Helix nebula (NGC 7293), infrared Spitzer Space Telescope image. This is a planetary nebula, a series of shells of gas cast off by a dying star. 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 varying energy: high (blue), medium (green) and low (red). The gas in the central regions has been ionised more than the outer gas, which is shielded from the stellar core. The most highly ionised areas (cometary heads) are round the edge of the central void. Foreground and background stars are blue. The Helix nebula is 650 light years from Earth, in the constellation of Aquarius. Image data obtained by the IRAC sensor in October 2004.