The continuing fascination of young people with celestial bodies leads them to pose challenging questions to their science teachers, such as how was the universe born? How were the stars formed? In this paper we present an extremely inexpensive but highly engaging activity to teach the basics of spectroscopy. Guided by the question "how do scientists know what a star is made of?", this simple activity allows a wide range of physical content to be taught, based on the analysis of the emission spectrum of a star's light. It is possible to see the configuration of lines of each chemical element as a fingerprint, thereby inferring each one's presence in the composition of the celestial body. As many countries do not have access to even inexpensive technologies, such as simple computers or digital cameras, this alternative could teach and motivate students of different ages to understand and enjoy the beauty of the nature of light.