Background: Natural Sources of Radio Waves By now you and your classmates have become familiar with the basics of how electromagnetic radiation transfers energy from one location to another. You may have even practiced generating your own waves by oscillating a current in a wire producing a constantly changing electric and magnetic field, and thus an EM wave. Astronomers have long been using their laboratory observations of these types of phenomena in order to explain observations that they are making of the light coming from distant objects. When scientists first began to look at the universe in wavelengths other than the visible wavelengths, they have been continually surprised to find many more features than what were visible to our unaided eye. The detection of naturally occurring radio sources has greatly improved our understanding of what our universe is made of and how it behaves in astrophysical systems. By observing incoming radiation in the radio frequency part of the spectrum (long wavelength), astronomers gain a significant advantage over using other frequencies. First, radio frequencies have very little disruption as they travel through our atmosphere. Therefore, we can observe sources from the ground both day and night and under both cloudy and clear conditions. Secondly, radio frequencies are less likely to be disrupted by interstellar gases and debris. So again, these frequencies can travel to us directly from very distant sources without being changed. This is a very valuable tool to the scientists who are studying the properties of those sources. In this unit, you will review the fundamental processes of radio frequency emission reviewing both electromagnetism and spectroscopy. You will have a chance to look at some real data recorded while observing some sources of radio emissions here in our milky way galaxy. You will also be able to use that data to infer both the rotational velocity of the galaxy as well as the total mass of the galaxy. This can be accomplished by astronomers designing systems to collect the radio waves that have been travelling for millions of years through our universe and applying fundamental knowledge about the nature of matter so as to interpret the observations and gain new information from them.