As defined by Wikipedia:
"Astrophysics (Greek: Astro - meaning "star", and Greek: physis – φύσις - meaning "nature") is the branch of astronomy that deals with the physics of the universe, including the physical properties of celestial objects, as well as their interactions and behavior."
Persons to remember:
Aristarchus of Samos (c. 310-250 BC) first gave the idea that the motions of the celestial bodies could be explained by assuming that the Earth and all the other planets in the Solar System orbited the Sun. It was deemed heretical at that time when most believed that the Earth is the center of the Universe.
Ptolemy (83-161 AD) Develops the Geocentric model of the Universe where the Earth is the center of everything.
Seleucus of Seleucia - Babylonian astronomer who is said to have proved the Sun-centered hypothesis through reasoning in the 2nd century BC. He used the phenomenon of tides to support that, which he correctly theorized to be caused by the attraction to the Moon and notes that the height of the tides depends on the Moon's position relative to the Sun.
Ibn al-Haytham (Alhazen) - wrote the Maqala fi daw al-qamar (On the Light of the Moon) some time before 1021. He concluded that it "emits light from those portions of its surface which the sun's light strikes."
Nicolaus Copernicus - in the 16th century, revived the Heliocentric idea that the Sun is the center of the Universe.
Galileo Galilei - Discovered the four brightest moons of Jupiter in 1610, and documented their orbits about that planet, which contradicted the geocentric (earth-centered universe) doctrine of the Catholic Church of his time, and escaped serious punishment only by maintaining that his astronomy was a work of mathematics, not of natural philosophy (physics), and therefore purely abstract.
A Cornell University website explained the difference between Astronomy and Astrophysics in a simple way:
"Technically speaking, astronomy is the science of measuring the positions and characteristics of heavenly bodies, and astrophysics is the application of physics to understand astronomy. However, nowadays, the two terms are more or less interchangeable since all astronomers use physics to understand their findings."
Image of the cosmic microwave background radiation
Wisegeek.org gives us some insight on what Astrophysicists do:
Astrophysicists are known for studying such phenomena as black holes, galaxies, superclusters, neutron stars, quasars, the Big Bang, dark matter and energy, cosmic strings, stellar evolution, the cosmic microwave background radiation, and many others. The cosmos is a good arena for studying pure physics because on such large scales, the particular type of element making up objects becomes less significant, and more general variables such as mass and velocity take primacy. Sometimes astrophysics is called "the study of the very large and the very small."
Well known Astrophysicists
|Neil DeGrasse Tyson|
You can check out more astrophysicists by clicking the link in the reference section.