Astrophotography by Bernhard Hubl
A nebula is an interstellar cloud of dust, hydrogen gas and plasma. It is the first stage of a star's cycle or it is a result of a star's death. The nebulae can be divided in three big classes:
An emission nebula is a cloud of ionized gas emitting light of various colors. The most common source for ionization are high-energy photons emitted from a nearby hot star. Most emission nebulae show a dominant red color (Halpha line in HII regions). Nebulae, which are associated with a star's death (planetary nebulae or supernova remnants) can also show strong emission in the OIII line, whereby parts of the nebulae appear in a bluish/greenish color.
2. Reflection nebulae
Reflection nebulae are clouds
of dust which are simply reflecting the light of a nearby star or stars.
The nearby star or stars are not hot enough to cause ionization in the gas
of the nebula like in emission nebulae but are bright enough to give
sufficient scattering to make the dust visible.
A dark nebula is a type of interstellar cloud that is so dense that it obscures the light from the background emission or reflection nebula (e.g., the Horsehead Nebula) or that it blocks out background stars (e.g., the Coalsack Nebula).