Round 11,300 years in the past, a large star teetered on the precipice of annihilation. It pulsed with power because it expelled its outer layers, shedding the fabric into house. Finally it exploded as a supernova, and its remnant is among the most studied supernova remnants (SNR). It is known as Cassiopeia A (Cas A) and new observations with the Chandra X-ray telescope are revealing extra particulars about its demise.
Cas A’s progenitor star had between about 15 to twenty photo voltaic lots, although some estimates vary as excessive as 30 photo voltaic lots. It was probably a pink supergiant, although there’s debate about its nature and the trail it adopted to exploding as a supernova. Some astrophysicists assume it could have been a Wolf-Rayet star.
In any case, it will definitely exploded as a core-collapse supernova. As soon as it constructed up an iron core, the star might not assist itself and exploded. The sunshine from Cas A’s demise reached Earth across the 1660s.
There are not any definitive data of observers seeing the supernova explosion within the sky, however astronomers have studied the Cas A SNR in nice element in trendy instances and throughout a number of wavelengths.
It is a composite false color picture of Cassiopeia A. It accommodates knowledge from the Hubble Area Telescope, the Spitzer Area Telescope, and the Chandra X-ray telescope. Picture Credit score: NASA/JPL-Caltech
New analysis in The Astrophysical Journal explains Chandra’s new findings. It is titled “Inhomogeneous Stellar Mixing in the Final Hours before the Cassiopeia A Supernova.” The lead writer is Toshiki Sato of Meiji College in Japan.
“It looks as if every time we carefully take a look at Chandra knowledge of Cas A, we study one thing new and thrilling,” stated lead writer Sato in a press release. “Now we’ve taken that invaluable X-ray knowledge, mixed it with highly effective laptop fashions, and located one thing extraordinary.”
One of many issues with learning supernovae is that their eventual explosions are what set off our observations. An in depth understanding of the ultimate moments earlier than a supernova explodes is tough to acquire. “In recent times, theorists have paid a lot consideration to the ultimate inside processes inside large stars, as they are often important for revealing neutrino-driven supernova mechanisms and different potential transients of large star collapse,” the authors write of their paper. “Nevertheless, it’s difficult to watch immediately the final hours of a large star earlier than explosion, since it’s the supernova occasion that triggers the beginning of intense observational research.”
The lead as much as the SN explosion of a large star includes the nucleosynthesis of more and more heavy parts deeper into its inside. The floor layer is hydrogen, then helium is subsequent, then carbon and even heavier parts below the outer layers. Finally, the star creates iron. However iron is a barrier to this course of, as a result of whereas lighter parts launch power once they fuse, iron requires extra power to endure additional fusion. The iron builds up within the core, and as soon as the core reaches about 1.4 photo voltaic lots, there’s not sufficient outward stress to forestall collapse. Gravity wins, the core collapses, and the star explodes.
This high-definition picture from NASA’s James Webb Area Telescope’s NIRCam (Close to-Infrared Digicam) unveils intricate particulars of supernova remnant Cassiopeia A (Cas A), and reveals the increasing shell of fabric slamming into the gasoline shed by the star earlier than it exploded. Picture Credit score: NASA, ESA, CSA, STScI, Danny Milisavljevic (Purdue College), Ilse De Looze (UGhent), Tea Temim (Princeton College)
Chandra’s observations, mixed with modelling, are giving astrophysicists a glance contained in the star throughout its ultimate moments earlier than collapse.
“Our analysis reveals that simply earlier than the star in Cas A collapsed, a part of an inside layer with giant quantities of silicon traveled outwards and broke right into a neighboring layer with a lot of neon,” stated co-author Kai Matsunaga of Kyoto College in Japan. “It is a violent occasion the place the barrier between these two layers disappears.”
The outcomes have been two-fold. Silicon-rich materials travelled outward, whereas neon-rich materials travelled inward. This created inhomogeneous mixing of the weather, and small areas wealthy in silicon have been discovered close to small areas wealthy in neon.
Inhomogeneous elemental distribution in Cas A noticed by Chandra. The distinction within the mixing ratio of blue and inexperienced colours clearly reveals the totally different composition within the O-rich ejecta. The pink, inexperienced, and blue embody emission inside power bands of 6.54–6.92 keV (Fe Heα), 1.76–1.94 keV (Si Heα), and 0.60–0.85 keV (O traces), respectively. The ejecta highlighted in pink and inexperienced are merchandise of explosive nucleosynthesis, whereas the ejecta in blue and emerald inexperienced replicate stellar nucleosynthesis. The circles within the small panels are O-rich areas used for spectral evaluation. The areas of excessive and low X-ray depth within the Si band are indicated by the magenta and cyan circles, respectively. Picture Credit score: Toshiki Sato et al 2025 ApJ 990 103
That is a part of what the researchers name a ‘shell merger’. They are saying it is the ultimate section of stellar exercise. It is an intense burning the place the oxygen burning shell swallows the outer Carbon and Neon burning shell deep contained in the star’s inside. This occurs solely moments earlier than the star explodes as a supernova. “Within the violent convective layer created by the shell merger, Ne, which is considerable within the stellar O-rich layer, is burned as it’s pulled inward, and Si, which is synthesized inside, is transported outward,” the authors clarify of their analysis.
This schematic reveals the inside of a large star within the technique of a ‘shell merger.’ It reveals each the downward plumes of Neon-rich materials and the upward plumes of silicon-rich materials. Picture Credit score: Toshiki Sato et al 2025 ApJ 990 103
The intermingled silicon-rich and neon-rich areas are proof of this course of. The authors clarify that the the silicon and neon didn’t combine with the opposite parts both instantly earlier than or instantly after the explosion. Although astrophysical fashions have predicted this, it is by no means been noticed earlier than. “Our outcomes present the primary observational proof that the ultimate stellar burning course of quickly alters the interior construction, leaving a pre-supernova asymmetry,” the researchers clarify of their paper.
For many years, astrophysicists thought that SN explosions have been symmetrical. Early observations supported the thought, and the essential concept behind core-collapse supernovae additionally supported symmetry. However this analysis modifications the elemental understanding of supernova explosions as asymmetrical. “The coexistence of compact ejecta areas in each the “O-/Ne-rich” and “O-/Si-rich” regimes implies that the merger didn’t absolutely homogenize the O-rich layer previous to collapse, abandoning multiscale compositional inhomogeneities and uneven velocity fields,” the researchers write of their conclusion.
This asymmetry may clarify how the neutron stars left behind get their acceleration kick and result in high-velocity neutron stars.
These ultimate moments in a supernova’s life might also set off the explosion itself, in accordance with the authors. The turbulence created by the inside turmoil could have aided the star’s explosion.
“Maybe a very powerful impact of this modification within the star’s construction is that it could have helped set off the explosion itself,” stated co-author Hiroyuki Uchida additionally of Kyoto College. “Such ultimate inner exercise of a star could change its destiny—whether or not it should shine as a supernova or not.”
“For a very long time within the historical past of astronomy, it has been a dream to check the interior construction of stars,” the researchers write of their paper’s conclusion. This analysis has given astrophysicists a essential glimpse right into a progenitor star’s ultimate moments earlier than explosion. “This second not solely has a big influence on the destiny of a star, but additionally creates a extra uneven supernova explosion,” they conclude.
