Final answer:
The more luminous star out of two main sequence stars is the more massive one, adhering to the mass-luminosity relation observed for main sequence stars, with greater mass leading to higher luminosity and temperature.
Step-by-step explanation:
When comparing two main sequence stars, if one is more luminous than the other, we can say with certainty that the more luminous star is more massive. This concept is known as the mass-luminosity relation, where a direct correlation between a star's mass and its luminosity is observed. Stars on the main sequence that are more massive have higher luminosities and higher surface temperatures. For example, spectral type O stars on the main sequence are very massive, blue, and incredibly luminous, often exceeding 100 times the Sun's mass and a million times its luminosity.
The main sequence star with lesser mass would be cooler and less luminous, residing in the lower right of the Hertzsprung-Russell (H-R) diagram, which plots luminosity against surface temperature or spectral type. Therefore, the H-R diagram aids in understanding stellar properties and the mass-luminosity relation, both central to stellar astronomy. Knowing that hotter stars are more luminous than cooler ones adds to our toolset for estimating stellar characteristics, even at vast distances.