A huge star that moved soundly through the cliff of extinction about 11 or 300 years ago. The banishment of the outer layer pulsated with energy, causing material to flow into space. It eventually explodes as a supernova, and its remains are one of the most studied supernova remnants (SNRs). This is called Cassiopeia A (CAS A), and new observations on the Chandra X-Ray telescope reveal more details about its end.
The progenitor stars in Cas A had solar masses of about 15-20, but some estimates range from 30 solar masses. It could have been a red supertitan, but there is debate about its nature and the path it will subsequently explode as a supernova. Some astrophysicists believe it could have been a star of Wolf Rayette.
In any case, it eventually exploded as a Core Collapse Supernova. Once it built an iron core, the stars could no longer support themselves and explode. The light from the end of Cas A reached Earth around the 1660s.
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Although there is no conclusive record of observers looking at supernova explosions in the sky, astronomers have studied CAS A SNR in great detail, both modern and across multiple wavelengths.
A new study in the Astrophysical Journal describes Chandra’s new discoveries. It is titled “Uneven star mixing at the final time before Cassiopeia A supernova.” The main author is Sato Sato from Meiji University in Japan.
“Every time you look closely at Chandra data from CAS A, you learn something new and exciting,” said lead author Sato in a press release. “Now we’ve taken that incredibly valuable X-ray data and combined it with a powerful computer model to find something extraordinary.”
One of the issues with researching supernova is that their final explosion is what triggers our observations. It is difficult to get a detailed understanding of the last moment before the supernova explodes. “In recent years theorists have paid much attention to the final internal processes within large stars as they are essential to uncover neutrino-driven supernova mechanisms and other potential transients of large-scale star collapse,” the author writes in his paper. “However, it is a supernova event that causes the initiation of intense observational studies, so it is difficult to directly observe the last time of a large star before the explosion.”
Nuclear synthesis involves the deeper and deeper elements inside, leading to large-scale star SN explosions. The surface layer is hydrogen, with helium being the next, then carbon and even the heavier element beneath the outer layer. In the end, the stars make iron. However, iron is a barrier to this process. This is because lighter elements release energy when fused, but iron requires more energy to receive further fusion. Iron accumulates in the core, and once the core reaches a solar mass of about 1.4, there is not enough outward pressure to prevent collapse. Gravity wins, the core collapses, and the stars explode.
Chandra’s observation, combined with modeling, sees the inside of the star to the astrophysicist at the final moments before the collapse.
“Our research shows that just before the CAS star collapsed, some of the inner layer with a large amount of silicon moved outwards, invading many neon and adjacent layers,” said Matsuyama kai of Kyoto University in Japan. “This is a violent event in which the barrier between these two layers disappears.”
There were two results. The silicon-rich materials moved outwards, and the neon-rich materials moved inwards. This resulted in a heterogeneous mixing of elements, and small areas rich in silicon were discovered near small areas rich in neon.
This is part of what researchers call “shell mergers.” They say it is the final stage of the stellar activity. It’s a fierce burning of oxygen-burning shells that engulf the outer carbon and neon burning shells deep inside the star. This only happens when the star explodes as a supernova. “In the violent convection layer created by the merger of shells, rich NEs are burned when pulled inward into the star’s O-rich layer, and the SIs synthesized inside are transported outward,” the authors explain in their study.
The neon-rich area is evidence of this process. The author explains that silicon and neon were not mixed with other elements just before or after the explosion. Astrophysical models predict this, but have never been observed before. “Our results provide the first observational evidence that the final star combustion process rapidly changes internal structure and leaves behind asymmetry before Spanova,” the researchers explain in their paper.
For decades, astrophysicists have thought SN explosions were symmetrical. Early observations supported this idea, and the basic ideas behind the Core Collapse Supernova also supported symmetry. However, this study changes the fundamental understanding of supernova explosions as asymmetric. “The coexistence of compact EJECTA regions in both the “O-/NE rich” and “O-/SI rich” regimes means that the merger did not completely homogenize the O-rich layer prior to collapse, leaving behind multi-scale compositional heterogeneity and asymmetric velocity fields.
This asymmetry can also explain how neutron stars are left behind and how they lead to fast neutron stars.
According to the author, these final moments in the life of a supernova can also cause the explosion itself. The turbulence caused by the inner chaos may have helped the star explode.
“The most important effect of this change in star structure is that it may have helped to cause the explosion itself,” said Uchida, a co-author at Kyoto University. “That ultimate internal activity of a star may change its fate, whether it shines as a supernova or not.”
“For a long time in the history of astronomy, studying the inner structure of stars has been a dream,” the researchers wrote in the paper’s conclusion. This study gave astrophysicists a critical glimpse into the final moments of the precursor star before the explosion. “This moment not only has a major impact on the fate of the stars, but also creates a more asymmetric supernova explosion,” they conclude.
The original version of this article was published today in Universe.
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