▁of ▁the ▁oldest ▁meteor ites , ▁and ▁models ▁of ▁the ▁evolution ▁of ▁the ▁Sol ar ▁System . ▁The ▁composition ▁in ▁the ▁photos phere ▁of ▁the ▁modern - day ▁Sun , ▁by ▁mass , ▁is ▁ 7 4 . 9 % ▁hydro gen ▁and ▁ 2 3 . 8 % ▁hel ium . ▁All ▁heav ier ▁elements , ▁called ▁met als ▁in ▁astronom y , ▁account ▁for ▁less ▁than ▁ 2   percent ▁of ▁the ▁mass . ▁The ▁SS M ▁is ▁used ▁to ▁test ▁the ▁valid ity ▁of ▁stell ar ▁evolution ▁theory . ▁In ▁fact , ▁the ▁only ▁way ▁to ▁determine ▁the ▁two ▁free ▁parameters ▁of ▁the ▁stell ar ▁evolution ▁model , ▁the ▁hel ium ▁abund ance ▁and ▁the ▁mixing ▁length ▁parameter ▁( used ▁to ▁model ▁con ve ction ▁in ▁the ▁Sun ), ▁are ▁to ▁adjust ▁the ▁SS M ▁to ▁" fit " ▁the ▁observed ▁Sun . ▁ ▁A ▁cal ibr ated ▁solar ▁model ▁A ▁star ▁is ▁considered ▁to ▁be ▁at ▁zero ▁age ▁( proto stell ar ) ▁when ▁it ▁is ▁assumed ▁to ▁have ▁a ▁hom ogeneous ▁composition ▁and ▁to ▁be ▁just ▁beginning ▁to ▁derive ▁most ▁of ▁its ▁lum inos ity ▁from ▁nuclear ▁re actions ▁( so ▁neglect ing ▁the ▁period ▁of ▁contra ction ▁from ▁a ▁cloud ▁of ▁gas ▁and ▁dust ). ▁To ▁obtain ▁the ▁SS M , ▁a ▁one ▁solar ▁mass ▁() ▁stell ar ▁model ▁at ▁zero ▁age ▁is ▁evol ved ▁numer ically ▁to ▁the ▁age ▁of ▁the ▁Sun . ▁The ▁abund ance ▁of ▁elements ▁in ▁the ▁zero ▁age ▁solar ▁model ▁is ▁estimated