Astronomers Uncover Heavily Stripped Star Before Supernova Explosion

For the first time, astronomers have identified a heavily stripped star that shed its outer layers just before its demise in a supernova explosion. A team led by Steve Schulze from Northwestern University discovered spectral signatures of heavy elements typically concealed deep within stars, providing new insights into stellar evolution.

Inside a star, the process of nucleosynthesis fuses atomic nuclei to create heavier elements, releasing energy that balances the immense gravitational forces at play. As stars age, they undergo a transformation akin to layers of an onion, with hydrogen forming the outer shell and progressively heavier elements forming the core. Near the end of a star’s life cycle, elements such as silicon, sulfur, and argon fuse together, eventually leading to the formation of iron. Unlike lighter elements, iron does not release energy during fusion, which contributes to the star’s collapse and subsequent explosion.

Observing a star that has expelled its outer layers prior to its explosion is rare. When a supernova occurs, heavier elements that are generally hidden create absorption lines in the light spectrum, allowing astronomers to analyze the composition of these inner layers. While elements like carbon and oxygen have been observed in previous instances, direct evidence of deeper elements remained elusive until now.

On September 7, 2021, a notable observation emerged from a program at the Zwicky Transient Facility, led by Avishay Gal-Yam at the Weizmann Institute of Science in Israel. The team was monitoring the sky for signs of infant supernovae when they detected SN 2021yfj due to its rapid brightness increase. Schulze recalls the moment, stating, “We immediately contacted Alex Filippenko’s group at the University of California, Berkeley to ask whether they could obtain a spectrum of this supernova.”

The spectral analysis revealed distinctive absorption lines that were different from anything the team had previously encountered. “We initially had no idea that most of the features in the spectrum were produced by silicon, sulfur, and argon,” Schulze explained.

As the investigation continued, the team conducted simultaneous observations of SN 2021yfj using various ground and space telescopes. Once Gal-Yam completed the analysis, the data consistently confirmed a groundbreaking result: they had detected a supernova enveloped in a shell rich in silicon, sulfur, and argon. These elements, typically hidden beneath other materials, are formed shortly before a star’s death, making their observation particularly significant.

The findings indicate that SN 2021yfj had undergone a more extreme shedding of its outer layers than any previously documented star. “SN 2021yfj demonstrates that stars can die in far more extreme ways than previously imagined,” Schulze remarked. “It reveals that our understanding of how stars evolve and die is still not complete, despite billions of them having already been studied.”

This research, published in the journal Nature, opens new avenues for understanding the later stages of stellar evolution and the processes that lead to these dramatic cosmic events. As astronomers continue to study these findings, they aim to deepen our knowledge of the life cycles of stars and the remarkable phenomena that accompany their explosive ends.