Mass -rich stars have an onion -like structure shortly before their end – according to the theory. Due to the fusion of increasingly heavier elements, lighter elements collect on the outside, further inside, heavier to a star core made of solid iron. Now a new Supernova has also unveiled the innermost layers of a dying star and confirmed this shell structure. In the light spectrum of the explosion of around 2.2 billion light years away, astronomers identified a pattern of spectral lines of elements silicon, sulfur and argon that was never seen before. This suggested that the innermost layers exposed to this star before exploding. The SN 2021YFJ baptized star explosion thus represents a completely new type of supernova that the researching types have baptized. However, it is puzzling why this distant star lost its entire external layers, but still kept a rest of the helium.
Most of your life cycle, stars draw stars from the core fusion of hydrogen and later helium. It provides you with the energy for your glow. But with massive stars, the stock of these fusion fuels is quickly exhausted. They then begin to merge the heavier products of this fusion inside. Over time, concentric shells form more and more heavier elements inside – from helium to carbon, oxygen, neon and magnesium to silicon and sulfur. “In the last step, silicon and sulfur merge into iron, this leads to a collapse of the star core and either a supernova explosion or the direct formation of a black hole,” explain Steve Schulze from Northwestern University in Illinois and his colleagues. Supernovae of stars provide indications of this shell structure, which cut off their outer covers shortly before the explosion and in which the helium or even carbon oxygen layers expire. “So far, however, evidence of the innermost layers that are more difficult to produce elements has been lacking than oxygen,” continued the astronomers.
Spectral signature even the innermost layers
The team around Schulze has now found this document. The starting point was a supernova detected by Zwicky Transient Facility (ZTF) in California in September 2021. It showed itself as an extremely bright point of light in a galaxy, which was around 2.2 billion light years away. 24 hours later, astronomers managed to capture a first spectrum of this supernova at the Keck-Observatory in Hawaii. “We almost immediately realized that this was something that we had never seen before,” says Schulze. Because in this spectrum there were numerous spectral lines of ionized silicon, sulfur and argon, but the signatures of lighter elements, such as hydrogen and nitrogen, which are otherwise typical of supernovae, were missing or were only very weakly represented as with helium and carbon. This unusual signature was also confirmed when Schulze and his team observed the re -gluing of this SN 2021YFJ star explosion for another 120 days. “This star must have lost most of the material that it produced in the course of his life,” explains Schulze. “We only see the elements that were created in the last months before the explosion.”
The spectral signatures of SN 2021YFJ suggest that only the innermost, heaviest layers of this star are left. “This is the first time that we see a star that has literally exposed to the bones,” says the astronomer. “This tells us how stars are structured – and for the first time it proves that they can not only lose their outer layers before an explosion, but also further inside shells.” At the same time, this star explosion presents the astronomers with a mystery. Because their course and its spectral features do not fit any guy’s type known so far. “SN 2021YFJ is most likely the first representative of a previously unknown supernova class,” the researchers write. You have baptized this new class type. Type1 refers to Supernovae of stars that have used up their supply of hydrogen. The “N” characterizes precursor stars, which are surrounded by a dense cover made of circuit changes, as Schulze and his team explain. When the shock wave of the Supernova hits this shell, it heats up the material and lines leaves lines in the spectrum. The letter between 1 and n describes what this material is made of. B stands for Helium, C for carbon/oxygen. The naming type1en classifies the newly discovered supernova as a star explosion, in which the star is surrounded by a circuit conversation shell made of silicon and sulfur.
(Video: Keck Observatory/ Adam Makarenko)
Prehistory of Supernova is still puzzling
But how can he explain this star exposed to his inner one? “Something very dramatic has to happen,” says Schulze. The astronomers played out different scenarios for their study, but did not find a clear explanation. So there are massive stars that develop extreme star winds shortly before their end and cut their outer layers in a series of outbreaks. “But exposure down to the oxygen/silicon shell is difficult to explain,” writes the team. In her view, a pulsating couple instability could be most likely to fit. According to the theory, this occurs with very massive stars from 70 to 140 solar masses. When the oxygen burning begins, the enormous pressure in the internal electron-positron pairs-electrons and their anti-particles are created. These extinguish each other and thus create repeated energy checks that trigger new phases of the nuclear fusion and blow away parts of the star. “The resulting interaction between the layers of the circuit change material can produce luminous transients, the properties of which match those of SN 2021YFJ,” write the astronomers.
However, a characteristic in the light spectrum of the supernova does not fit into the picture: a star that is such a strongly decimated star should actually no longer have a helium. But in the spectrum of SN 2021YFJ, the spectral signature of the helium is weak, but clearly recognizable. “This detection of Helium cannot be explained easily,” explain Schulze and his colleagues. “At the moment we can only speculate about the origin of these helium signatures.” A possible explanation would be that the star has an undetected companion whose helium -containing star wind has contaminated the supernova with this element. It would also be conceivable that the interior of the star has been asymmetrical and that only small remains of helium -rich material have remained in a region at the Stellar Equator, as the astronomers explain. But these are just scenarios. “This star shows us that our ideas and theories for the stellar development are too closely set,” says senior author Steve Miller from Northwestern University. “Our textbooks are not wrong, but they obviously do not capture the entire spectrum of these events. There must be other exotic paths through which the life cycle of a massive star ends.” It is all the more important to track down more of these rare supernovae.
Source: nature, DOI: 10.1038/S41586-025-09375-3
