system
Given the following post, respond as if you're commenting on Reddit:
human
How rapidly (or not) does a newborn star "ignite" once it reaches critical mass? Hours, days, years?
gpt
It happens so gradually there isn't a clear moment of "ignition".

Protostars are glowing hot long before any fusion starts. As they collapse under their own weight they heat up, as any gas will when you compress it. At this phase, the protostar is converting its gravitational potential energy into heat, which provides the pressure that slows its collapse.

As the core continues to compress and heat up, it gradually approaches temperatures where fusion reactions start to become possible. Over many thousands of years, fusion slowly becomes the dominant energy source, replacing the collapse in heating the star. This causes the surface to stop contracting, and the star achieves a stable equilibrium where the fusion at its core produces enough thermal pressure to exactly balance the weight of its gas. This lasts until it runs out of hydrogen to fuse.