Definition

A reflection nebula shines because of reflected starlight. Dark nebulae are identical to reflection nebula except that they are not lit by any star. They can be seen only when silhouetted against something brighter. The best example is the Coal Sack.

Mechanism

In this case it is the myriad tiny motes of dust that are normally mixed with the gas that causes the nebula to be visible. The dust is lit up like smoke in sunlight. Reflection nebulae are always fainter than the star that illuminates them, unless the star is hidden from view by a denser part of the nebula. However, some reflection nebula, in particular the so-called cirrus that was first brought to astronomers attention by the Infrared Astronomical Satellite, are lit not by an individual star but by the combined light of the whole galaxy, much like the clouds at night above a distant city. Reflection nebulae usually appear blue on photographs. A physical principle dictates that the nebula must be bluer than the star that illuminates it.

Examples

M 45

M 78


NGC 7129 - A Reflection Nebula in Cepheus

NGC 7023 Iris Nebula