Dark Nebulae are clouds of dust in space that obscure the stars behind them.
Emission nebulae are clouds of glowing ionized gas.
Reflection nebulae do not shine by their own light, but are visible because they reflect the light of nearby stars. Along with the Orion Nebula, M42, it is part of a very large complex that is a stellar nursery where stars are forming out of the dust and gas. Located about 1,500 light years away, this complex is the closest star forming region to our own solar system. To the lower left of the Horsehead is the blue reflection nebula NGC 2023. Other reflection nebulae in the photo are IC 435, IC 432 and IC 431.