Main sequence stars are stars in the phase where they are fusing hydrogen atoms to form helium. Once a star runs out of hydrogen and starts fusing helium to form heavier elements, a star enters its dying phase.
Stars begin their main sequence phases with cores filled with 74-percent hydrogen, 25-percent helium and 1-percent other elements. The sun is still in its main sequence phase and has been at this stage for 5 billion years. Its core consists of 29-percent hydrogen, 70-percent helium and 1-percent other elements. The sun will continue fusing hydrogen into helium for another 5 billion years. About 90 percent of known stars are in their main sequence stages.