An astronomer is a scientist in the field of astronomy who concentrates their studies on a specific question or field outside the scope of Earth. They look at stars, planets, moons, comets and galaxies, as well as many other celestial objects — either in observational astronomy, in analyzing the data, or in theoretical astronomy. Examples of topics or fields astronomers work on include: planetary science, solar astronomy, the origin or evolution of stars, or the formation of galaxies. There are also related but distinct subjects like physical cosmology which studies the Universe as a whole. Astronomers usually fit into two types: observational and theoretical. Observational astronomers make direct observations of planets, stars and galaxies, and analyze the data. 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 have to observe snapshots of different systems at unique points in their evolution to determine how they form, evolve and die. They use these data to create models or simulations to theorize how different celestial bodies work. There are further subcategories inside these two main branches of astronomy such as planetary astronomy, galactic astronomy or physical cosmology.
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