Models are at the heart of any physics discipline, and in recent decades physics education has shifted heavily toward teaching students how to think using models. Involving students in an investigation of a real phenomenon through measurements and the applications of models is an ideal learning experience for a physics student and is the aim of project-based learning. Astronomy provides a rich discipline in which to teach physics. This paper describes the scope and sequence of an investigation of the Milky Way Galaxy using a radio telescope in a high school physics course. Specifically, by analyzing HI spectra collected with a horn radio telescope at the school, the students were able to develop a simple model of the structure of the Milky Way Galaxy. Utilizing a radio telescope in physics and astronomy courses is not a new concept. However, the telescope described here is very affordable and can be built by any teacher or student interested in doing so.