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 that we observe. The majority of stars, including our Sun, are classified as main-sequence stars. These stars vary in size, temperature, and brightness, depending on their mass. They are typically plotted on the Hertzsprung-Russell diagram, which illustrates the relationship between a star's luminosity and its surface temperature. Main-sequence stars will eventually exhaust their hydrogen fuel and evolve into different types of stars, depending on their mass.