What is the primary factor dictating a star's lifespan on the main sequence?

Initial mass

The duration a star spends fusing hydrogen in its core, known as the main sequence life, is fundamentally determined by the quantity of material it possessed when it first formed. This initial mass sets the stage for the entire stellar evolution path. A larger mass translates directly into greater gravitational forces compressing the core. To maintain hydrostatic equilibrium against this crushing weight, the core must achieve significantly higher temperatures and pressures than in a less massive star. This elevated state drives nuclear fusion reactions, particularly hydrogen consumption, at an exponentially faster pace, thereby ensuring a shorter overall lifespan despite having more total fuel available initially.