<P> Stars spend about 90 % of their existence fusing hydrogen into helium in high - temperature and high - pressure reactions near the core . Such stars are said to be on the main sequence , and are called dwarf stars . Starting at zero - age main sequence , the proportion of helium in a star 's core will steadily increase , the rate of nuclear fusion at the core will slowly increase , as will the star 's temperature and luminosity . The Sun , for example , is estimated to have increased in luminosity by about 40 % since it reached the main sequence 4.6 billion ( 4.6 × 10 ) years ago . </P>
<P> Stars spend about 90 % of their existence fusing hydrogen into helium in high - temperature and high - pressure reactions near the core . Such stars are said to be on the main sequence , and are called dwarf stars . Starting at zero - age main sequence , the proportion of helium in a star 's core will steadily increase , the rate of nuclear fusion at the core will slowly increase , as will the star 's temperature and luminosity . The Sun , for example , is estimated to have increased in luminosity by about 40 % since it reached the main sequence 4.6 billion ( 4.6 × 10 ) years ago . </P>
what is the true power source for the sun and other main sequence stars
what is the true power source for the sun and other main sequence stars
hydrogen
hydrogen