It is important to first establish the definition of these terms:
Intrinsic Brightness
also called 'Luminosity'. This is a measure of the
total amount of light emitted by that object (such as a star). It is NOT dependent
on distance. For instance, a 100 Watt light bulb is always 100 Watts, not matter
where it is located!
Flux
This is a measure of the amount of light we receive from an object
(such as a star). If the object is very far away, the amount of flux received might
be very small, even if the object is intrinsically very bright.
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The Flux we recieve from an object and the Luminosity (Intrinsic Brightness) of
that object is related by the distance in this way:
Once you know the distance to the star, D, you can determine its intrinsic luminosity, L, based on how bright it is (the flux, F,) on earth and the inverse square law. |
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Notice in the illustration above, if you double the distance away, the area shadowed increases by a factor of FOUR. This is the same as saying you have one fourth the flux (energy per square inch) at twice the distance.
It's all because of that (Distance)2 term!