Artist’s impression of SN 2021yfj, a new kind of supernova, ejecting material rich in silicon (grey), sulfur (yellow) and argon (purple). Credit: W.M. Keck Observatory/Adam Makarenko
A group of international scientists has detected a never-before-seen silicon, sulphur and argon-rich supernova.
The supernova observations provide primary evidence of what happens in the inner layers and deep cores of giant stars and offer insight into the crucial moments before their explosive deaths.
When stars 10 to 100 times the mass of Earth’s Sun reach the end of their lives, they collapse in on themselves, causing a massively bright but brief supernova explosion.
The outer layers of these massive stars are mostly made from lighter elements, like hydrogen and helium. But through a process of nuclear fusion in the star’s core, heavier elements like carbon and oxygen are created and form the inner shells.
Although this stellar process is well understood, astronomers studying supernova explosions usually only find strong signatures of lighter elements from the outer layers, making it difficult to definitively prove what happens in the inner cores.
However, when the team observed a newly discovered supernova, they did not notice these lighter elements. They witnessed the inner silicon and sulphur rich layers exploding instead.
“This event quite literally looks like nothing anyone has ever seen before,” says Adam Miller, a senior author of the study and assistant professor at Northwestern University in the US.
The observations of the supernova, named SN2021yfj, have been published in Nature.
“This is the first time we have seen a star that was essentially stripped to the bone,” says Steve Schulze, lead author of the study from Northwestern University.
“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.”
Previous studies have witnessed supernova explosions after the outer hydrogen layer had been stripped, with scientists observing layers of helium or carbon and oxygen. But up until this point, astrophysicists have never witnessed heavier elements.
The team used a wide-field camera at the Zwicky Transient Facility in California to discover SN2021yfj in September 2021. They noticed an extremely luminous object located 2.2 billion light years away from Earth.
The team came across a challenge, though, as they were unable to find a clear enough image that they could use for a spectrum analysis.
A spectrum analysis breaks down the light coming from an object into its different colours. Each colour represents a different element, allowing scientists to uncover which elements are present during an explosion.
“We thought we had fully lost our opportunity to obtain these observations,” says Miller.
“So, we went to bed disappointed. But the next morning, a colleague at [the University of California] Berkeley unexpectedly provided a spectrum. Without that spectrum we may have never realised that this was a strange and unusual explosion.”
The spectrum had been gathered by this colleague using instruments at the W.M Keck Observatory in Hawai’i.
“Almost instantly, we realised it was something we had never seen before, so we needed to study it with all available resources,” says Schulze.
The team could only observe materials formed in the last few months of the star’s life before exploding. They suggest that some violent event must have occurred to strip the star of its outer hydrogen and helium layers.
“This star is telling us that our ideas and theories for how stars evolve are too narrow,” says Miller.
“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.”
While the team doesn’t know for certain the cause of this event, they do offer some suggestions. Potential scenarios include unusually strong stellar winds, influence from a companion star or a massive pre-supernova eruption.
However, the team suggests that this is most likely the result of the massive star tearing itself apart. As the star’s iron core collapses inward under its own gravity, it becomes extremely hot and dense. This creates the environment needed to set off nuclear fusion with such force it could push out the top layers of the star.
“While we have a theory for how nature created this particular explosion,” says Miller, “I wouldn’t bet my life that it’s correct.”
“This star really underscores the need to uncover more of these rare supernovae to better understand their nature and how they form.”