Nuclear Ignition

When the core temperature reaches about 10 million Kelvin, hydrogen atoms move fast enough to overcome their mutual electrical repulsion and fuse into helium. This is the moment of nuclear ignition — the protostar becomes a true star. The outward pressure from fusion balances the inward pull of gravity, and the star stabilizes.

Not every collapsing cloud fragment makes it. If the mass is below about 0.08 solar masses, the core never gets hot enough for sustained fusion. These failed stars are called brown dwarfs — too massive to be planets, too small to be true stars.