What critical physical threshold must the density of a collapsing core cross to trap radiation and halt the initial free-fall phase?
Answer
The gas becomes opaque to its own radiation
During the very first phase of core collapse, the material is initially low density and transparent, allowing energy generated by compression to escape freely as photons leak out. This prevents pressure from building up, allowing gravity to drive a relentless free-fall inward. This phase ends abruptly when the core density rises high enough that the material achieves an opacity threshold. At this point, the newly generated photons are trapped within the core. This trapping causes the interior temperature to spike dramatically because the energy cannot escape easily, finally allowing internal pressure to accumulate sufficiently to resist the relentless inward pull of gravity.
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