MAPOW Nr 7: A false color Flaming Star Nebula

Auriga is really a treasure box of bright astronomical objects: star clusters, bright nebulas and dark nebulas. Among them all, IC 405, the Flaming Star Nebula, is one of the most photographed objects.

This image is an unusual Hα–SII bicolor composition in false color. The Hα and SII emission lines both lie in deep red, so it is impossible to render colour differences in a realistic way: the only option is a false color composition.

In this image hydrogen emission has the usual pinkish color due to the Hα and Hβ emission lines. As said before, the SII emission line lies in deep red but, on Earth, crystalline sulfur has yellow hues, so my goal was to render the SII in yellow.

To accomplish my goal I used the PixelMath tool in PixInsight. The composition formula was:

R channel:  Hα
G channel:  SII
B channel:  0.25*Hα - 0.75*SII

The B channel formula is very simple: the 0.25*Hα accounts for the Hβ emission, giving the hydrogen emission its usual pinkish color; subtracting the SII emission shifts the color hue of the sulfur-rich zones toward yellow. The 0.75 factor is an arbitrary value chosen to render the wanted color.

All the image processing was done in PixInsight.

IC 405, The Flaming Star Nebula