We observe the stars and galaxies through the electromagnetic radiation they emit. We receive this radiation over a wide range of frequencies. In this chapter we will define terms such as intensity, flux density, and luminosity that characterize the observed radiation. The total amount of radiation emitted by a source per unit time is called its luminosity. This is a measure of the intrinsic brightness of the source. The units of luminosity are Joules per second (J/s), also called Watts (W). We denote the luminosity of any object with the symbol L. We are often interested in the radiation emitted in a narrow band of frequencies, centered at some frequency ν. This is characterized by the specific luminosity Lν , which is defined as the energy emitted per unit time per unit frequency interval. The unit of frequency is Hz (Hertz). Hence the unit of Lν is W/Hz.