Why Are Astronomers Much More Interested in the Luminosity of a Star

Multiple Choice

Why are astronomers much more interested in the luminosity of a star than its apparent brightness?

A) because luminosity can be measured exactly, but apparent brightness 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 look from Earth
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's core, while apparent brightness only tells us about its outside layers
E) you can't fool me, there is no difference between luminosity and apparent brightness; they are merely different terms for the same property of a star
Section 17.2: Colors of Stars