Cosmic nebulae: Contrast matters
With a simple image processing method, it is possible to reproduce luminous gas nebulae with high contrast without distorting the colors of other objects, such as stars. An experienced astrophotographer describes the individual steps. Visual observers will also receive valuable tips in this practice report: Read how you can marvel at emission nebulae with the eye on the telescope!
Stars, including our sun, emit radiation at all wavelengths of visible light. Such objects are also referred to as continuum emitters. The situation is completely different with cosmic gas nebulae, such as emission nebulae, planetary nebulae and supernova remnants: They only emit their light at certain wavelengths. If you break down the light of such nebulae into its individual colors, you get a spectrum in which bright lines appear. They are also called emission lines. This line radiation is characteristic of the chemical elements that occur in the cosmic gas clouds and of the respective excited state of the gas atoms contained therein. The nebulae shine particularly brightly in the light of the emission lines H-alpha and H-beta of hydrogen, in the two lines of doubly ionized oxygen (O III) and the line of singly ionized sulfur (S II).
The clear differences between continuum and line emitters are used by astrophotographers, but also by visual observers, to photograph cosmic nebulae or observe them with a telescope. If you take a photo of the starry sky with a filter that only lets through the light of a certain emission line, for example the H-alpha light of hydrogen, the luminous nebulae of star-forming regions appear rich in contrast, since the filter effectively blocks all other wavelengths (see graphic below).
Such an image not only shows the nebulae, but always also stars - because a narrow band filter is not able to suppress the light that the stars emit in its narrow pass band. Amateur astronomers worldwide have been using this technique of nebula photography successfully for a long time - but it gets even better…
