Supernova remnant. Artwork of the brightly- coloured wisps of gas of a supernova remnant, seen from a nearby planet. The planet's moon is seen at upper left. The red and green strands of nebulosity are the remains of a massive star which ended its life in an enormous supernova explosion. The ejected gas may travel at thousands of kilometres per second after the explosion. It is heated by the shock wave produced as it collides with the interstellar medium, which leads to its emissions. Supernovae leave behind the exposed core of the old star, which exists as a neutron star or a black hole depending on its mass.