SN 2021yfj is a new kind of supernova. Its progenitor lost its outer shells well before the supernova happened, unlike any known star in the Milky Way. The dying star experienced extreme mass loss episodes that led to the ejecting of material rich in silicon (shown as gray), sulfur (yellow), and argon (purple).
Credit: W. M. Keck Observatory/Adam Makarenko
An international team of scientists, led by Northwestern University astrophysicists, has detected a never-before-seen type of exploding star, or supernova, that is rich with silicon, sulfur, and argon. Astronomers long have theorized that massive stars (10 to 100 times heavier than our Sun) have a layered structure. The outermost layers are made of the lightest elements. As the layers move inward, the elements become heavier.
When massive stars explode, their spectra typically show light elements, such as hydrogen and helium. But the newly discovered supernova, dubbed SN2021yfj, displayed a surprising chemical signature. The observations suggest it somehow lost its outer hydrogen, helium, and carbon layers — exposing the inner silicon- and sulfur-rich layers — before exploding. This finding offers direct evidence of the layered structure of stellar giants.
“This is the first time we have seen a star that was essentially stripped to the bone,” said Steve Schulze, a research associate at Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics, who led the study. “It shows us how stars are structured and proves that stars can lose a lot of material before they explode. Not only can they lose their outermost layers, but they can be completely stripped all the way down and still produce a brilliant explosion that we can observe from very, very far distances.”
“This event quite literally looks like nothing anyone has ever seen before,” added Northwestern’s Adam Miller, an assistant professor of physics and astronomy at Northwestern’s Weinberg College of Arts and Sciences, a senior author on the study. “It was almost so weird that we thought maybe we didn’t observe the correct object. This star is telling us that our ideas and theories for how stars evolve are too narrow. It’s not that our textbooks are incorrect, but they 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.”
Layers of elements
Massive stars are powered by nuclear fusion where extreme pressure and heat in their cores cause lighter elements to fuse, generating heavier elements. As the star evolves, successively heavier elements are fused in a series of shells surrounding the core. This process continues, eventually leading to a core of iron. When the iron core collapses, it triggers a supernova.
Although massive stars typically shed layers before exploding, SN2021yfj ejected far more material than scientists had ever detected. Other observations of “stripped stars” have revealed layers of helium, carbon, and oxygen — exposed after the outer hydrogen envelope was lost. But astrophysicists had never glimpsed anything deeper than that, hinting that something violent must have taken place.
Looking at its light
Schulze and their team discovered SN2021yfj in September 2021, using Northwestern’s access to the Zwicky Transient Facility (ZTF). Located just east of San Diego, ZTF uses a wide-field camera to scan the entire visible night sky. Since its launch, ZTF has become the world’s primary discovery engine for fleeting phenomena like supernovae.
After looking through ZTF data, Schulze spotted an extremely luminous object in a star-forming region located 2.2 billion light-years from Earth. To gain more information about the mysterious object, the team wanted to obtain its spectrum. The spectrum came from an astronomy colleague, who captured it using instruments at the W. M. Keck Observatory in Hawai‘i.
A weird explosion
Instead of helium, carbon, nitrogen and oxygen — found in the spectra of other supernovae — N2021yfj’s spectrum was dominated by strong lines of silicon, sulfur, and argon. These heavier elements form as shells around the core during a massive star’s 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.”
The science team is exploring possible reasons, including interactions with a potential companion star, a massive pre-supernova eruption, or even unusually strong stellar winds. But most likely, this mysterious supernova is the result of a massive star literally tearing itself apart. As the star’s energy output lessens and gravity can then shrink it, the core becomes even hotter and denser. This reignites nuclear fusion of heavier elements, creating powerful bursts of energy that push away the star’s outer layers. Each time the star undergoes a new episode, the corresponding pulse sheds more material.
“One of the most recent shell ejections collided with a pre-existing shell, which produced the brilliant emission that we saw as SN2021yfj,” Schulze said.
“While we have a theory for how nature created this particular explosion,” Miller said, “I wouldn’t bet my life that it’s correct, because we still only have one discovered example. This star really underscores the need to uncover more of these rare supernovae to better understand their nature and how they form.”
