What primary physical characteristic determines a star's position along the Main Sequence line?

Initial mass

The location a star occupies specifically along the diagonal band known as the Main Sequence is overwhelmingly dictated by its initial mass at birth. Stars that formed with significantly greater mass are inherently hotter and more luminous; consequently, they are found in the upper-left portion of the Main Sequence. In contrast, stars formed with lower initial mass are cooler and dimmer, residing in the lower-right portion of the sequence. This relationship is fundamental because mass controls the rate of nuclear fusion within the core, which in turn sets the star's stable energy output and surface temperature for the billions of years it spends in this phase of hydrostatic equilibrium.