Astronomers have recently made groundbreaking discoveries about the life cycle of massive stars by studying a unique type of extreme supernova. This extraordinary event, designated SN2021yfj, involved a massive star that was stripped down to its core, providing invaluable insights into the processes of stellar evolution. This supernova, located an astonishing 2.2 billion light-years away from Earth, has revealed a different chemical signature compared to typical supernovae, leading scientists to rethink current models of stellar life and death.
Typically, when massive stars undergo supernova explosions, astronomers observe strong signals of light elements such as hydrogen and helium, which are found on the star's surface. However, in the case of SN2021yfj, researchers discovered traces of much heavier elements like silicon, sulfur, and argon, which originate from the star's inner layers. This finding supports the prevailing theory that massive stars have an onion-like structure, with lighter elements on the outer layers and heavier elements closer to their iron cores. The loss of the star's outer layers allowed for these inner, heavier elements to be exposed before the supernova explosion.
According to team leader and Northwestern University scientist Steve Schulze, this represents the first observation of a star essentially stripped to the bone. “It shows us how stars are structured and proves that stars can lose a lot of material before they explode,” Schulze stated. This phenomenon not only confirms the layered structure of massive stars but also offers a rare glimpse into the interior of a star just prior to its explosive demise.
SN2021yfj was initially spotted in September 2021 by the Zwicky Transient Facility (ZTF). The discovery suggests that while existing models of stellar life and death may be accurate, they do not fully encompass the explosive end of all stars. “This event looks like nothing anyone has ever seen before,” said Northwestern University researcher Adam Miller. He emphasized that the findings indicate a need to broaden the current understanding of stellar evolution, suggesting there may be more exotic pathways for how massive stars end their lives.
The progenitor stars of supernovas typically range between 10 and 100 times the mass of the Sun and generate energy through the nuclear fusion of lighter elements into heavier ones at their cores. As these stars evolve, they can fuse progressively heavier elements until they reach iron. Once the core is composed entirely of iron, it collapses, triggering a supernova that blasts away the outer layers. The subsequent collapse can result in the formation of a neutron star or, in the case of the most massive stars, a black hole.
To analyze the characteristics of SN2021yfj, astronomers employed a technique known as s spectroscopy, using the W.M. Keck Observatory in Hawaii. Initially, the team feared they had lost the opportunity to gather essential observations, but a colleague unexpectedly provided critical spectroscopic data, which revealed the unique nature of the supernova. Unlike previous supernovae, which typically revealed outer layers rich in elements such as carbon or oxygen, SN2021yfj displayed dominant signals of heavier elements, indicating that it had lost most of the material generated throughout its lifetime.
The cause of the extreme behavior exhibited by SN2021yfj remains somewhat of a mystery, with several potential explanations. These include a massive pre-supernova eruption, exceptionally strong stellar winds, or even a companion star stripping outer material away from the dying star. However, the research team posits that the most plausible scenario involves multiple episodes of pair instability, during which nuclear fusion reignites, producing powerful bursts of energy that strip away the star's outer shells. This violent process could be likened to the star effectively tearing itself apart before the supernova event.
The bright emissions that enabled SN2021yfj to be detected by the ZTF were likely caused by ejected material colliding with previously expelled shells. "While we have a theory for how nature created this particular explosion, I wouldn't bet my life that it's correct," Miller concluded. He emphasized the importance of uncovering more examples of such rare supernovae to deepen our understanding of their nature and formation. The study of SN2021yfj not only enhances our knowledge of supernovae but also challenges existing theories about the lifecycle of massive stars, indicating that the universe may hold even more extraordinary phenomena yet to be discovered.
