This image shows the nebula as seen by the Very Large Array in New Mexico. In this part of the spectrum, the nebula appears larger than it does at any other wavelength. This is because the electrons have the most energy (so emit at the shortest wavelengths) when they leave the central pulsar. As they move out through the nebula they lose energy and emit at longer and longer wavelengths until they reach the outher part of the nebula.
Image credit: VLA