Red indicates those regions where the gas is ionized once by the central star. Green light arises in regions where the gas is doubly ionized. The blue colors arise in regions in which the light is dominated by continuous emission, such as starlight. Click for a bigger image . On some monitors this bigger and brighter image might look even better.
Red regions are faint zones in which ionized hydrogen emits light.
Green implies regions where the gas is doubly ionized. Blue regions are the brighter portions the red image. Click for a bigger image . On some monitors this bigger and brighter image might look even better.