The appearance of a cloud of dust and gas can depend entirely on the relative position of the cloud, observer and light source, as noted above. The nebulae IC 432 and NGC 2023 appear as small blue reflection nebulae. The diagram below shows the geometry of an absorption nebula, such as the Horsehead, and a reflection nebula…
 
 

In a reflection nebula, the light from nearby stars is reflected off dust within the nebula: little radiation is emitted by the dust itself (certainly not enough to cause light to be emitted). This may seem to be the end of the story, but, in fact, the colour of the light reflected is strongly affected by the size of the dust grains present in the nebula and gives us an insight into the composition of such phenomena.
 
 

If the dust particle size is comparable to the wavelength of the light being scattered, the predominant scattering mechanism is called Mie scattering. In this process, the intensity of the scattered light is inversely proportional to the wavelength of the radiation incident on the particles, ie.

Sincel_RED is about twice that of l_BLUEblue light is scattered twice as effectively as red light in this case. So the predominance of blue reflection nebulae may imply a predominance of blue wavelength-sized dust particles.
 
 

Reflection nebulae are also blue because they are most noticeable when the brighteststars shine into the dust. A glance at a Hertzsprung-Russell diagram indicates that the most luminous stars have the highest surface temperatures and emit their radiation predominantly in the UV-blue end of the spectrum. The majority of the stars in the Orion complex (of which the Horsehead region is a small part) are hot blue stars, so the region is full of predominantly blue reflection nebulae.
 
 

It is worth pointing out that when the particle size is much smaller than the incident wavelength, Rayleigh scattering predominates. In this case,