What is the luminosity class of main sequence stars on the H-R diagram?

The luminosity class of main sequence stars on the H-R diagram is identified as class V. This classification pertains to stars that are in the stable phase of their evolution where they are fusing hydrogen into helium in their cores.

Main sequence stars cover a wide range of masses and temperatures, but they all fall under this class because they exhibit similar characteristics in terms of brightness (luminosity) and temperature. The main sequence runs diagonally across the H-R diagram, from the upper left region with hot, luminous stars to the bottom right with cooler, less luminous stars.

The classification system, which includes various luminosity classes, recognizes that while other classes (such as giants or supergiants) represent different phases of stellar evolution, class V distinctly marks those stars that are actively fusing hydrogen, thereby maintaining a stable and long-lasting state on the main sequence. This key feature differentiates them from other stars that have progressed beyond this stage.