Brightness is described and measured in terms of
radiant flux received from a celestial object. Radiant flux is the total amount of light energy (joules) that crosses one square meter per second. Radiant flux,
F, is measured in watts.
Radiant flux received from an object depends both on its distance, and on its luminosity.
Luminosity is a measure of how much energy an object emits per second.
Radiant flux and luminosity can be related by the equation:
F = L/(4πr
2)
This comes from imagining a star with luminosity L, surrounded by a huge spherical shell with radius r, and assuming no energy is lost between the star and the shell.
The luminosity of the star is intrinsic and does not depend on the distance from the shell, but the radiant flux is inversely proportional to the square of the distance from the star. This is known as the
inverse square law for light.
