▁fully ▁within ▁hydro static ▁equilibrium — a ▁condition ▁where ▁the ▁in ward ▁force ▁of ▁gravity ▁is ▁matched ▁by ▁the ▁out ward ▁pressure ▁of ▁the ▁gas — then ▁the ▁star ▁can ▁not ▁exceed ▁the ▁radius ▁defined ▁by ▁the ▁Hay ashi ▁limit . ▁This ▁has ▁important ▁imp lications ▁for ▁the ▁evolution ▁of ▁a ▁star , ▁both ▁during ▁the ▁form ulative ▁contra ction ▁period ▁and ▁later ▁when ▁the ▁star ▁has ▁consum ed ▁most ▁of ▁its ▁hydro gen ▁supply ▁through ▁nuclear ▁fusion . ▁ ▁A ▁H ert z spr ung - Russ ell ▁diagram ▁displays ▁a ▁plot ▁of ▁a ▁star ' s ▁surface ▁temperature ▁against ▁the ▁lum inos ity . ▁On ▁this ▁diagram , ▁the ▁Hay ashi ▁limit ▁forms ▁a ▁nearly ▁vertical ▁line ▁at ▁about ▁ 3 , 5 0 0 ▁K . ▁The ▁outer ▁layers ▁of ▁low ▁temperature ▁stars ▁are ▁always ▁con ve ct ive , ▁and ▁models ▁of ▁stell ar ▁structure ▁for ▁fully ▁con ve ct ive ▁stars ▁do ▁not ▁provide ▁a ▁solution ▁to ▁the ▁right ▁of ▁this ▁line . ▁Thus ▁stars ▁are ▁const rained ▁to ▁remain ▁to ▁the ▁left ▁of ▁this ▁limit ▁during ▁all ▁periods ▁when ▁they ▁are ▁in ▁hydro static ▁equilibrium , ▁and ▁the ▁region ▁to ▁the ▁right ▁of ▁the ▁line ▁forms ▁a ▁type ▁of ▁" for b idden ▁zone ". ▁Note , ▁however , ▁that ▁there ▁are ▁exceptions ▁to ▁the ▁Hay ashi ▁limit . ▁These ▁include ▁col laps ing ▁proto st ars , ▁as ▁well ▁as ▁stars ▁with ▁magnetic ▁fields ▁that ▁inter f ere ▁with ▁the ▁internal ▁transport ▁of ▁energy ▁through ▁con