International study led by Northwestern University and the Weizmann Institute of Science has identified SN2021yfj – a supernova rich in iron, sulfur and argon – confirming the “onion model” of giant stellar structure; featured on the cover of Nature and observed from a distance of 2.2 billion light-years
Giant stars are built in the onion method – they are made up of layers, with the outer shells consisting mainly of light chemical elements, and as you go deeper, the elements become heavier and heavier until they reach an iron core at their center. At least that is the accepted theory, but observations of giant stars that have exploded – a phenomenon known as a supernova – have so far detected almost exclusively signs of light elements.New research, which will be published tomorrow (August 21) on the cover of the scientific journal NatureAn international team of scientists from Northwestern University and the Weizmann Institute of Science has identified a new type of supernova – rich in heavy elements such as silicon, sulfur and argon. The findings indicate that the giant star shed its outer layers “while still alive,” and with its final explosion, provided a rare glimpse into its depths and significant confirmation of the onion structure theory.
"For the first time, we have seen a star exposed to its core," says Dr. Steve Schulze, a former member of Prof. Avishai Gal-Yam at the Weizmann Institute and now a researcher at Northwestern University, who led the study. "The findings teach us how giant stars are built and reveal that they can lose many layers of material and still produce a powerful explosion that can be observed and studied from Earth."
Supernova on the spectrum
Despite its enormous size – 10 to 100 times the mass of our Sun – a giant star collapses in a fraction of a second, and the bright light emitted in the explosion can usually be observed for several weeks. The scintillating light of the current supernova, which is named SN2021yfj, was detected by Dr. Schulze in September 2021 using the Zwicky Transient Facility (ZTF) – a telescope located east of San Diego, California, and equipped with a wide-field camera that scans the entire visible night sky. Dr. Schulze noticed in the telescope images an especially luminous object that was located in a nebula where new stars are being born about 2.2 billion light-years from Earth.
The spectrum of the nucleus
To learn about the mysterious object, the research team tried to get their hands on the spectrum of the light emitted from it. Analyzing the spectrum allows researchers to know which chemical elements were present in the explosion, since each color of light in the spectrum is characteristic of the burning of a different chemical element. Dr. Schulze and his collaborators acted quickly so as not to miss the window of opportunity, but attempts to obtain the spectrum were initially in vain – telescopes around the world were either unavailable or unable to penetrate the cloud layer to obtain a clear image. Finally, a colleague from the University of California, Berkeley, was able to provide the requested information, and the scientists were amazed to discover that instead of the helium, carbon, nitrogen and oxygen – usually found in naked supernovae – the spectrum of SN2021yfj was saturated with strong signals of silicon, sulfur and argon.
In previous cases, giant stars have also shed their outer layers before the explosion that ended their lives. However, previous observations have revealed stars that have shed their hydrogen layer – the lightest element in nature and the one located in the outermost part of the star; when those stars exploded, layers of slightly heavier elements such as helium, carbon and oxygen were exposed. The star at the center of the new study, SN2021yfj, shed more outer layers than any star observed before, providing a glimpse into deeper layers than ever before and allowing the identification of heavier elements.
"The star prematurely lost most of the material it had created throughout its life, so we could actually only see the material that was created in the months leading up to the explosion. Something very violent must have happened to cause this," explains Dr. Schulze. "Exposing such an inner layer challenges existing theories about mass loss and layer exfoliation in giant stars, and indicates that this is an unusual supernova that occurred by an unknown mechanism," adds Dr. Ofer Yaron, a staff scientist in Prof. Gal-Yam's group and a world leader in the field of supernova databases.
Scientists have examined several possible scenarios that led to this type of supernova, including an interaction with a nearby companion star, a massive burst of energy that occurred before the star exploded and ended its life, and even extremely strong gusts of stellar wind that caused it to be stripped of its shells. However, the most likely explanation is that the star "torn itself apart." Late in its life, its core began to collapse inward under its own gravity and became extremely hot and dense. Waves of inward collapse and extreme conditions ignited powerful processes of nuclear fusion and bursts of energy that pushed the outer layers of the star away from it. Scientists also believe that the exceptionally bright explosion observed on Earth is actually the result of a collision of one of the shells that was thrown out of the star with another shell that was thrown out earlier. At this point, these are only estimates, and the exact reason why a giant star lost its outer shells remains a mystery.
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