Nebulae
There are several different types of nebulae,and there are several classification schemes. One is based on spectroscopy and the light which is seen from the nebulae:
Emission Nebulae: Emit light because the atoms in their gases are excited by high energy radiation of stars involved. They show emission line spectra.
Reflection Nebulae: Reflect light of nearby stars by their dust particles. Therefore, their spectra are the same as those of the stars, typically continuous spectra.
Dark Nebulae: Absorb light - Their gas component can be seen as absorption spectra in the light of background stars, their dust component by absorbing and reddening background light.
For more information see http://www.seds.org/messier/nebula.html
These images are normally captured using limited spectrum (Narrow Band) filters. Typically Ha, SII and OIII filters are used to represent Green, Red and Blue elements of the visible spectrum. Sometimes a Red Continuum is employed. Exposure durations are from 3 to 16 hours. The filters vary in spectrum width from 3 to 5 nanometers. Narrow band imaging has an important advantage over normal RGB imaging. The impact of light pollution is nearly eliminated. This enables me to image in a suburban setting.
Carina Nebula
The Heart Nebula
Western Veil Nebula
Messier 101 The Tadpole Nebula
The Jellyfish Nebula
Thor's Helmut
The Tapezium - Orion's Nebula
The Pacman Nebula
