




If we think of a beam of light as a stream of particles (like a stream of water, for example) we can speak of a flux , or flow of light. Flux is the amount of light that comes from a certain area (usually one square meter) in a certain amount of time (usually one second). The amount of flux given off by an object depends only on its temperature, according to the StefanBoltzmann Law: where T is the temperature (in K) and the Greek letter Sigma is a constant term called the StefanBoltzmann Constant. Its value is unimportant for this class, as we shall see. Flux is not the true measure of an object's energy output. For example, a flashlight and a searchlight have similar temperatures, therefore similar fluxes. But from a distance of 100 yards, the searchlight is the brighter of the two. Why? Because the searchlight is bigger! We call the total energy output per second of an object its luminosity. It depends not only on Flux (temperature) but also on size (or, more accurately, surface area). Stars are for the most part spherical, so we can compute their surface areas easily, using A = 4(pi)R^{2}, where R is the radius of the sphere. Therefore While it is possible to compute the exact values of luminosities, it requires that we know the value of Sigma. We can get around this by comparing the luminosities of two objects, either two different objects, or the same object before or after some great change in temperature, radius, or both: Sigma and 4 and pi all drop out, leaving us with: Or, to arrange it another way: Often we will use the Sun as object #2, comparing its luminosity to that of another star.

Updated
8/27/99
By James
E. Heath
Copyright Ó 1999 Austin Community College 