system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do Atomic Fusion stops at Iron?

Hello, 

i look at many cool Videos about the fusion processes inside of Sun's.   
A Sun begin's to die when she can't fuse elements anymore.   
This given Element is Iron, but why?
gpt
When you fuse small atoms, like hydrogen, it releases energy. This is how the sun works for example. But doing it in reverse (splitting helium to hydrogen) takes energy.

When you split big atoms, like uranium, it releases energy. This is how nuclear reactors work. But again, doing it in reverse (combining xenon and strontium into uranium) takes energy.

Iron is the crossover point between the two, where fusing it goes from producing energy, to consuming energy. So most stars stop there.