Hydrogen in Orion
Taken from light-polluted Tyneside with an unmodified DSLR by using a 7nm H-alpha filter
Canon 5D3 ISO 12800 50mm f/1.4 lens at f/2.8 7nm Baader Ha filter
HEQ5 mount (unguided) 123 x 64s = 131.2 minutes. Exposures by APT. Stacked in GRIP
This slide demonstrates a further technique: putting a narrowband filter in front of the camera to pick up a specific wavelength of light, which works even in light-polluted suburbia. It shows more clearly the nebulae in Orion. (But the band across the bottom is still due to light pollution. Orion never gets high enough in the sky to completely avoid this at my latitude.)
In the direction of Orion there are clouds of hydrogen gas between the stars. They are rarified but many light years across, so have a significant cumulative effect. The radiation (especially ultraviolet) from some of the stars ionises hydrogen atoms. The electrons separate from the protons but being oppositely charged they attract each other again. As an electron falls down through the energy levels of a hydrogen atom it emits photons (light) of specific wavelengths. The brightest of these in the optical band is called Hydrogen-alpha emission. A narrow band filter around the H-alpha wavelength, which is red, allows through this emission and not much else. That is what we see here.