A Main-Sequence Star is a type of star that is in the stable phase of its life cycle, where it fuses hydrogen into helium in its core. This process generates energy, allowing the star to shine brightly for billions of years. The Sun is a prime example of a main-sequence star, classified as a G-type star. Main-sequence stars vary in size, temperature, and brightness, and they are categorized into different spectral types, ranging from O-type (the hottest) to M-type (the coolest). As they exhaust their hydrogen fuel, they will eventually evolve into red giants or other stellar remnants, marking the end of their life cycle.