system
Given the following post, respond as if you're commenting on Reddit:
human
When a star runs out of hydrogen, it begins using helium as fuel. Is this shift a gradual process or does it happen suddenly?

Does the star begin to use helium gradually, as the hydrogen is depleted? Or does the process of fusing helium into carbon not start until the last hydrogen atom has been spent? Or do I misunderstand completely?
gpt
It's a relatively sudden shift. 

When hydrogen in the core runs out, the radiative pressure that's keeping the star puffed up disappears, and it starts collapsing under it's own gravity. 

This creates tremendous heat and pressure in the core, allowing the fusion of helium.