Main sequence stars are a category of stars that are in a stable phase of their life cycle, where they fuse hydrogen into helium in their cores. This process generates energy, which creates the light and heat we observe. The majority of stars, including our Sun, are classified as main sequence stars, and they can vary in size, temperature, and brightness. These stars are plotted on the Hertzsprung-Russell diagram, a graph that shows the relationship between a star's luminosity and its temperature. Main sequence stars range from small, cool red dwarfs to massive, hot blue giants. Their position on the diagram helps astronomers understand their age and evolutionary stage.