Expert: Ed - 1/31/2012

Question
Star A and star B appear equally bright in the sky. Star A is twice as far away from Earth as star B. How do the luminosities of stars A and B compare?

A. Star A is four times as luminous as star B.
     
B. Star A is twice as luminous as star B.
     
C. Star B is twice as luminous as star A.
     
D. Star B is four times as luminous as star A.

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Star C and star D are equally luminous. Star C is twice as far away from Earth as star D. How do the brightness level of stars C and D compare?

A. Star C appears four times as bright as star D.
     
B. Star C appears twice as bright as star D.
     
C. Star D appears twice as bright as star C.
     
D. Star D appears four times as bright as star C

Answer
Patrick, the thing to remember is that the apparent brightness of objects with the same inherent luminosity varies with the square of the distance to the objects.  Look at this page:
https://www.e-education.psu.edu/astro801/content/l4_p4.html

To save you some time, this is the main idea you need to know:

a light source will appear four times fainter if you are twice as far away from it as someone else, and it will appear nine times fainter if you are three times as far away from it as someone else.

Keep looking up,
-Ed