This image, taken by the Hubble Space Telescope, shows the Crab Nebula. This nebula is left over from a supernova explosion; at its center sits the Crab Pulsar. Pulsars are neutron stars that form after giant stars explode in supernovae, and regularly emit pulsing beams of radiation. Although the nebula was first observed in 1731, uncertainties in calculations mean that we cannot be sure just how far it is from Earth.

Photo courtesy of NASA's Hubble Space Telescope