Imagine that a light bulb emits a brief burst of light in all directions. That light moves away from the bulb like the expanding surface of a balloon. As the surface expands the light must become more dilute because it has more to cover. The surface area of a sphere is where R is the radius of the sphere or, in this case, the distance from the light bulb. This law quantifies the amount of energy emitted by one unit of surface area of a hot blackbody per second.
Energy flux is energy radiated per unit area second.
Note that the amount of energy emitted depends very strongly on the temperature. The luminosity of a star is a measure of the total amount of energy it emits. Since stars behave like blackbodies we can use the Stefan-Boltzmann law for the amount of energy emitted by one square centimeter of the surface of the star with the equation for the surface area of a sphere. The surface area of a sphere, i . Suppose the size of a star doubles while it maintains the same temperature.