m=-2.5log(F/F0) 註:織女星的星等=0, 月亮的星等=-2.5
設某顆星亮度為S
7.5=-2.5log(S/F0 ) 星等愈大愈暗
logS/F0 =-3
S/F0 =10-3
故此星的亮度是織女星的1/1000倍