The gas and dust particles that occupy the space between the individual stars and galaxies are called interstellar matter. There are different types of interstellar matter, such as reflecting nebulae, emission nebulae and dark nebulae, which vary in their composition and concentration.
Reflecting nebulae are dimly luminescent nebulae with a diffuse appearance. They are probably caused by light scattering of dust particles in the vicinity of not very hot stars. A reflecting nebulae itself has very little energy and cannot itself cause the gas particles, which are also present, to glow. The dust particles do, however, glow in the scattered light.
The nebula surrounding the Pleiades and certain parts of the Orion nebula are examples of reflecting nebulae. They are probably made up of matter left over from the birth of a star.
Emission nebulae are bright, shining gas nebulae of hydrogen (and smaller amounts of helium, oxygen, carbon etc.) that unlike reflecting nebulae do produce their own light. This effect is created by the radiation from nearby hot and brightly luminescent stars.
The Orion nebula M42 is again an example of this category, as it contains both emission and reflecting nebulae.
Dark nebulae are areas without stars within the Milky_Way. They are dust clouds that do not permit us to see through them deeper into space. They are found at a great distance to neighboring stars and cannot reflect their light. The stars that lie behind them are also not visible giving the impression that there are large areas in space without stars. Examples of this category are the Horse Head nebula in Orion, the Northern Coal Sack in the region of the constellations Aquila and Cygnus, and the Coal Sack in the southern hemisphere. In all, about a third of the Milky Way is covered by dark nebulae.
