Vela and Puppis A supernova remnants: coloured X-ray image taken by the ROSAT satellite. The Vela remnant is the large, round, orange-red shell occupying most of the picture. Puppis A is the small light blue area at upper right centre. 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 red and orange. The emission from the two remnants is mainly due to interstellar medium being heated by the shock wave originating from the supernova explosions.