▁objects , ▁one ▁of ▁which ▁is ▁used ▁as ▁a ▁reference ▁or ▁bas eline , ▁whose ▁intens ities ▁( br ight ness es ) ▁measured ▁from ▁Earth ▁in ▁units ▁of ▁power ▁per ▁unit ▁area ▁( such ▁as ▁w att s ▁per ▁square ▁met re , ▁W   m − 2 ) ▁are ▁ ▁and ▁, ▁will ▁have ▁magn itudes ▁ ▁and ▁ ▁related ▁by ▁ ▁Using ▁this ▁formula , ▁the ▁magnitude ▁scale ▁can ▁be ▁extended ▁beyond ▁the ▁ancient ▁magnitude ▁ 1 – 6 ▁range , ▁and ▁it ▁becomes ▁a ▁precise ▁measure ▁of ▁bright ness ▁rather ▁than ▁simply ▁a ▁classification ▁system . ▁Astronom ers ▁now ▁measure ▁differences ▁as ▁small ▁as ▁one - h undred th ▁of ▁a ▁magnitude . ▁Stars ▁that ▁have ▁magn itudes ▁between ▁ 1 . 5 ▁and ▁ 2 . 5 ▁are ▁called ▁second - m agn itude ; ▁there ▁are ▁some ▁ 2 0 ▁stars ▁br ighter ▁than ▁ 1 . 5 , ▁which ▁are ▁first - m agn itude ▁stars ▁( see ▁the ▁list ▁of ▁bright est ▁stars ). ▁For ▁example , ▁Si ri us ▁is ▁magnitude ▁− 1 . 4 6 , ▁Ar ct urus ▁is ▁− 0 . 0 4 , ▁Al de bar an ▁is ▁ 0 . 8 5 , ▁Sp ica ▁is ▁ 1 . 0 4 , ▁and ▁Pro cy on ▁is ▁ 0 . 3 4 . ▁Under ▁the ▁ancient ▁magnitude ▁system , ▁all ▁of ▁these ▁stars ▁might ▁have ▁been ▁class ified ▁as ▁" st ars ▁of ▁the ▁first ▁magnitude ". ▁ ▁Magn