“Gravity causes light to travel on curved paths, leading to a phenomenon called gravitational lensing. This effect allows us to view far away galaxies, the back sides of neutron stars, and the regions near black holes! Neutron stars are tiny stars with ultra-strong magnetic and gravitational fields and densities larger than nuclear. Their small size and large average densities allow them to spin at very rapid rates, with surface velocities that are a large fraction of the speed of light. The pulsed X-ray emission from hot spots on the surface of a rotating neutron star contains encoded information about the neutron star’s gravitational field and the properties of the spot’s emission pattern. NASA’s NICER (Neutron Star Interior Composition ExploreR) X-ray telescope is mounted on the International Space Station and is designed to do detailed observations of neutron stars. In this talk I will show how we used the observations of one X-ray pulsar to create a map of its surface emission and to determine other properties of the star. I will also talk about some of the other interesting observations of black holes and neutron stars made by NICER.”