Astronomers have observed what they call a new type of supernova.
A study detailing surprising findings published Wednesday in the journal Nature.
A giant star is like an onion in the heavens. The outermost layer consists of lightweight elements such as hydrogen and helium, but with a heavier layer of elements below.
These stars can be 10-100 times heavier than our Sun, but it is the process of fusing lighter elements to create heavier ones.
Adam Miller, a researcher at Northwestern University’s Physics and Astronomy professor Adam Miller, says the stars start with a composition of about 75% hydrogen and 25% helium, and a small amount of carbon, nitrogen, silicon and other elements.
Through fusions occurring at the centre of the star with the highest temperature and density, hydrogen is converted to helium, creating an outer layer of onion structure. Throughout the life of the star, the process continues, fusing lighter elements to form heavier elements, and over time adds an inner layer of silicon, sulfur, oxygen, neon, magnesium and carbon underneath helium and hydrogen.
At the end of the star’s life, after all gas layers were formed, Miller explained that the star’s iron nucleus formed.
Fusion releases energy. This creates pressure to prevent the stars from collapse on their own due to gravity, Miller said. However, when the star tries to fuse the iron in its core into a heavier element, it does not have enough energy to continue to provide pressure. As a result, the star’s core collapses under gravity, leading to a stellar explosion.
However, when astronomers observed the first supernova named SN2021YFJ, nothing happened as expected. At one point long before the explosion, the star had already lost its outer layer of hydrogen, helium and carbon. Then, just before it exploded, the star released a typical hidden layer of relatively heavy elements such as silicon, sulfur, and argon, which are not often seen on dying stars.
The explosion of stars “littles” the exiled layers of silicon, sulfur and argon, Miller said.
“This is the first time I’ve seen a star essentially stripped of bone,” Steve Schulze, a researcher at the Center for Astrophysics Research at Northwestern University’s Center for Interdisciplinary Exploration and Research, said in a statement.
“It shows how the stars are structured and proves that they can lose a lot of material before the stars explode. They can not only lose the outermost layer, but they can peel completely downwards and cause great explosions that can be observed from very long distances.”
This discovery provides direct evidence of the difficult yet difficult to observe the inner structure of a giant star for a long time. It also challenges traditional ways astronomers can understand the evolution of stars.
“This event literally looks like no one has ever seen it,” Miller said in a statement.
“It was pretty much odd, so we didn’t expect to observe the right objects. This star says that our ideas and theories about how stars evolve are too narrow. The textbook is not wrong, but we should not fully capture everything that was produced in nature.
The study authors don’t know exactly what kind of stars existed before the supernova, but they believe they have a mass about 60 times heavier than the sun, Schulze and Miller said. However, the hydrogen layer outside the star had already been stripped before the explosion, so the star mass was smaller when it became a supernova than when it was born, Miller added.
It is known that a giant star removes the outer layer of material before it explodes, but this star has lost much more than previously observed. For example, astronomers have seen stars stripped of their hydrogen layers but still covered in helium, carbon and oxygen.
“The stars experience very strong instability,” Schulze said. “These instability is so violent that we can make the stars contract, and then suddenly release so much energy that flows through the outermost layer. You can do this multiple times.”
In some large-scale star explosions, we can observe that elements such as silicon and sulfur are “mixed” with all other elements as part of the ejected material, but have not been previously seen before the supernova.
The team estimates that stars need to release three times the mass of the Sun during their lifetime to leave their silicon and sulfur shells behind, suggesting that some stars will experience extreme mass losses later in their lives.
In this unique supernova, the team observed a thick shell of silicon and sulfur being exiled just before the death of the star. When the star exploded, the material exploded from its core. It collided with the gas shell, and the heat of the collision sparkled the silicon and sulfur layers.
“The star lost most of the material it produced throughout its lifetime,” Schulze said. “So we could only see the material that had been formed in the months just before the explosion. Something must have been very violent to cause it.”
The team discovered the supernova in September 2021 while using the Zwicky Transient facility at the Palomar Observatory in Southern California. Scanning the night sky with a wide field camera, Zwicky has a reputation for allowing astronomers to discover transient, or fleeting space phenomena, such as quickly burning and fading Supernova.
Schulze looked at the data for supernova evidence and noticed an object that had rapidly increased 2.2 billion light years from Earth. (A single light year is a measure of how long it takes light to travel to Earth, so the brightness increase occurred 2.2 billion years ago.)
To better understand what they were seeing, the team wanted to see the spectrum of objects – the wavelength of colored light, each color means different elements. Capture the spectrum Zwicky only measures the overall brightness change, so it was not possible with Zwicky. At first, it seemed that other telescopes could not capture a clear image of the supernova. However, Yi Yang, now an assistant professor at Tingua University in China, found objects and captured the spectrum while observing at the WM Keck Observatory in Hawaii.
Typically, supernova searches are done with small telescopes like Zwicky that measure brightness, and large telescopes like Keck are used to understand the chemical composition of the gases emitted by the explosion, according to Miller.
“If it wasn’t for that spectrum,” Miller said, “you may never have noticed this is a strange, unusual explosion.”
The team shared the spectrum with Avishay Gal-Yam, dean of the Department of Physics at Wiseman Institute of Science in Israel and professor of particle physics. Garuyam, a co-author of the study and a leading expert in supernova science, identified the mystical features of the spectrum that were found to be silicon, sulfur and argon, Schultz said.
The team remains unsure what was triggered for the stars to release The silicon and sulfur shells are examining the possibility that the star interacted with potential companion stars, experienced extremely strong star winds, or experienced a massive pre-Supernova explosion.
However, the research authors leaned towards the idea that the stars tore themselves.
Whatever the cause, the team designated the discovery as an entirely new type of supernova called the Type IEN (pronounced One-en) supernova, Miller said.
The supernova classification is based on the presence of different elements. Type II supernovae contain hydrogen, while Type IB has helium but no hydrogen. Type ICs have oxygen but no helium or hydrogen. Each type of Supernova exposes a deeper layer of stars.
“We tend to think of large stars forming sequences,” Miller said. “Silicon, sulfur and argon are only present in the deepest and innermost layers of the giant stars, so we call this new discovery.”
Stefano Valenti, an associate professor in the Faculty of Physics and Astronomy at the University of California, Davis, had never seen a spectrum like this study. Valenti is studying an unusual supernova, but he was not involved in this study.
“It’s clearly new,” Valenti said. “This finding shows that astronomical transient zoos are not yet complete, and that large broad research such as (Rubin Observatory) provides the opportunity to discover perhaps a new type of transient.”
Having examples of supernova types underscores the need to find other cases of this type, but said it is difficult. The Vera C. Rubin Observatory can find at least one million supernovaes, but it cannot measure their spectra. In that paper, the team showed that simple machine learning models did not identify supernova as rare based solely on their brightness.
“The big open question for me is, how often do such explosions occur in space?” Miller emailed, “Did we just happen to be incredibly and incredibly lucky? Or are we not searching the right way to find more?”
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