Stars have layers like onions, based on principle. The layers are made of various parts, progressing from mild to heavy the deeper the layers are. Whereas the idea is powerful, observing the interior layers of a star has been principally inconceivable.
Till now.
Astronomers working with the Keck Observatory in Hawaii have obtained spectroscopic information from a supernova found by the Zwicky Transient Facility in 2021. The exploding star is 2.2 billion mild years away, and is known as SN 2021yf. The Keck information confirmed the presence of ionized silicon, sulphur, and argon. These have by no means been noticed in a supernova earlier than, as a result of they’re normally buried beneath different layers.
This discovery each confirms some theoretical understanding of exploding stars and challenges others.
Astronomers know that as an enormous star ages and evolves in direction of exploding as a supernova, it ejects materials from its outer layers. They’ve noticed it occurring, and these observations present it occurring. However this star has ejected extra materials than any beforehand noticed star.
The observations are introduced in a brand new paper titled “A cosmic formation site of silicon and sulphur revealed by a new type of supernova explosion.” The lead creator is Steve Schulze, a analysis affiliate at Northwestern College’s Heart for Interdisciplinary Exploration and Analysis in Astrophysics (CIERA).
“This occasion fairly actually seems to be like nothing anybody has ever seen earlier than.” – Adam Miller, Northwestern College.
“That is the primary time we have now seen a star that was basically stripped to the bone,” stated lead creator Schulze in a press release. “It exhibits us how stars are structured and proves that stars can lose quite a lot of materials earlier than they explode. Not solely can they lose their outermost layers, however they are often utterly stripped all the way in which all the way down to the core and nonetheless produce a superb explosion that we are able to observe from very, very far distances.”
When astronomers observe a supernova, they usually see sturdy indications of sunshine parts like hydrogen and helium. Layers of parts together with carbon, oxygen, neon, and magnesium are additionally generally noticed if the outermost hydrogen and helium have been shed. Deeper layers containing heavier parts like silicon, sulfur, and argon are obscured.
The long-standing theoretical understanding of stars says that huge stars are layered like onions, and these new Keck observations assist that. By seeing an enormous star’s interior layers proper earlier than it explodes, the invention is the primary time that observations assist this principle so clearly.
Nonetheless, the observations additionally pose a problem. Astrophysicists know that huge stars eject materials earlier than exploding as supernova. Shock waves from the expelled matter work together with the encompassing medium, heating it and creating observable mild signatures. However SN 2021yf will need to have ejected rather more materials than thought attainable, as a result of it is stripped all the way down to its core.
This determine exhibits the weather detected within the spectrum from SN 2021yf with the Keck’s Low Decision Imaging Spectrometer at some point after the ZTF noticed it. “The spectrum reveals slender emission traces of extremely ionized species of silicon, sulphur, and argon, which have by no means been seen in any SN earlier than, in addition to doubly ionized carbon, singly ionised magnesium, and impartial helium,” the authors write. The observations additionally present that a few of these parts are transferring at about 3,000 km/sec in an ejected circumstellar medium. Picture Credit score: Schulze et al. 2025.
“This occasion fairly actually seems to be like nothing anybody has ever seen earlier than,” added Adam Miller, an assistant professor of physics and astronomy at Northwestern and senior creator on the examine. “This star is telling us that our concepts and theories for a way stars evolve are too slender. It’s not that our textbooks are incorrect, however they clearly don’t absolutely seize every little thing produced in nature. There have to be extra unique pathways for an enormous star to finish its life that we hadn’t thought-about.”
Huge stars have the ability to fuse lighter parts into heavier parts in a course of referred to as nucleosynthesis. (With out stellar nucleosynthesis, the one parts within the Universe could be these created throughout the Big Bang.) All through its lifetime of fusion, an enormous star burns lighter parts like hydrogen and helium in its outer shells, whereas in its core it burns successively heavier parts in its deeper layers. Finally, a star finally ends up with an iron core. Iron cannot be burned to launch extra power, so as soon as the core is dominated by iron, fusion nearly ceases. With out the outward strain from fusion, the star collapses in on itself and explodes as a supernova.
