Since gravity affects the propagation of light, one can imagine a very dense and compact star whose gravitational force is so strong that light cannot escape from its surface. Such an object is called a black hole.
Do such objects exist in nature? If light cannot escape from it, the object will be invisible. How can we tell it is there?
These and related questions will be discussed in our fourth year course in General Relativity. You can also visit hubblesite.org. The Hubble Space Telescope has been responsible for making observations that help to confirm the existence of black holes.
Click on the images for an enlarged view.