Why are astronom ers much more interested in the luminosity of a star than its apparent brightness?
a. because luminosity can be measured exactly, but apparentbrightness can only be roughly estimated
b. because the luminosity tells us how bright a star really is, while apparent brightness only tells us how bright it happens to lo ok from E arth
c. because the luminosity also tells us what elements the star is made of, while apparent brightness cannot tell us a star's chemical make-up
d. because luminosity can tell us how bright it is inside the star, while apparent brightness only tells us about its outside layers
e. there is no difference between lumino sity and apparent brightness