The inverse square law and stellar distance

Overview

Recall questions on The inverse square law and stellar distance.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Define the luminosity of a star.

   Model answer

   Luminosity is the total power radiated by the star in all directions, measured in watts.

2. 2

   Define the intensity of the radiation received from a star.

   Model answer

   Intensity is the power received per unit area at the observer, measured in W m⁻².

3. 3

   Give the inverse square law relating intensity to luminosity and distance, and define every symbol.

   Model answer

   I = L/(4πd²), where I is the intensity in W m⁻², L the luminosity in watts and d the distance to the star in metres.

4. 4

   Where does the 4πd² in that expression come from?

   Model answer

   It is the surface area of a sphere of radius d, over which the star's whole output is spread by the time it reaches the observer.

5. 5

   What happens to the intensity received if the distance to a star is doubled?

   Model answer

   It falls to a quarter, since intensity is inversely proportional to the square of the distance.

6. 6

   What assumption does the inverse square law make about the space between the star and the observer?

   Model answer

   That nothing absorbs or scatters the radiation on the way, so all the power that leaves the star arrives spread over the sphere.

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