Young Carl Sagan told his grandfather that he wanted to practice astronomy.
Sagan later recalled that, at about age 12, his grandfather asked what he wanted to do when he grew up. Sagan answered “an astronomer,” prompting his grandfather to ask how such a career could provide a living. The question reflected how unfamiliar professional astronomy was to many families at the time.
Sagan’s fascination began even earlier, when he learned that stars were distant suns. He eventually studied at the University of Chicago, earned degrees in physics, and received a doctorate in astronomy and astrophysics in 1960.
Although he became famous as a science communicator through Cosmos, Sagan’s research focused heavily on planetary science, extraterrestrial life, and the conditions on worlds such as Venus and Mars. Astronomy is therefore the right answer, even though his career crossed several related scientific fields.