Supernovae are spectacular events. A single star briefly shines brighter than the whole galaxy during an explosion marking its end. It also sews heavy elements like oxygen, silicon and iron into space as cosmic seeds for future star and planet formation.
The dramatic increase in brightness coming from a star in its final breath is how a supernova is detected and identified by scientists like Steve Schulze of Northwestern University, who led this study published in Nature.
Supernova2021yfj is the event in question, observed in 2021 by Steve Schulze and his team just before death. Core-collapse supernovae like this one only happen for stars over 8 times the mass of the sun. Elemental layers build up in the star as fusion combines smaller atoms to larger ones, with the core of the star containing the heaviest elements, like iron. The lightest elements like hydrogen and helium undergo fusion most readily, then once there is insufficient fusion occurring the star collapses under its own weight.
Explosion usually tails the collapse by mere milliseconds. The scientists managed to see inside the star during this tiny window. Finally the distinct rings suggested by models of supernovae have now been confirmed by observation.
“21yfj answered those questions, but it opened an entire new universe to us,” he said.
His colleagues at UC Berkeley used spectroscopic analysis to map the interior of the star from the outside, taking elemental ‘fingerprints’ from the star's light. Schulze recalled analysing the data for the first time, “I wake up and in my inbox is a spectrum of something that we had never observed before. So it was absolutely mind-blowing.”
More of the hydrogen and helium outer-layers had been lost right before explosion than models had predicted, revealing its silicon, sulfur and argon-rich insides. This “hiccup shortly before it exploded,” as Schulze described, allowed measurements of inner layers never before seen in a supernova. A true peek behind the cosmic curtain.
This is beyond conformation of theory. It shows that stars are more dynamic and complex than previously predicted and the answers it provided are now followed by as many new unanswered questions.
The scientists are now looking into how this outer-shedding was possible without immediate explosion. This momentary instability would require a huge amount of energy which Schulze theorises was provided by the star from “some very peculiar processes or rare processes at the end of its lifetime.”
This type of supernova is “one in over 10,000,” Schulze says. “It is definitely a very rare object... But it is not so rare that it is something like a once in a lifetime opportunity.” He hopes to find another like it that can help explain the cause of this brilliant boom.
