a brilliant explosion that we can observe from very, very far distances.”
When massive stars explode, astrophysicists typically find strong signatures of light elements, such as hydrogen and helium. But the newly discovered supernova, dubbed SN 2021yfj, displayed a startling different chemical signature containing silicon, sulfur and argon, suggesting it somehow lost its outer hydrogen, helium, carbon, oxygen, neon, and magnesium layers — exposing the inner silicon and sulfur-rich layers — immediately before exploding. Astronomers have long proposed that massive stars possess an onion-like layered structure, with the lightest elements in the outermost shells and progressively heavier elements toward the center. The discovery of SN 2021yfj provides direct evidence of long-hypothesized internal layering in giant stars. It offers an unprecedented glimpse inside a stellar giant’s interior — captured just moments before its explosive death.
“This event quite literally looks like nothing anyone has ever seen before,” added Adam Miller, an assistant professor of physics and astronomy at Northwestern and senior author on the study. “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.”
A hot, burning onion
Weighing in at 10 to 100 times heavier than our sun, massive stars are powered by nuclear fusion. In that process, intense pressure and extreme heat in the stellar core cause lighter elements to fuse together, generating heavier elements. As the star evolves over time, successively heavier elements are burned in the core, while lighter elements are burned in a series of shells around the core. This process continues, eventually leading to a core of iron. When the iron core collapses, it triggers a supernova or forms a black hole.
Although massive stars typically shed layers before exploding, SN 2021yfj ejected far more material than scientists had ever previously detected. Other observations of “stripped stars” have revealed inner layers of helium or carbon and oxygen — exposed after the outer hydrogen envelope was lost. But astrophysicists had never glimpsed anything deeper than that.
“Stars experience very strong instabilities,” Schulze said. “These instabilities are so violent that they can cause the star to contract. Then, it suddenly liberates so much energy that it sheds its outermost layers. It can do this multiple times.”
Chasing down a cosmic oddity
Schulze and their team discovered SN 2021yfj in September 2021, using the wide-field camera on the Zwicky Transient Facility (ZTF) based on Palomar Mountain in Southern California. After looking through ZTF data, Schulze spotted an extremely luminous object in a star-forming region located 2.2 billion light-years from Earth. Without any idea what it was, but recognizing they had never seen it before, Schulze and Miller sought to obtain the object’s spectrum to determine which elements were present in the explosion.
Schulze contacted Yi Yang, then a postdoctoral scholar in Alex Filippenko’s group and now an assistant professor at Tsinghua University, China. Filippenko, a distinguished professor of astronomy at the University of California, Berkeley, had been very interested in infant supernovae himself and was intrigued by the object. Fortuitously, he happened to be observing at Keck Observatory and was able to quickly pivot to capture the spectrum of this newly discovered celestial transient using Keck Observatory’s Low-Resolution Imaging Spectrograph (LRIS). “It’s so exciting to discover a new class of exploding star, especially one that provides a confirmation of some of our theories of how massive stars evolve with time yet also reveals interesting new puzzles,” said Filippenko. “It was very fortunate that my team was using the Keck I telescope the night SN 2021yfj was discovered — we were able to obtain a spectrum that directly led to the realization that this was an incredibly special new type of supernova. Opportunities of this kind are rare!”
‘Something very violent must have happened’
Instead of typical carbon and oxygen — found in other stripped supernovae — the spectrum was dominated by strong signals of silicon, sulfur and argon. Nuclear fusion produces these heavier elements within a massive star’s deep interior during its final stages of life.
“This star lost most of the material that it produced throughout its lifetime,” Schulze said. “So, we could only see the material formed during the months right before its explosion. Something very violent must have happened to cause that.”
While the precise cause of this phenomenon remains an open question, Schulze, Miller, Yang, Filippenko, and their colleagues propose a rare and powerful process was at play. They are exploring multiple scenarios, including interactions with a potential companion star, a massive pre-supernova eruption, or even unusually strong stellar winds.
But, most likely, the team posits this mysterious supernova is the result of a massive star literally tearing itself apart. As the star’s core squeezes inward under its own gravity, it becomes even hotter and denser. The extreme heat and density then reignite nuclear fusion with such incredible intensity that it causes a powerful burst of energy that pushes away the star’s outer layers. Each time the star undergoes this powerful pulse, it sheds more material.
“We still don’t fully understand how nature created this particular explosion,” Miller said. “This star underscores the need to uncover more of these rare supernovae, so we can continue to study them.”
Filippenko’s research at Keck Observatory is supported by many donors, including Clark and Sharon Winslow, Gary and Cynthia Bengier, Briggs and Kathleen Wood, Christopher R. Redlich, Alan Eustace and Kathy Kwan, Timothy and Melissa Draper, William Draper, and Sanford Robertson.
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ABOUT LRIS
The Low Resolution Imaging Spectrometer (LRIS) is a very versatile and ultra-sensitive visible-wavelength imager and spectrograph built at the California Institute of Technology by a team led by Prof. Bev Oke and Prof. Judy Cohen and commissioned in 1993. Since then it has seen two major upgrades to further enhance its capabilities: the addition of a second, blue arm optimized for shorter wavelengths of light and the installation of detectors that are much more sensitive at the longest (red) wavelengths. Each arm is optimized for the wavelengths it covers. This large range of wavelength coverage, combined with the instrument’s high sensitivity, allows the study of everything from comets (which have interesting features in the ultraviolet part of the spectrum), to the blue light from star formation, to the red light of very distant objects. LRIS also records the spectra of up to 50 objects simultaneously, especially useful for studies of clusters of galaxies in the most distant reaches, and earliest times, of the universe. LRIS was used in observing distant supernovae by astronomers who received the Nobel Prize in Physics in 2011 for research determining that the universe was speeding up in its expansion.