▁from ▁prim ord ial ▁meteor ites . ▁Al ong ▁with ▁this ▁abund ance ▁information , ▁a ▁reasonable ▁guess ▁at ▁the ▁zero - age ▁lum inos ity ▁( such ▁as ▁the ▁present - day ▁Sun ' s ▁lum inos ity ) ▁is ▁then ▁converted ▁by ▁an ▁iter ative ▁procedure ▁into ▁the ▁correct ▁value ▁for ▁the ▁model , ▁and ▁the ▁temperature , ▁pressure ▁and ▁density ▁throughout ▁the ▁model ▁calculated ▁by ▁solving ▁the ▁equations ▁of ▁stell ar ▁structure ▁numer ically ▁assuming ▁the ▁star ▁to ▁be ▁in ▁a ▁steady ▁state . ▁The ▁model ▁is ▁then ▁evol ved ▁numer ically ▁up ▁to ▁the ▁age ▁of ▁the ▁Sun . ▁Any ▁dis cre p ancy ▁from ▁the ▁measured ▁values ▁of ▁the ▁Sun ' s ▁lum inos ity , ▁surface ▁abund ances , ▁etc . ▁can ▁then ▁be ▁used ▁to ▁ref ine ▁the ▁model . ▁For ▁example , ▁since ▁the ▁Sun ▁formed , ▁some ▁of ▁the ▁hel ium ▁and ▁heavy ▁elements ▁have ▁settled ▁out ▁of ▁the ▁photos phere ▁by ▁diffusion . ▁As ▁a ▁result , ▁the ▁Sol ar ▁photos phere ▁now ▁contains ▁about ▁ 8 7 % ▁as ▁much ▁hel ium ▁and ▁heavy ▁elements ▁as ▁the ▁proto stell ar ▁photos phere ▁had ; ▁the ▁proto stell ar ▁Sol ar ▁photos phere ▁was ▁ 7 1 . 1 % ▁hydro gen , ▁ 2 7 . 4 % ▁hel ium , ▁and ▁ 1 . 5 % ▁met als . ▁A ▁measure ▁of ▁heavy - element ▁sett ling ▁by ▁diffusion ▁is ▁required ▁for ▁a ▁more ▁accurate ▁model . ▁ ▁N umer ical