▁a ▁degree , ▁or ▁ ▁the ▁diameter ▁of ▁the ▁full ▁moon ), ▁with ▁second ▁through ▁sixth ▁magnitude ▁stars ▁meas uring ▁ ′ , ▁ ′ , ▁ ′ , ▁ ′ , ▁and ▁ ′ , ▁respectively . ▁The ▁development ▁of ▁the ▁teles cope ▁showed ▁that ▁these ▁large ▁sizes ▁were ▁ill us ory — st ars ▁appeared ▁much ▁smaller ▁through ▁the ▁teles cope . ▁However , ▁early ▁teles cop es ▁produced ▁a ▁sp ur ious ▁disk - like ▁image ▁of ▁a ▁star ▁that ▁was ▁larger ▁for ▁br ighter ▁stars ▁and ▁smaller ▁for ▁f ain ter ▁ones . ▁Astronom ers ▁from ▁Gal ile o ▁to ▁Ja ques ▁Cass ini ▁mist ook ▁these ▁sp ur ious ▁dis ks ▁for ▁the ▁physical ▁bodies ▁of ▁stars , ▁and ▁thus ▁into ▁the ▁eigh teenth ▁century ▁continued ▁to ▁think ▁of ▁magnitude ▁in ▁terms ▁of ▁the ▁physical ▁size ▁of ▁a ▁star . ▁Johannes ▁He vel ius ▁produced ▁a ▁very ▁precise ▁table ▁of ▁star ▁sizes ▁measured ▁teles cop ically , ▁but ▁now ▁the ▁measured ▁diam eters ▁r anged ▁from ▁just ▁over ▁six ▁seconds ▁of ▁arc ▁for ▁first ▁magnitude ▁down ▁to ▁just ▁under ▁ 2 ▁seconds ▁for ▁sixth ▁magnitude . ▁By ▁the ▁time ▁of ▁William ▁H ersch el ▁astronom ers ▁recognized ▁that ▁the ▁teles cop ic ▁dis ks ▁of ▁stars ▁were ▁sp ur ious ▁and ▁a ▁function ▁of ▁the ▁teles cope ▁as ▁well ▁as ▁the ▁bright ness ▁of ▁the ▁stars , ▁but ▁still ▁spoke ▁in ▁terms ▁of ▁a ▁star ' s ▁size ▁more ▁than ▁its ▁bright ness . ▁Even ▁well ▁into ▁the