These images were acquired using filters which isolate specific wavelengths of light
emitted by ionized gasses when electrons that have been stripped off recombine,
emitting a photon of a specific energy.
The specific wavelengths used here are:
Oxygen III (O III) at 495.9nm and 500.7nm with a 15nm-wide passband
Hydrogen-alpha (Hα) at 656.3nm with a 9nm-wide passband
Sulfur II (S II) at 672.4nm with a 9nm-wide passband
The passbands of the filters used are plotted below:
The color images are created by assigning the wavelengths to color channels such that:
O III = blue, Hα = green and S II = red
called the HST palette, as Hubble Space Telescope images are combined this way.