1. Nuclear Fusion of Hydrogen: Main sequence stars are defined by the process happening in their core: the fusion of hydrogen into helium. This process releases tremendous amounts of energy, which is what makes the stars shine.

2. Stable Equilibrium: Main sequence stars are in a state of hydrostatic equilibrium. This means the outward pressure from nuclear fusion in the core perfectly balances the inward pressure of gravity. This equilibrium allows the star to maintain its size and temperature for a significant portion of its life.

3. Luminosity and Temperature Relationship: There is a direct relationship between the mass, luminosity, and surface temperature of main sequence stars. More massive stars are hotter and brighter. This relationship is captured in the Hertzsprung-Russell diagram, which shows a clear band of main sequence stars based on their characteristics.