Astrophysicists have noticed layers of helium, carbon, and oxygen in exploding stars earlier than, that are seen after the star has ejected its outer layer of hydrogen. By observing the silicon, sulphur, and argon, it implies that this star has ejected not solely its outer helium layer, however different layers as properly. This possible occurred in a number of episodes somewhat than suddenly.
Artist’s illustration of the supernova’s aftermath. Even after the star was stripped all the way down to its core, it continued to expertise violent mass-loss episodes, ejecting shells of fabric wealthy in silicon (gray), sulfur (yellow), and argon (purple). The catastrophic collision of those huge shells, as depicted on this illustration, generated a superb supernova explosion seen throughout 2.2 billion light-years of house. Credit score: Keck Observatory/Adam Makarenko.
“Stars expertise very sturdy instabilities,” Schulze stated. “These instabilities are so violent that they will trigger the star to contract. Then, it abruptly liberates a lot power that it sheds its outermost layers. It may possibly do that a number of occasions.”
Alex Filippenko is a professor of astronomy at UC Berkeley and a co-author of the paper. He occurred to be working with the Keck when SN 2021yf was found, and shortly pivoted to seize its spectrum withe the Keck’s LRIS. “It’s so thrilling to find a brand new class of exploding star, particularly one that gives a affirmation of a few of our theories of how huge stars evolve with time but additionally reveals attention-grabbing new puzzles,” stated Filippenko. “It was very lucky that my crew was utilizing the Keck I telescope the night time SN 2021yfj was found — we have been in a position to acquire a spectrum that immediately led to the conclusion that this was an extremely particular new sort of supernova. Alternatives of this type are uncommon!”
The silicon, sulfur and argon within the star weren’t all the time current. These parts have been created by way of nucleosynthesis within the star’s inside because it approached the top of its life.
“This star misplaced many of the materials that it produced all through its lifetime,” Schulze stated. “So, we may solely see the fabric fashioned throughout the months proper earlier than its explosion. One thing very violent will need to have occurred to trigger that.”
That query is on the coronary heart of this discovery. Is SN 2021yfj a brand new sort of supernova outlined by a strong new course of that stripped it of its outer layers? A few of the explanations the crew is contemplating are interactions with a companion star, uncommon and very highly effective stellar winds, and an enormous eruption that preceded the supernova explosion.
“Huge stars can lose a considerable quantity of their delivery mass via stellar winds, eruptions, and interplay with a companion star,” the researchers write of their paper. The presence of helium on this star’s circumstellar materials is puzzling, since helium is normally ejected earlier within the SN course of. “Since huge stars are likely to stay in binary programs, it is probably not too unlikely to have a helium-star companion with a powerful wind,” they write. This might clarify the helium.
The researchers assume that the almost certainly rationalization is that this huge star merely tore itself aside. Stellar cores of huge stars are beneath intense gravitational strain that raises their core temperatures till nuclear fusion is reignited, producing a strong explosion. The explosion blasts away the star’s outer layers. The method is repetitive, and every time it occurs, extra materials is ejected till the deeper core is seen.
Supernovae are categorized based on spectroscopy, and classifications are centered on hydrogen. Sort 1 present helium however no hydrogen and Sort 2 present hydrogen. Then there are sub-types beneath every of these classifications based mostly on different spectral traces. The sequence of labelled sub-types displays the quantity of stripping within the progenitor stars.
This desk exhibits supernova taxonomy. SN 2021yf might be a brand new sort referred to as Sort Ien. The ‘e’ signifies the place of the silicon/sulphur layer within the star’s construction, and the “n” signifies that the emission traces are slender. Picture Credit score: Wikipedia.
“Our observations … counsel that SN 2021yfj is certainly the primary instance of a Sort Ien SN,” the researchers write of their paper. This can be a new sort that lacks traces from hydrogen or helium, and is as an alternative dominated by emission traces from extremely ionized silicon, sulphur and argon.
Since there’s just one instance of this kind, there are nonetheless many questions. As is commonly the case in astronomy, a bigger dataset will possible result in some solutions.
“Whereas we have now a principle for a way nature created this explicit explosion,” Miller stated, “I wouldn’t wager my life that it’s appropriate, as a result of we nonetheless solely have one found instance. We nonetheless don’t absolutely perceive how nature created this explicit explosion. This star underscores the necessity to uncover extra of those uncommon supernovae, so we are able to proceed to check them.”
