Astrophysics is a field of academic study, a branch of astronomy, which deals with the physics of astronomical objects in the universe. A person who studies in this field is referred to as an astrophysicist.
Astrophysics was a required course of study at Starfleet Academy during the 23rd and 24th centuries. Hikaru Sulu became fascinated with the subject and initially pursued as a career option following his graduation. (TOS - My Brother's Keeper novel: Constitution) During his time at the Academy, Tom Paris achieved a major in astrophysics. (VOY episode: "Future's End")
During Sulu's first year aboard the USS Enterprise he served as the ship's chief astrophysicist. However, he soon became bored of his role and requested a transfer to another Constitution-class vessel where he could assumea helmsmans position. After discussing the matter with Captain Kirk, he remained aboard the Enterprise as helmsman, after suggesting that Tammy O'Shea assume his former position. (TOS episode: "Where No Man Has Gone Before"; TOS - My Brother's Keeper novel: Constitution)