SIMBAD references

The interpretation of observations over different wavelength domains, which now exist over a large fraction of the sky, will be used to determine relationships between a nebula and its illuminating source.The illuminating source of a high latitude nebula must be behind the cloud and e.g. at close angular distance. It is either a point source behind the nebula or the average radiation field created by all the background stars. In the latter case it is possible to estimate the maximum surface brightness a nebula can reach. Illumination by the galactic plane is always negligible in up to date observations. The red color of some nebulae, when they are not HII regions, is more likely to be due to large column densities than to an emission process. Concerning interstellar grain properties, the same data will be used to support the well known property that grains scatter starlight in forward direction.