Never-Before-Seen Supernova Proves Crucial Fact On The Interior Of Stars

Researchers report the discovery of supernova SN2021yfj, a first-of-its-kind supernova that changes our knowledge about stars' evolution and death, while confirming something that has been suspected for a very long time. Massive, evolved stars are like onions and ogres: they have layers.

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Stars start by fusing hydrogen into helium; while they do that, they are in the main sequence. Once the hydrogen in the core is gone, there’s a mini-collapse, which leads to increased temperatures in the core and the fusing of helium into heavier elements. The star also swells up into a red giant star whose outer layers are loosely held together by gravity. The more massive stars continue fusing into heavier and heavier elements and end up forming layers of different elements.

The observations of SN2021yfj have shown that the star, which was originally up to 100 times the mass of the Sun, has lost most of its outer layers. No hydrogen, no helium, and no carbon. What the explosion showed was a collision between shells of material, strong signals of silicon, sulfur, and argon.

“This is the first time we have seen a star that was essentially stripped to the bone,” lead author Steve Schulze, from Northwestern University, said in a statement. “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, 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.”

The signal seen, now in its own class of Type Ien supernovae, was generated by ejecting shells rich in silicon, sulfur, and argon. The powerful interaction between two shells generated the brilliant supernova flash. But what of the star? It is currently unclear what has happened. Astronomers have previously found stripped stars, lacking hydrogen, but none that were also lacking helium, carbon, oxygen, and nitrogen.

The extreme shell release might be a consequence of the squeezing of the core and a subsequent burst of energy, pushing out some layers at high speed. Alternatively, the star might have had a companion, creating the shell release; it might have simply shed layers and the star is still out there intact; or, in the team-favored scenario, the star tore itself asunder.

“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.”

A paper describing this new discovery is published in the journal Nature.