, ▁it ▁pl ung es ▁back ▁down ward ▁to ▁the ▁base ▁of ▁the ▁con ve ction ▁zone , ▁to ▁receive ▁more ▁heat ▁from ▁the ▁top ▁of ▁the ▁radi ative ▁zone . ▁ ▁In ▁a ▁solar ▁model , ▁as ▁described ▁in ▁stell ar ▁structure , ▁one ▁cons iders ▁the ▁density ▁, ▁temperature ▁T ( r ), ▁total ▁pressure ▁( m atter ▁plus ▁radiation ) ▁P ( r ), ▁lum inos ity ▁l ( r ) ▁and ▁energy ▁generation ▁rate ▁per ▁unit ▁mass ▁ ε ( r ) ▁in ▁a ▁spher ical ▁shell ▁of ▁a ▁thick ness ▁dr ▁at ▁a ▁distance ▁r ▁from ▁the ▁center ▁of ▁the ▁star . ▁ ▁Rad i ative ▁transport ▁of ▁energy ▁is ▁described ▁by ▁the ▁radi ative ▁temperature ▁gradient ▁equation : ▁ ▁where ▁ κ ▁is ▁the ▁opacity ▁of ▁the ▁matter , ▁ σ ▁is ▁the ▁Stefan - B olt z mann ▁constant , ▁and ▁the ▁Bol tz mann ▁constant ▁is ▁set ▁to ▁one . ▁ ▁Con ve ction ▁is ▁described ▁using ▁mixing ▁length ▁theory ▁and ▁the ▁corresponding ▁temperature ▁gradient ▁equation ▁( for ▁ad i ab atic ▁con ve ction ) ▁is : ▁ ▁where ▁ γ ▁= ▁cp ▁/ ▁cv ▁is ▁the ▁ad i ab atic ▁index , ▁the ▁ratio ▁of ▁specific ▁he ats ▁in ▁the ▁gas . ▁( For ▁a ▁fully ▁ion ized ▁ideal ▁gas , ▁ γ ▁= ▁ 5 / 3 .) ▁ ▁Near ▁the ▁base ▁of ▁the ▁Sun ' s ▁con ve ction ▁zone , ▁the ▁con ve ction ▁is ▁ad i ab atic ,