Imaging Cygnus in Hydrogen Alpha

Emission nebulae are given this name because unlike reflection nebulae
they are emitting their own light. This is due to the atoms of gas
absorbing energy from nearby stars and then re-emitting the energy at
a very specific wavelengths. In most cases the dominant emission line
is hydrogen-alpha, or H-alpha that is found in the red part of the
spectrum.

This 9 page web site shows what can be achieved with a fairly small
telescope equiped with a narrow band filter.

http://www.martin-nicholson.info/ast...ogenalpha1.htm