What stellar process defines stars currently residing on the Main Sequence?

Fusing hydrogen into helium in their cores

The Main Sequence is the principal, most stable phase of a star's life cycle, encompassing approximately 90% of all observed stars. The defining characteristic of any star located on this diagonal band is the sustained nuclear fusion reaction occurring within its core, specifically the conversion of hydrogen fuel into helium. This process generates the outward pressure necessary to counteract the inward force of gravity, maintaining a state known as hydrostatic equilibrium. While stars on the Main Sequence range widely in mass, temperature, and luminosity, the underlying physics driving their stability is the ongoing hydrogen core burning. Only when this hydrogen is exhausted do stars migrate off the Main Sequence to begin evolving into giants or eventually collapse into remnants.