▁nin ete enth ▁century ▁the ▁magnitude ▁system ▁continued ▁to ▁be ▁described ▁in ▁terms ▁of ▁six ▁classes ▁determined ▁by ▁apparent ▁size , ▁in ▁which ▁▁ ▁There ▁is ▁no ▁other ▁rule ▁for ▁class ing ▁the ▁stars ▁but ▁the ▁estimation ▁of ▁the ▁observer ; ▁and ▁hence ▁it ▁is ▁that ▁some ▁astronom ers ▁reck on ▁those ▁stars ▁of ▁the ▁first ▁magnitude ▁which ▁others ▁este em ▁to ▁be ▁of ▁the ▁second . ▁However , ▁by ▁the ▁mid - n in ete enth ▁century ▁astronom ers ▁had ▁measured ▁the ▁distances ▁to ▁stars ▁via ▁stell ar ▁par alla x , ▁and ▁so ▁understood ▁that ▁stars ▁are ▁so ▁far ▁away ▁as ▁to ▁essentially ▁appear ▁as ▁point ▁sources ▁of ▁light . ▁Following ▁adv ances ▁in ▁understanding ▁the ▁diff raction ▁of ▁light ▁and ▁astronom ical ▁seeing , ▁astronom ers ▁fully ▁understood ▁both ▁that ▁the ▁apparent ▁sizes ▁of ▁stars ▁were ▁sp ur ious ▁and ▁how ▁those ▁sizes ▁dep ended ▁on ▁the ▁intensity ▁of ▁light ▁coming ▁from ▁a ▁star ▁( this ▁is ▁the ▁star ' s ▁apparent ▁bright ness , ▁which ▁can ▁be ▁measured ▁in ▁units ▁such ▁as ▁w att s / cm 2 ) ▁so ▁that ▁br ighter ▁stars ▁appeared ▁larger . ▁ ▁Modern ▁definition ▁▁ ▁Early ▁phot ometric ▁measurements ▁( made , ▁for ▁example , ▁by ▁using ▁a ▁light ▁to ▁project ▁an ▁artificial ▁“ star ” ▁into ▁a ▁teles cope ' s ▁field ▁of ▁view ▁and ▁adjust ing ▁it ▁to ▁match ▁real ▁stars ▁in ▁bright ness ) ▁demonstrated ▁that ▁first ▁magnitude ▁stars ▁are ▁about ▁ 1 0 0 ▁times ▁br ighter ▁than ▁sixth