TL;DR: Northwestern University astrophysicists have made a discovery that challenges current models of stellar evolution. Leading an international team, the researchers detected a previously unseen type of exploding star that reveals the deep interior layers of massive stellar giants just moments before their catastrophic deaths.
The newly discovered supernova, SN2021yfj, exhibits a chemical signature unlike anything astronomers have seen before. Whereas typical massive star explosions are dominated by light elements such as hydrogen and helium, this event is rich in heavier elements – including silicon, sulfur, and argon – that usually remain buried deep within a star's core.
Northwestern's Steve Schulze, who led the study, highlighted how the supernova offers an unprecedented glimpse into the internal structure and mass loss of massive stars before they explode.
"This is the first time we have seen a star that was essentially stripped to the bone," Schulze explained. "It shows us how stars are structured and proves that stars can lose a lot of material before they explode."
The discovery offers the first direct evidence of the long-theorized onion-like structure of massive stars. Astronomers have proposed that these giants consist of progressively heavier layers, from the hydrogen- and helium-rich outer envelope down to the dense iron core at the center.
Adam Miller, a senior author on the study published in Nature, emphasized the observation's extraordinary significance.
"This event quite literally looks like nothing anyone has ever seen before. This star is telling us that our ideas and theories for how stars evolve are too narrow."
The team discovered SN2021yfj in September 2021 using the Zwicky Transient Facility, located east of San Diego. The supernova appeared as an exceptionally bright object in a star-forming region 2.2 billion light-years from Earth, making it visible despite the vast cosmic distance.
Obtaining crucial spectral data proved difficult when several telescopes worldwide were unavailable or obscured by clouds. A colleague at the University of California, Berkeley, unexpectedly stepped in, providing spectrum observations from the W.M. Keck Observatory in Hawaii and salvaging what the team feared would be a lost opportunity.
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"Without that spectrum, we may have never realized that this was a strange and unusual explosion," Miller noted.
The spectral analysis indicated that an extraordinarily violent event must have stripped the star's outer layers completely. Researchers suggest several possible mechanisms, including interactions with a companion star, massive pre-supernova eruptions, or unusually powerful stellar winds, any of which could explain the extreme material loss.
The most likely explanation involves a process called pair-instability, in which the massive star tears itself apart through repeated bursts of violent energy. These pulses gradually strip away the outer layers, leaving the deep interior exposed just before the star's final explosive death.
"This star lost most of the material that it produced throughout its lifetime," Schulze explained. "So, we could only see the material formed during the months right before its explosion."
The discovery highlights the importance of identifying more of these rare supernovae to gain a better understanding of how the universe produces such extreme cosmic events. Supported by the National Science Foundation and Northwestern's Center for Interdisciplinary Exploration and Research in Astrophysics, the research opens new avenues for studying the final moments of massive stellar evolution.