What core temperature must be surpassed for nuclear fusion to ignite and a star to officially join the main sequence?

Answer

10 million Kelvin

When the internal temperature and pressure within a collapsing protostar reach a critical threshold, specifically surpassing approximately 10 million Kelvin, nuclear fusion begins, initiating the stable main sequence phase.

What core temperature must be surpassed for nuclear fusion to ignite and a star to officially join the main sequence?
Elementlife cyclestarFusionnucleosynthesis