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 light and heat that we observe. 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 lifetimes in the main sequence phase. The duration of this phase depends on the star's mass; more massive stars burn through their hydrogen fuel quickly, while smaller stars can remain in this phase for billions of years.