What continuous process defines a star's stable main sequence phase deep within its core?

Fusion of hydrogen atoms into helium

The main sequence phase, which constitutes the majority of a star's life, is fundamentally characterized by a single, sustained nuclear reaction occurring in its core. This reaction involves the process where lighter atomic nuclei, specifically hydrogen atoms, undergo fusion to form heavier atomic nuclei, which in this case is helium. This conversion releases enormous amounts of energy, creating the outward pressure necessary to stabilize the star against the inward pull of its own immense gravity. As long as this core hydrogen fuel is available to sustain this precise chain reaction, the star maintains its steady state and remains fixed on the main sequence, serving as the foundational benchmark for tracking stellar evolution.