The four O stars of the Trapezium are identified and their luminosities determined. A reflection nebula shines by reflected starlight from nearby stars. Reflection nebulae depend heavily on the presence of dust in the cloud. Gas alone does not reflect light efficiently, and without the presence of dust particles, most nebulae would be essentially invisible. In all reflection nebulae it is the dust which is most reflective.
Dust can also obscure our view of a nebula.
A cold, dusty region in front of a reflection nebula will absorb the light from that nebula, and make that region appear dark. This effect is sometimes called a dark nebula . An emission nebula is formed of gas and shines because it is hot. Vast amounts of ultra-violet radiation are required to make the gas hot enough to shine. An example of a nebula in which the gas itself glows is M42, The Orion nebula , located in the sword of Orion.