This is a follow up to: [Is there a theoretical maximum size limit for a star?](https://astronomy.stackexchange.com/questions/328/is-there-a-theoretical-maximum-size-limit-for-a-star)

The answer there talks about the radiation pressure preventing a star from forming. 

![enter image description here](https://i.stack.imgur.com/a172j.jpg)

What reaction is causing this radiation pressure and why does this potentially prevent a star from forming?