itudes . ▁ ▁However , ▁in ▁the ▁late ▁tw enti eth ▁century ▁V ega ▁was ▁found ▁to ▁vary ▁in ▁bright ness ▁making ▁it ▁un su itable ▁for ▁an ▁absolute ▁reference , ▁so ▁the ▁reference ▁system ▁was ▁modern ized ▁to ▁not ▁depend ▁on ▁any ▁particular ▁star ' s ▁stability . ▁ ▁This ▁is ▁why ▁the ▁modern ▁value ▁for ▁V ega ' ▁magnitude ▁is ▁close ▁to , ▁but ▁no ▁longer ▁exactly ▁zero , ▁but ▁rather ▁ 0 . 0 3 ▁in ▁the ▁V ▁( visual ) ▁band . ▁ ▁Current ▁absolute ▁reference ▁systems ▁include ▁the ▁AB ▁magnitude ▁system , ▁in ▁which ▁the ▁reference ▁is ▁a ▁source ▁with ▁a ▁constant ▁flux ▁density ▁per ▁unit ▁frequency , ▁and ▁the ▁ST MA G ▁system , ▁in ▁which ▁the ▁reference ▁source ▁is ▁instead ▁defined ▁to ▁have ▁constant ▁flux ▁density ▁per ▁unit ▁w avel ength . ▁ ▁Problem s ▁▁ ▁The ▁human ▁eye ▁is ▁easily ▁foo led , ▁and ▁Hi pp arch us ' s ▁scale ▁has ▁had ▁problems . ▁For ▁example , ▁the ▁human ▁eye ▁is ▁more ▁sensitive ▁to ▁yellow ▁and ▁red ▁light ▁than ▁to ▁blue , ▁and ▁phot ographic ▁film ▁more ▁to ▁blue ▁than ▁to ▁yellow / red , ▁giving ▁different ▁values ▁of ▁visual ▁magnitude ▁and ▁phot ographic ▁magnitude . ▁Appar ent ▁magnitude ▁can ▁also ▁be ▁affected ▁by ▁factors ▁such ▁as ▁dust ▁in ▁the ▁atmosphere ▁or ▁light ▁cloud ▁cover ▁absor bing ▁some ▁of ▁the ▁light . ▁ ▁See ▁also ▁▁▁ ▁AB ▁magnitude ▁ ▁Color – color ▁diagram ▁ ▁List ▁of ▁bright est ▁stars ▁ ▁Phot ometric - standard