Observations and theories of interstellar dust

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Abstract
Interstellar dust has a major effect on the Universe and our view of it by strongly affecting the physical and chemical processes within the interstellar medium. Its properties are best determined by the wavelength dependence of its extinction (scattering plus absorption), reflection from nearby stars, emissions (especially in certain near-infrared bands, along with continuum), and polarization of starlight by aligned grains. There are other clues as well, such as the depletion of certain elements from the gas phase onto the grains. The extinction varies strongly with direction, typically with the outer parts of molecular clouds having less extinction in the ultraviolet (relative to the visual) than the diffuse interstellar medium. There is a rather well defined one-parameter mean extinction law extending over the whole range of wavelengths for which observations are available. There are several rival theories regarding the nature of the grains and of their evolution. Most have a common base: that amorphous silicate materials and hydrogenated carbon are almost surely important constituents of grains. Various ideas regarding the arrangements are reviewed.