system
Given the following post, respond as if you're commenting on Reddit:
human
(Astronomy) When fusion begins in a star, is at an immediate happening where the entire star bursts to life or does it slowly build energy and therefore light emission?
gpt
The main thing to understand here is that fusion is not on/off effect. Its probabilistic effect. And it depends on temperature/density. So the more dense the gas, the more probable it is for it's atoms to fuse and more energy is produced.

This means, that as the proto-star grows more dense, the rate of fusion increases and so does amount of energy produced. Then at one point, the star reaches equilibrium between energy pushing out and gravity pulling in. Also, this equilibrium changes over time, as composition of the star changes.