The flux of a star is a measure of its brightness as observed from Earth. The formula for star flux (F) is F = L / (4πd^2), where L is the star's luminosity (the total amount of energy it emits per unit of time) and d is the distance from the observer to the star. This formula shows that the flux decreases with the square of the distance, meaning that a star will appear fainter the farther away it is.