Astronomers study the ways that different forms of matter and energy react with one another. If you become a theoretical astronomer, you may study heady topics like the nature of time or the origin of the universe. You might spent time developing scientific theories and models that try to define properties about celestial bodies. You could then come up with scientific experiments to test your different theories.
Astronomers study planets, stars and galaxies, using ground-based equipment like radios and optical telescopes as well as space-based equipment like the Hubble Space Telescope. Some astronomers will develop a career around research on particular objects like the sun or one of the planets in our solar system. Others may study distant stars, galaxies, and black holes.