Cassiopeia A, one of the most studied remnants in the night sky, began as a massive star that lived for more than a million years. As with other massive stars, its interior formed onion-like layers of hydrogen, helium, carbon, and heavier elements. When iron accumulated at the core, it collapsed under its own weight, initiating the explosion about three centuries ago.
Chandra's X-ray data, combined with advanced simulations, revealed that part of the silicon-rich inner layer broke outward into a neon-rich layer in the star's final hours. This disruption forced silicon to move outward and neon to move inward, leaving clear evidence in Cas A's debris field: regions with abundant silicon but little neon adjacent to areas with the opposite composition.
"These findings show a violent event where the barrier between layers disappears," said Kai Matsunaga of Kyoto University, a co-author of the study. The survival of these unmixed regions confirms predictions from detailed models of stellar interiors near collapse.
The consequences of this rearrangement are profound. It likely produced Cas A's lopsided shape and gave a strong recoil to the surviving neutron star, which now speeds away from the explosion site. Moreover, the turbulence from these late-stage flows may have amplified the supernova shock wave itself.
"Perhaps the most important effect of this change in the star's structure is that it may have helped trigger the explosion itself," noted co-author Hiroyuki Uchida of Kyoto University. Lead author Toshiki Sato of Meiji University added, "Each time we closely look at Chandra data of Cas A, we learn something new and exciting."
Research Report:Inhomogeneous stellar mixing in the final hours before the Cassiopeia A supernova
Related Links
Chandra X-ray Center
Stellar Chemistry, The Universe And All Within It
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