system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5; What makes iron so special that stars stop fusing elements heavier than it?
gpt
Iron takes more energy to fuse than is released in the process of fusion.

Take a look at hydrogen: the mass of the constituent hydrogen atoms  (four electrons, four protons) is more than the mass of the helium atom that comes out the other side (four electrons, two protons, two neutrons).

The missing mass is converted into the energy that sustains the star, and which can lead to further fusion chains.

Silicon-to-iron is the final stage in stellar fusion. Fusing silicon into iron takes more energy than the star gets out of it, and so it begins losing energy, ultimately leading to its death.