Coloured X-ray image taken by the ROSAT satellite of the Vela and Puppis A supernova remnants. The Vela remnant is seen as a yellow-red, round shell occupying the centre of the frame and most of the lefthand side. Puppis A is visible as a round blue area in the top right corner. It appears to be smaller because it is further away (4500 light years) from the Earth than the Vela remnant (1630 light years). Blue shows the highest level of X- ray emission declining through yellow and red. The emission from the two remnants mainly derives from interstellar medium heated by the shock wave originating from the supernova explosions.