A main-sequence star is a type of star that is in a stable phase of its life cycle, where it fuses hydrogen into helium in its core. This process generates energy, which creates the outward pressure needed to balance the gravitational pull trying to collapse the star. Main-sequence stars vary in size, temperature, and brightness, and they are classified into different spectral types, such as O, B, A, F, G, K, and M. Most stars, including our Sun, spend the majority of their lives in the main sequence phase, which can last billions of years. The position of a main-sequence star on the Hertzsprung-Russell diagram indicates its temperature and luminosity. As a star exhausts its hydrogen fuel, it will eventually leave the main sequence and evolve into a different type of star, such as a red giant or a supernova.