During the rapid collapse phase, what size does the core shrink down to in milliseconds?

Answer

The size of a small city, perhaps only 10 to 20 kilometers in diameter

The core undergoes an astonishingly fast transformation, shrinking from an Earth-sized object down to approximately 10 to 20 kilometers in diameter within milliseconds.

During the rapid collapse phase, what size does the core shrink down to in milliseconds?
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