BDanielMayfield wrote:Why do the colors in this apod look so unusual? The nebulas are very recognizable by shape, but not by color.
The colors look strange because they have been produced with narrowband photography. Narrowband photography does not produce colors the way the human eye is built to detect it. We have
blue-, green- and red-sensitive "color detectors", so called cones. Their ranges of sensitivity overlap, which is why we can detect an enormous range of hues.
Narrowband photography means that you photograph only extremely specific wavelengths, for example
hydrogen alpha at 656 nm. Hydrogen alpha emission completely dominates almost all emission nebulae. This means that if you were to photograph the region of Cygnus near Deneb and Sadr through a hydrogen alpha filter, your entire image would be flooded by red, all of it exactly the same shade of red.
There is another emission line whose color is almost exactly the same as hydrogen alpha,
ionized sulphur, SII. SII is just a little deeper into the red part of the spectrum than Ha. Visually, there is actually no difference between them. However, SII emission is generally fainter than Ha, and more importantly, it represents a lower grade of ionization than Ha. Where you have SII emission but no Ha, you are likely to be relatively far away from the ionization source.
For scientific purposes, then, it might be useful to separate the Ha emission from the SII, but you can't do that using ordinary RGB astrophotography. However, with narrowband photography you can indeed separate the two. You can take one image through a Ha filter and one image through an SII filter, and they will indeed look different. You can then combine the two images into one picture. But when doing so, it is customary to assign different colors to the Ha picture and the SII picture. In reality both are red. But to separate them, it is customary, using the Hubble palette, to assign green color to the Ha image.
There is another emission line which is of interest, and that is the OIII line. Whereas the SII line represents are fairly low degree of ionization, OIII is only found where ionization is high. Therefore it is popular to make images through an OIII emission to isolate regions where the degree of ionization is very high. The true color of OIII emission is blue-green, but when OIII images are combined with Ha and SII images, the OIII image is colored blue.
Here is a (very large) picture of a nebula in Cygnus which has been produced with narrowband photography in the Hubble palette. Blue represents high-ionization OIII emission, green is Ha and red is low-ionization SII.
Do you want to know what today's APOD would look like to our eyes, if our eyes were sensitive enough to detect color in nebulae?
This is what it would look like.
Ann
Clouds in Cygnus
