Flux (or radiant flux), F, is the total amount of energy that crosses a unit area per unit time. Flux is measured in joules per square metre per second (joules/m2/s), or watts per square metre (watts/m2).


The flux of an astronomical source depends on the luminosity of the object and its distance from the Earth, according to the inverse square law:


$\displaystyle  F=\frac{L}{4\pi r^2} $

where F = flux measured at distance r,
L = luminosity of the source,
r= distance to the source.

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