Helix nebula (NGC 7293), infrared Spitzer Space Telescope image. This is a planetary nebula, a series of shells of gas cast from a central star towards the end of its life. The ejection of the star's outer layers exposes its hot core, whose radiation ionises the shells, causing them to glow. This image was taken at wavelengths of 3.6 microns (blue), 4.5 microns (green) and 8 microns (red). Longer wavelengths correspond to lower energies, and the gas near the central star is seen to be more energetic that the outer gas, as the outer gas is shielded from some of the radiation by the inner shells. Inward-pointing cometary heads" (blue tips) are dense regions of gas that take the brunt of the core's energy."