Interstellar Dust and Extinction Annu. All rights reserved 4. However, the relative locations of the illuminating sources, scattering grains, and the observer are very important in determining the actual intensity of scattered radiation. Its advantage is that the geometry of the sources and scatterers is well known, in contrast to reflection nebulae.
Grains exhibit strong forward throwing scattering in the optical.
This translates into a sky brightness which is comparable to that observed. Clearly, the properties of grains or the intensity of any extragalactic component cannot be analyzed until the scattered intensity is known. Reflection nebulae, in which a bright star illuminates a nearby scattering cloud, have been analyzed in some detail by various authors. Simple geometries used in modelling the scattering, such as plane-parallel or spherical, are not so appropriate as one would like. For clumpy objects, mean values of the albedo and g determined for the clump may not represent the values for a single scattering from an individual grain.