Final answer:
The Shapley-Curtis Debate centered on the size of the Milky Way and the existence of other galaxies, being influenced by discussions about cepheid variable stars and spiral nebulae.
Step-by-step explanation:
The Shapley-Curtis Debate of 1920 was a significant event in astronomy that revolved around two key issues: the size of our Galaxy, the Milky Way, and the existence of other galaxies. The two classes of celestial objects critical to this debate were cepheid variable stars and spiral nebulae. Harlow Shapley's work on cepheid variables helped establish a method for measuring astronomical distances, while Heber Curtis observed spiral nebulae and proposed that they were separate galaxies beyond the Milky Way. This debate marked a pivotal point in our understanding of the universe's structure.