Reflection nebulae are clouds of gas and dust which reflect the light of a nearby star. The nearby star is
not hot enough to ionize in the gas (as in emission nebulae) but is bright enough that scattered light from the tiny dust grains in the
cloud is visible. A reflection nebula spectrum is similar to that of the illuminating star but is bluer because the tiny dust grains
scatter small wavelengths much more efficiently than long wavelengths.
The illuminating star for IC 2118 (above) is Rigel, a bluish star, so this nebula is especially blue in
color. If the nebula was near a much cooler star (such as the red
giant Antares), the nebula will be more yellow in color since a cool star has so little blue light to reflect.
Spectrum of Reflection Nebula IC 2118