Supernova Reveals What The Inside of a Star Really Looks Like

A supernova marking the final moments of a star has shaken the Universe in a way astronomers have never previously witnessed.

SN2021yfj: an unprecedented supernova 2.2 billion light-years away

Back in 2021, researchers looked on in disbelief as a supernova known as SN2021yfj flared into view at a distance of 2.2 billion light-years. Its spectrum was dominated by silicon, sulphur, and argon - a combination never before identified so clearly in the debris of an exploding star.

According to a team led by astrophysicist Steve Schulze of Northwestern University in the US, this material amounts to the first direct observational evidence for the long-proposed, onion-like structure inside massive stars: concentric shells made up of different elements. In turn, that supports the accepted picture of the stellar life cycle while also broadening what we know about how the most massive stars meet their end.

"This event quite literally looks like nothing anyone has ever seen before," says astrophysicist Adam Miller of Northwestern University.

"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."

Why the chemistry is so surprising

A star’s lifetime is driven by nuclear fusion in its core, where temperatures and pressures become so extreme that atoms are forced together to form heavier elements. In a massive star, hydrogen fuses into helium, helium into carbon, and the process continues through progressively heavier nuclei, ultimately reaching the stage where sulphur and silicon fuse into iron.

Iron represents the stopping point. Fusing iron consumes more energy than it releases, removing the key power source that holds the star up - essentially sounding the star’s death knell. Throughout the star’s life, theory predicts that the elements it manufactures arrange themselves in distinct layers, like an onion: the heaviest elements concentrated at the centre, with lighter material - hydrogen and helium - forming the outermost envelope.

When supernovae are observed, astronomers usually detect signatures of these lighter outer layers in the material blasted into space, and they rarely see strong evidence for the deeper, heavier layers - generally not beyond carbon and oxygen. The fact that SN2021yfj is instead dominated by much heavier elements indicates that the lead-up to its destruction was far more violent and chaotic than is typical.

A star “stripped to the bone”

"This is the first time we have seen a star that was essentially stripped to the bone," Schulze says. "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."

In the unstable period before a massive star dies, it can shed substantial amounts of its outer layers through repeated outbursts ahead of the supernova. Because silicon, sulphur, and argon are expected to exist only relatively near the core late in the star’s lifetime, their prominence implies that the star behind SN2021yfj somehow managed to jettison far more mass than a typical star on the verge of explosion.

One possible mechanism - and why it is not yet confirmed

The researchers cannot yet say exactly how that level of mass loss occurred. However, they outline a possible sequence in which the star’s final convulsions effectively tear it apart bit by bit. As the core exhausts its fuel, the pressure provided by fusion diminishes, allowing gravity to compress the core more strongly.

That rising inward squeeze and the accompanying heat could then spark a renewed burst of fusion in an explosive episode, throwing off part of the star’s outer material. If this cycle repeats, most of the star’s mass could be peeled away - like shrugging off a coat - producing an expanding shell of matter moving outward from the star.

The team suggests that, when the final supernova blast occurred, the faster-moving ejecta would have caught up with this pre-existing shell and slammed into it. That collision would generate the exceptionally bright light detected from billions of light-years away. Even so, additional observations will be needed before this explanation can be treated as confirmed.

"While we have a theory for how nature created this particular explosion," Miller says. "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."

The research has been published in Nature.