A star that is fusing hydrogen into helium in its core, representing the majority of a star's lifetime.A main sequence star is a star that is currently fusing hydrogen into helium in its core. This is the longest and most stable phase in a star's life, lasting from millions to trillions of years depending on the star's mass. During this time, the star's internal pressure from fusion balances the inward pull of gravity, keeping it in a state of equilibrium. The vast majority of stars in the universe, including our Sun, are on the main sequence.
The concept was developed in the early 20th century, with key contributions from Ejnar Hertzsprung and Henry Norris Russell, who independently created the Hertzsprung–Russell diagram in the 1910s. The physical explanation of the main sequence as a hydrogen-burning phase was established later, notably by Arthur Eddington and others in the 1920s and 1930s.
The star's core reaches temperatures and pressures high enough for nuclear fusion to occur. Hydrogen nuclei (protons) combine through a series of reactions to form helium, releasing enormous amounts of energy in the process. This energy radiates outward, creating an outward pressure that counteracts the gravitational collapse. The star remains on the main sequence until it exhausts the hydrogen in its core, at which point it evolves into a red giant or another later-stage object.
Main sequence stars serve as standard candles and age indicators in astrophysics, helping astronomers measure distances and understand stellar evolution in galaxies.
The Sun has been on the main sequence for about 4.6 billion years and will remain there for roughly another 5 billion years before becoming a red giant.
