system
Given the following post, respond as if you're commenting on Reddit:
human
Do stars "turn on" immediately when fusion can occur, or do they gradually increase in brightness as they gain mass?

Basically, does the first fusion event "kick-start" the star into life in a violent, sudden reaction? Or does the star slowly grow in luminosity at the rate at which it is able to add mass to itself?
gpt
When the pressure and temperature of the collapsing cloud of hydrogen is sufficient to initiate fusion, it begins and the star lights up.  Some mass blows off during this, the T Tauri phase, and there is a pulsing effect as the star expands and contracts while it settles down, but as soon as the radiation from the region of fusion makes its way out it’ll light up.