Here the Crab nebula is seen in X-rays by the Chandra satellite. At these high energies (short wavelengths) the nebula appears more compact then it does optically or in the radio. This is because the electrons which are causing the emission have the most energy as they leave the pulsar in the centre of the nebula. As they travel out towards the edges, they lose energy and emit optically, then in the infra-red, then in the radio.
Image credit: Chandra