Scientists recently confirmed for the first time a new subclass of exploding star — or supernova — stripped to its core that also reduced down to the bare bones what we know about the massive stars, especially their life cycle.
The discovery challenges existing models of stellar evolution, revealing new pathways for how massive stars end their lives. It was made possible by using spectral data captured by W. M. Keck Observatory atop Mauna Kea on the Big Island.
“It shows us how stars are structured and proves that stars can lose a lot of material before they explode,” said Northwestern University Center for Interdisciplinary Exploration and Research in Astrophysics research associate and the study’s lead author Steve Schulze in an announcement about the “bare-bones” find.
He added that not only can these stars lose their outermost layers, they can be completely stripped all the way down to their core and still produce a brilliant explosion we can observe from extremely far distances.
The study was published Aug. 20 in the journal Nature.
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 this long-hypothesized internal layering in giant stars and 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,” said Northwestern assistant professor of physics and astronomy and senior author of the study Adam Miller in the announcement. “This star is telling us that our ideas and theories for how stars evolve are too narrow.”
It’s not so much that textbooks are incorrect, Miller added, they just 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,” he said.
Despite weighing in at 10 to 100 times heavier than our sun, these massive stars are powered by the same nuclear fusion process.
Intense pressure and extreme heat in the stellar core cause lighter elements to fuse together, generating heavier elements. As the star evolves with time, successively heavier elements are burned in the core as 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 ever previously detected.
Other observations of “stripped stars” 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.”
Schulze and their team discovered SN 2021yfj in September 2021 using the wide-field camera on the Zwicky Transient Facility 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 astronomy professor at University of California, Berkeley, had been very interested in infant supernovae himself and was intrigued by the object.
He and his team just so happened to be observing at Keck Observatory and was able to quickly pivot to capture the spectrum of this newly discovered celestial transient using the Big Island observatory’s low-resolution imaging spectrograph.
“It’s so exciting to discover a new class of exploding star,” said Filippenko in the discovery’s announcement, especially one that provides a confirmation of some of theories yet also reveals interesting new puzzles.
He agreed it was fortunate his 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,” Filippenko said. “Opportunities of this kind are rare!”
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.
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.”
They are exploring multiple scenarios, including interactions with a potential companion star, a massive pre-supernova eruption or even unusually strong stellar winds.
Most likely, however, 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 it causes a powerful burst of energy, pushing 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.”
(Except for the headline, this story has not been edited by PostX News and is published from a syndicated feed.)
