Before a giant star explodes in a supernova, it still produces the elements that make up the Earth and its curious inhabitants: Substances such as carbon, sulphur, oxygen, silicon and iron. However, these are not produced randomly anywhere, but in well-organised, stacked spherical shells made of different elements that become heavier and heavier towards the inside. A rare stellar explosion known as SN 2021yfj has now revealed the innermost, previously only theoretically predicted spherical shell for the first time - and at the same time raised new mysteries. As a team led by Steve Schulze from Northwestern University in Evanston reports in the scientific journal «Nature», a giant star had blown the innermost layer of silicon and sulphur into space as a coherent gas shell. In the subsequent supernova, spectroscopic measurements were able to distinguish this shell from the explosion itself. On the one hand, the analysis confirms the shell model of the evolution of giant stars, but on the other hand it also calls it into question. Because how the star exposed its innermost core is a complete mystery.
For several decades, experts have understood quite well what happens when a giant star of around 40 solar masses or more approaches the explosive end of its existence. When almost all the hydrogen has been burnt to helium, the core of the star shrinks and becomes hotter and denser until helium can fuse. For its products oxygen and carbon to fuse in turn, the conditions must become even more extreme. At ever higher temperatures and pressures, neon, magnesium, silicon, sulphur and finally iron are gradually formed, the final stage of stellar fusion. If this iron core becomes too large, it collapses into a neutron star or black hole and causes a gigantic explosion.
The progenitor star of the 2021yfj supernova was also a Wolf-Rayet star, a very rare class of giant stars that eject an extraordinary amount of mass before exploding. They reveal their interior in the process, so that their shells of oxygen and carbon have already been detected. However, gigantic cataclysms are necessary for a giant star to also eject its sulphur-silicon shell. Schulze's team calculated that the progenitor star of 2021yfj must have ejected around three times the mass of the sun in a single episode in order to explain the observed gas shell. There is currently no mechanism that could cause this. Another finding deepens the mystery surrounding the exotic supernova 2021yfj even further. Schulze's team detected small amounts of helium between the silicon and sulphur - but this should not be able to exist so deep in the star under any circumstances. So while the detection of the sulphur-silicon shell supports the decades-old model of the prelude to a supernova, the study also shows how many mysteries these very «ordinary» supernovae still pose.
