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2 articles on this topic.
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A white dwarf does not collapse, nothing falls inward, and it has no layers to begin with. The onion you see in the spectrum of a Type Ia supernova did not exist two seconds earlier — it is manufactured on the way out, and which layer you get is decided by nothing but how dense the gas was at the moment the flame arrived.
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Almost everything I thought I knew about how a massive star dies turned out to be a description of a different kind of supernova. The iron core does not start fusing. The collapse is not stopped by a void. The shock does not cascade outwards lighting each layer as it goes — it dies twenty milliseconds in, two hundred kilometres up, and something else has to come and rescue it.