What fundamental characteristic dictates a star's main sequence lifespan and luminosity?

Initial mass

The initial mass of a star when it first forms is the single most crucial factor determining every significant characteristic of its existence while it resides on the main sequence. This initial mass dictates the overall strength of the star's gravitational force. Consequently, a greater mass requires correspondingly higher core temperatures and pressures to generate sufficient outward pressure to achieve hydrostatic equilibrium and prevent immediate collapse. Because higher mass leads to higher core pressures, fusion rates accelerate dramatically; more massive stars burn their core hydrogen fuel supply at an astonishingly rapid rate. This leads directly to much shorter lifespans—mere millions of years for the most massive stars—compared to the billions or trillions of years afforded to lower-mass stars, while also dictating their extreme luminosity.