The term astrophysicist is largely interchangeable with astronomer, so any list of famous astrophysicists can rightly begin with Nicolaus Copernicus. Copernicus was the first modern scientist to develop a consistent model of the heliocentric solar system. His work was followed closely by the Italian astrophysicist Galileo Galilei, who formulated fundamental principles of physics and astronomy, discovered mountains on the Moon and first observed the moons of Jupiter.
Other notable astrophysicists of the 16th and 17th centuries were Tycho Brahe, who was perhaps the greatest observational astronomer of the age, and Johannes Kepler, who was arguably it's greatest theoretician. Christiaan Huygens was the first to accurately describe Saturn's rings, and Giovanni Cassini discovered four of the planet's moons and a gap in the rings that bears his name. Isaac Newton articulated the laws of motion and described gravity in scientific terms for the first time, and Edmund Halley was the first astrophysicist to predict the appearance of a comet.
Later centuries brought other great astrophysicists. Giuseppe Piazzi discovered Eros, the first observed asteroid, in 1801. Pierre-Simon Laplace developed the modern model for the formation of the solar system. Joseph von Fraunhofer identified the spectral signatures of various elements, which made it possible to determine the composition of stars.