What empirical relationship is the most successful indirect method for single star mass estimation?

Mass-Luminosity Relation

Because the vast majority of stars exist in isolation, direct gravitational calculations based on orbital mechanics are impossible. Astronomers must rely on indirect methods, the most successful of which is the Mass-Luminosity Relation. This empirical correlation establishes a strong link between a star's mass and its intrinsic brightness, or luminosity. The underlying physical reason is that more massive stars must sustain significantly higher core pressures to counteract their own gravity, leading to much faster rates of nuclear fusion, which manifests as far greater luminosity. For stars that are currently residing on the main sequence—the longest phase of stellar life—luminosity ($L$) is generally proportional to the mass ($M$) raised to a power, commonly approximated as $L imes M^{3.5}$. By accurately determining a star's luminosity (which requires knowing both apparent brightness and distance), astronomers can use this established relationship to estimate its mass.