Astronomers have used a new type of extreme supernova in which a massive star was stripped right “down to the bone” to better understand the process of stellar life and death.

When other massive stars die in supernova explosions, astronomers detect strong signals of light elements like hydrogen and helium that existed at the surface of the star. However, in this supernova, designated SN2021yfj and located 2.2 billion light-years from Earth, this team found a different chemical signature. This contained traces of heavier elements like silicon, sulfur, and argon that originate from deeper within the progenitor star.

If dying stars have onion-like structures with lighter elements at their surfaces and heavier elements toward their iron cores as astrophysicists currently theorize, then this star must have somehow lost its outer layers, thus exposing inner silicon and sulfur-rich layers before it “went nova.” This would not only confirm the layered structure of massive stars, but it also give stellar scientists a rare glimpse at the interior of a star prior to it exploding in a supernova.

“This is the first time we have seen a star that was essentially stripped to the bone,” team leader and Northwestern University scientist Steve Schulze 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.”

SN2021yfj, first spotted in September 2021 by the Zwicky Transient Facility (ZTF), suggests that while our models of stellar life and death and star structure may be correct, they may not fully describe the explosive death throes of all stars.

“This event quite literally looks like nothing anyone has ever seen before,” Northwestern University researcher and team member Adam Miller said. “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.”

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The team’s research was published on Wednesday (Aug. 20) in the journal Nature.

A burning onion in space

The progenitor stars of supernovas are between 10 and 100 times as massive as the sun, but still generate their energy via the nuclear fusion of lighter elements to heavier elements at their cores.

Whereas the sun will die when it has finished fusing its core hydrogen to helium in around 5 billion years, more massive stars have the pressures and temperatures at their cores to fuse progressively heavier and heavier elements right up to iron. As this process unfolds, lighter elements continue to undergo nuclear burning in the outer shells of massive stars.

When the cores of massive stars are hearts of pure iron, they collapse, and a supernova is triggered, ripping away the outer layers. The collapsing iron core eventually becomes a neutron star, or in the case of the most massive stars, a black hole.

To obtain information about supernovas, astronomers look for the signatures of chemical elements using a process called spectroscopy. The team was able to gain a spectroscopic picture of SN2021yfj using the W.M. Keck Observatory in Hawaii.

“We thought we had fully lost our opportunity to obtain these observations,” said Miller. “So, we went to bed disappointed. But the next morning, a colleague at UC Berkeley unexpectedly provided a spectrum. Without that spectrum, we may have never realized that this was a strange and unusual explosion.”

An illustration of a gamma-ray burst erupting from a dense environment around a collapsing massive star (Image credit: NASA, ESA and M. Kornmesser)

This revealed that SN2021yfj stands apart from other supernovas because the layers that were ripped away during its explosive end went deeper than what has been seen in the deaths of other massive stars. Astronomers have seen elements as heavy as carbon or oxygen during other supernovas due to the prior loss of stars’ outer hydrogen layers. However, no elements heavier than this, and thus from deeper within the progenitor stars, have been seen before.

“We saw an interesting explosion, but we had no idea what it was,” Schulze said. “Almost instantly, we realized it was something we had never seen before, so we needed to study it with all available resources.”

The spectrum of SN2021yfj didn’t just contain traces of heavy elements; it was dominated by strong signals of heavy elements like silicon, sulfur and argon. Thus, it became evident very early in this investigation that there was something particularly extreme and violent about SN2021yfj.

“This star lost most of the material that it produced throughout its lifetime,” Schulze explained. “So, we could only see the material formed during the months right before its explosion. Something very violent must have happened to cause that.”

What caused this particular supernova to be violent is still somewhat mysterious, with several possible scenarios including a massive pre-supernova eruption, unusually strong stellar winds, or even a companion star stripping outer material away from this dying star prior to its explosive death.

However, the team thinks the most likely explanation is multiple episodes of so-called “pair instability” during which nuclear fusion is reignited, causing powerful bursts of energy that blow away the outer shells of the star. This is akin to the massive star effectively ripping itself apart before its supernova death. The bright emission that allowed SN2021yfj to be spotted by the ZTF would have been caused by shells of ejected material catching up with and slamming into previously ejected shells.

“While we have a theory for how nature created this particular explosion,” Miller concluded. “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.”