Which initial characteristic almost entirely determines a star's main-sequence longevity?

Mass

The primary determinant governing how long a star will reside on the main sequence, the longest phase of its life, is its initial mass when it first formed. A star’s mass sets the stage for every subsequent physical parameter. Greater mass translates directly into a much stronger gravitational force pressing toward the center. To counteract this intensified inward crush and maintain hydrostatic balance, the core must reach significantly higher temperatures and pressures. This elevated energy requirement directly necessitates a vastly accelerated rate of hydrogen fusion, meaning the star burns through its core fuel reservoir at an exponentially faster rate compared to lower-mass stars. Therefore, there is a clear inverse relationship: higher mass results in higher luminosity and a dramatically shorter lifespan.