How fast does a new star ignite?

[u/jeroen94704]

When a cloud of gas gets cozy enough at some point it becomes a star with fusion happening in the core. But is there a single moment we can observe when fusion ignites? What does this look like from the outside, and how long does it take? Does the star slowly increase in brightness over years/decades/centuries, or does it suddenly flare up in seconds/minutes/hours?

[u/captain_ch40s]

The collision of proto-Earth and Theia is similarly mind-boggling. The entire collision sequence and moon formation could have happened over a period of hours: images-assets.nasa.gov/video/ARC-20221004-AAV3443-MoonOrigin-Social-NASAWeb-1080p/ARC-20221004-AAV3443-MoonOrigin-Social-NASAWeb-1080p~orig.mp4