What empirical rule relates brightness to mass for about 90% of main sequence stars?

Answer

The mass-luminosity relation ($L \propto M^{3.9}$ or $L \propto M^4$)

The mass-luminosity relation shows that for most main sequence stars, luminosity is proportional to mass raised to a high power, often approximated as the fourth power ($L \propto M^4$).

What empirical rule relates brightness to mass for about 90% of main sequence stars?
astronomystarmass