Reflection nebula

In astronomy, a reflection nebula is a cloud of dust which reflects the light of a star or many stars. The light from the nearby stars is not hot enough to ionize the gas of the nebula to make an emission nebula. However, is enough to make the dust visible because the light reflects off of the dust. 
Reflection nebulae are usually blue because the scattering works better for blue light than red (this is the same reason that the sky is blue and sunsets are red).
Reflection nebulae and emission nebulae are usually seen together and sometimes are called 'diffuse nebulae'. A good example of this is the Orion nebula.
Reflection nebulae may also be the site of star formation.