We are often so amazed by the colorful images we see of nebulae, galaxies and other deep space objects that we may sometimes forget that these colors are not the natural colors, or at least not the colors we would see if viewed directly by the naked eye.
Part of the reason to color these objects so dramatically is to distinguish the different components of the object. That might be different elements or molecules, densities of matter or different energy levels, say from x-rays to infrared to gamma rays.
It really all comes down to the energy of the photons that the CCDs (use to be photographic film) receive. Typically these images are in black and white and are obtained by passing the light from the object through filters, which allow only certain frequencies or energies of electromagnetic radiation to reach the detector. Below are three images of the Eagle Nebula taken (from left to right) in the green, red and deep red parts of the visible spectrum:
You can see some subtle differences in the structure. Interesting, but not too exciting. But, if we assign colors to these filtered images so that the green light emitted by the doubly-ionized oxygen is blue, the red light from hydrogen is green and the deep red light from ionized sulfur is red. This helps to distinguish sulfur from hydrogen, which otherwise would both look red.
This is beginning to look a bit more interesting! Now, by blending these images into one we get the final product:
These color choices show the bluish background of hydrogen and oxygen atoms surrounding the columns of dust and gas containing sulfur. Pretty to the eye and much more informative as to the components that make up the nebula.
These images come from a NASA site “Behind the Pictures”. Check it out. It won’t take long to go through the different pages that talk about using color as a tool, filters and the shapes of the images. There are also many more examples of how this process is applied to learn more about these amazing celestial objects.
Till next time,