Some of the most exotic objects in the universe result from gravitational collapse. These compact objects include white dwarfs, neutron stars, black holes formed from stellar evolution in our Galaxy and the supermassive black holes in the centres of other galaxies which are thought to power the Quasars and other Active Galactic Nuclei. They are all intrinsically rather dim (especially black holes!), so the easiest way to study them is through their effects on nearby material. While detailed observations of orbiting stars or gas/dust can reveal the presence of a compact object, a far more spectacular signature arises if the material falls into the enourmous gravitational field. The huge amount of potential energy released by accretion often emerges in the form of X-ray radiation.
X-rays cannot penetrate through the Earths atmosphere, so observing high energy emission from celestial sources only became possible in the 1960's with the development of rocket and satellite technology, where X-ray instruments could be lifted above most of the atmosphere. A nice introduction to X-ray astronomy is given at the NASA/Goddard Space Flight Center Imagine site, and at the Harvard Smithsonian Center for Astrophysics X-ray Astronomy Field Guide.
Below is a guide to the Durham X-ray group research interests in this field. You will need to know something about the radiation processes which produce X-ray emission and before you read the sections on accretion onto: