 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 × 109 ) years ago . 
 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 <a href='null_genetive' title='1'><mark>'s</mark></a>core will steadily increase , the rate of nuclear fusion at the core will slowly increase , as will the star <a href='null_genetive' title='2'><mark>'s</mark></a>temperature and luminosity . The Sun , for example , is estimated to <a href='been_done' title='3'><mark>have</mark></a> increased in luminosity by about 40 % since it<a href='been_done' title='4'><mark></mark></a> reached the main sequence 4 @.@ 6 billion ( 4 @.@ 6 × 109 ) years ago . 
Stars spend about 90 % of their existence fusing hydrogen into helium in high @-@ temperature and high @-@ pressure reactions near the core . Such stars r said to b on the main sequence , and arralled dwarf stars . Starting at zero @-@ age main sequence , da proportion of helium in a star  core will steadily increase , tdarate of nuclear fusion at thdaore gon slowly increase , as will thedaar  temperature and luminosity . The da , fir example , is estimated to done increased in luminosity by bout 40 % since it done reached the mda sequence 4 @.@ 6 billion ( 4 @.@ 6 × 109 ) years ago .