Planetary nebulae like NGC 40 are formed when aging stars eject a shell of material. Note that the top two images are brighter near the edges, as there is more gas along our line of sight in these regions.
The image in the upper right is a photograph of the nebula in the light of the optical H alpha spectral line. The dark circle in the center, not a feature of NGC 40, was used to block light from the central star so that the nebula could be photographed better. Light from this star stimulates the gas to emit energy, much of it in the H alpha line.
The gas in planetary nebulae also emit radio waves. In the upper left is a radio image (at a wavelength of 6 centimeters) of
