The Inverse Square Law
The intensity of light observed from a source of constant intrinsic luminosity falls off as the square of the distance from the object. This is known as the inverse square law for light intensity.

The inverse square law for intensity

Thus, if you double the distance to a light source the observed intensity is decreased to (1/2)2 = 1/4 of its original value. Generally, the ratio of intensities at distances d1 and d2 are

Because of this fall of in light intensity with distance, the intrinsic energy output of stars (also called their luminosity can only be known if their distances are known. Later on this term we will be measuring distances to nearby stars using the Parallax applet.

Thus, if you have a known distance to a star and are able to measure the flux at that distance, the total energy output of the star can easily be ascertained (this will be shown in class).

A simple way to think about this, is the following: