▁energ ies , ▁which ▁are ▁ext rap ol ated ▁back ▁to ▁the ▁lower ▁energ ies ▁of ▁stell ar ▁inter iors ▁( the ▁Sun ▁burn s ▁hydro gen ▁rather ▁slowly ). ▁Histor ically , ▁errors ▁in ▁the ▁nuclear ▁reaction ▁rates ▁have ▁been ▁one ▁of ▁the ▁biggest ▁sources ▁of ▁error ▁in ▁stell ar ▁mod elling . ▁Comput ers ▁are ▁employed ▁to ▁calculate ▁the ▁varying ▁abund ances ▁( us ually ▁by ▁mass ▁fraction ) ▁of ▁the ▁nuclear ▁species . ▁A ▁particular ▁species ▁will ▁have ▁a ▁rate ▁of ▁production ▁and ▁a ▁rate ▁of ▁destruction , ▁so ▁both ▁are ▁needed ▁to ▁calculate ▁its ▁abund ance ▁over ▁time , ▁at ▁varying ▁conditions ▁of ▁temperature ▁and ▁density . ▁Since ▁there ▁are ▁many ▁nuclear ▁species , ▁a ▁computer ised ▁reaction ▁network ▁is ▁needed ▁to ▁keep ▁track ▁of ▁how ▁all ▁the ▁abund ances ▁vary ▁together . ▁ ▁According ▁to ▁the ▁Vog t - Russ ell ▁theorem , ▁the ▁mass ▁and ▁the ▁composition ▁structure ▁throughout ▁a ▁star ▁uniqu ely ▁determine ▁its ▁radius , ▁lum inos ity , ▁and ▁internal ▁structure , ▁as ▁well ▁as ▁its ▁subsequent ▁evolution ▁( though ▁this ▁" theorem " ▁was ▁only ▁intended ▁to ▁apply ▁to ▁the ▁slow , ▁stable ▁phases ▁of ▁stell ar ▁evolution ▁and ▁certainly ▁does ▁not ▁apply ▁to ▁the ▁trans itions ▁between ▁stages ▁and ▁rapid ▁evolution ary ▁stages ). ▁The ▁information ▁about ▁the ▁varying ▁abund ances ▁of ▁nuclear ▁species ▁over ▁time , ▁along ▁with ▁the ▁equations ▁of ▁state , ▁is ▁sufficient ▁for ▁a ▁numerical ▁solution ▁by ▁taking ▁sufficiently ▁small ▁time ▁incre ments ▁and ▁using ▁iteration ▁to ▁find