Black holes & Co.
The more compact and massive an object, the stronger the gravitational influence it exerts on its direct neighborhood - and the stronger the deviations between general relativity's predictions and those of Newtonian gravity.
This chapter of Elementary Einstein is devoted to the most compact objects there are. First we'll examine neutron stars, super-dense remnants of stars, and their visible avatars, pulsars. Secondly, we'll discuss a class of compact objects whose existence was predicted using general relativity: black holes. Black holes started out as a rather exotic theoretical idea, but nowadays, they're an important building block of astrophysical models (for stellar evolution and for the active nuclei of certain galaxies).