Iron Kills the Star That Made It

Added: 2026-06-02

[00:00:00]Iron is the most tightly bound nucleus in nature. Every reaction up to iron releases energy, and that energy is what holds the star up against gravity.

[00:00:10]Fusing iron doesn't release energy, it absorbs it. The furnace doesn't slow down, it reverses. What follows takes less than a second. The iron core, about 1.4 solar masses packed into a volume the size of Earth, loses all outward pressure support. It collapses. In under a quarter of a second, that core crushes down to a sphere roughly 20 km across, a neutron star. Protons and electrons smash together into neutrons by raw gravitational compression.

[00:00:47]The outer layers, still falling inward, slam into this suddenly rigid surface.

[00:00:52]They bounce. A shockwave rips outward through every layer of the onion at tens of thousands of kilometers per second.

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