system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it that Iron is the worst element to a star? Why is it that it causes the end of a star?
gpt
Elements can be divided into two categories. The first category contains elements that require outside energy to push together (fusion) and give off energy when they split apart (fission).  The second category is the reverse, giving off energy when they fuse, and requiring extra energy to sustain fission. 

Heavier elements, like plutonium, tend to be in that first group. Lighter elements, like hydrogen are in the second. The dividing line falls roughly at Iron. 

Since stars are basically giant fusion reactors in space, they can run on everything fusion produces, up to Iron. After Iron, they aren't getting enough out of the fusion to keep the cycle going. And so the reaction stops.