Gamma ray image of the Crab Nebula (centre) and the gamma ray source known as Geminga (top left). The colour code runs from black for the dim background through red and yellow for the areas where the gamma emission is stronger. The Crab Nebula (M1) was formed by a supernova explosion and at its centre lies a pulsar, a rapidly spinning neutron star, which represents the dense core of the exploded star. The gamma source Geminga is believed to be an isolated neutron star. The image was taken in the energy range 100- 500MeV by the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Observatory satellite.