In contrast, theoretical astronomers create and investigate models of things that cannot be observed. Because it takes millions to billions of years for a system of stars or a galaxy to complete a life cycle, astronomers must observe snapshots of different systems at unique points in their evolution to determine how they form, evolve, and die.
In comparison, theoretical astronomers create and explore models of entities that cannot be observed. Given that it takes millions to billions of years for a star system or a galaxy to complete its life cycle, astronomers must look at various systems at particular stages in their evolution to determine how they are created, change, and eventually die.