What describes a main sequence star's lifespan in relation to hydrogen?

A main sequence star's lifespan is primarily characterized by the continuous fusion of hydrogen into helium in its core. This process occurs in a stable state over billions of years, where the outward pressure from the fusion reactions balances the gravitational forces trying to collapse the star. Throughout this phase, the star steadily converts hydrogen into helium, harnessing the energy released during fusion to sustain itself.

As the hydrogen in the core gradually depletes, the star transitions from the main sequence phase, eventually leading to changes such as expansion into a red giant and further fusion of heavier elements. The concept that a star continually fuses hydrogen until it runs out accurately reflects the primary energy-generating process during this critical phase of stellar evolution.