▁the ▁unique ▁internal ▁structure ▁of ▁the ▁star ▁at ▁each ▁stage . ▁ ▁Pur pose ▁of ▁the ▁standard ▁solar ▁model ▁The ▁SS M ▁serves ▁two ▁purposes : ▁ ▁it ▁provides ▁estimates ▁for ▁the ▁hel ium ▁abund ance ▁and ▁mixing ▁length ▁parameter ▁by ▁forcing ▁the ▁stell ar ▁model ▁to ▁have ▁the ▁correct ▁lum inos ity ▁and ▁radius ▁at ▁the ▁Sun ' s ▁age , ▁ ▁it ▁provides ▁a ▁way ▁to ▁evaluate ▁more ▁complex ▁models ▁with ▁additional ▁physics , ▁such ▁as ▁rotation , ▁magnetic ▁fields ▁and ▁diffusion ▁or ▁improvements ▁to ▁the ▁treatment ▁of ▁con ve ction , ▁such ▁as ▁mod elling ▁tur bul ence , ▁and ▁con ve ct ive ▁over sh oot ing . ▁ ▁Like ▁the ▁Standard ▁Model ▁of ▁particle ▁physics ▁and ▁the ▁standard ▁cosm ology ▁model ▁the ▁SS M ▁changes ▁over ▁time ▁in ▁response ▁to ▁relevant ▁new ▁theoretical ▁or ▁experimental ▁physics ▁discover ies . ▁ ▁Energy ▁transport ▁in ▁the ▁Sun ▁As ▁described ▁in ▁the ▁Sun ▁article , ▁the ▁Sun ▁has ▁a ▁radi ative ▁core ▁and ▁a ▁con ve ct ive ▁outer ▁en velope . ▁In ▁the ▁core , ▁the ▁lum inos ity ▁due ▁to ▁nuclear ▁re actions ▁is ▁transm itted ▁to ▁outer ▁layers ▁princip ally ▁by ▁radiation . ▁However , ▁in ▁the ▁outer ▁layers ▁the ▁temperature ▁gradient ▁is ▁so ▁great ▁that ▁radiation ▁cannot ▁transport ▁enough ▁energy . ▁As ▁a ▁result , ▁thermal ▁con ve ction ▁occurs ▁as ▁thermal ▁columns ▁carry ▁hot ▁material ▁to ▁the ▁surface ▁( ph otos phere ) ▁of ▁the ▁Sun . ▁Once ▁the ▁material ▁co ols ▁off ▁at ▁the ▁surface