Has it already happened?
It has certainly happened. We know of stars that are younger than humans.
But has someone directly observed this happening? Well, it depends. As already noted, star formation takes a long time. It’s not like dark, cold dust suddenly collapses and ignites into a star. During the formation process, the dust is compressed, and as you might recall, as volume decreases on a mass, pressure and temperature increase. As it heats up, it starts emitting radiation. So it’s possible to observe this even before becoming a true main sequence star. And we have definitely observed a lot of those, and if you go outside and point your sufficiently powerful telescope in the right direction, you too can see one with your own eyes!
There’s not really one instant where a new star goes from not shining to shining where a human looking up at night would see a star “turn on.” The process of turning on takes between 10,000 - 1,000,000 years, depending on the size of the star and how many stages you include in the process.
It doesn’t really matter how long it takes. There will be a day in that period when it becomes visible.
Well, not exactly…
Sure, hair grows long over time - but there isn’t really a day where you’d look at someone and suddenly see the “Long hair” … It happens gradually…
Hair grows over time and yesterday it was under 3 inches today it’s over 3 inches. A tiny change over a threshold.
There’s a threshold of visibility for stars. Yesterday it wasn’t visible to the eye. Today it’s barely visible.


