When a star is formed what happens? Does it reach a particular mass and then just turn on like a light switch?
or does it slowly get hotter and brighter?
IIRC, under the influence of gravity matter gets more dense until you get a critical mass. That's the tipping point. That's when nuclear fusion starts and is self sustaining. The energy released can grow until it hits a more or less steady state, like the Sun with its variations. Any corrections are appreciated.