Assuming that we can measure the apparent brightness of a star, ...

[hubes95]

Assuming that we can measure the apparent brightness of a star, what does the inverse square law for light allow us to do?
 
  A) determine the distance to the star from its apparent brightness
  B) determine both the star's distance and luminosity from its apparent brightness
  C) calculate the star's surface temperature if we know either its luminosity or its distance
  D) calculate the star's luminosity if we know its distance, or calculate its distance if we know its luminosity

Question 2

What is the Tully-Fisher relation?
 
  What will be an ideal response?

[frejo]

Answer to Question 1

D

Answer to Question 2

A galaxy's total luminosity is determined by its mass, which can be observed from its rotational speed. The faster it is spinning, the more luminous it is.

[hubes95]

Thank you!

[frejo]

Always glad to help...