Galactic Nebulae

Galactic nebulae are gas or dust regions that are stimulated to shine by hot stars (emission nebula, mostly red) or reflect their light (reflection nebula, mostly blue, Wikipedia).

A dark nebula or absorption nebula is a type of interstellar cloud, particularly molecular clouds, that is so dense that it obscures the visible wavelengths of light from objects behind it, such as background stars and emission or reflection nebulae, Wikipedia.