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Dust is everywhere in the Universe: it is a ubiquitous feature of the cosmos, impinging directly or indirectly on most fields of modern astronomy. It occurs in a wide variety of astrophysical regions, ranging from the local environment of the Earth to distant galaxies and quasars: from meteorites originated in the asteroid belt, the most pristine solar system objects - comets, and stratospherically collected interplanetary dust particles (IDPs) of either cometary or asteroidal origin to external galaxies (both normal and active, nearby and distant) and circumnuclear tori around active galactic nuclei; from circumstellar envelopes around evolved stars (cool red giants, AGB stars) and Wolf-Rayet stars, planetary nebulae, nova and supernova ejecta, and supernova remnants to interstellar clouds and star-forming regions; from the terrestrial zodiacal cloud to protoplanetary disks around young stellar objects and debris disks around main-sequence stars ...

Dust plays an increasingly important role in astrophysics. It has a dramatic effect on the Universe by affecting the physical conditions and processes taking place within the Universe and shaping the appearance of dusty objects (e.g. cometary comae, reflection nebulae, dust disks, and galaxies) (i) as an absorber, scatterer, polarizer, and emitter of electromagnetic radiation; (ii) as a revealer of heavily obscured objects (e.g. IR sources) of which we might otherwise be unaware; (iii) as a driver for the mass loss of evolved stars; (iv) as a sink of heavy elements which if otherwise in the gas phase, would profoundly affect the interstellar gas chemistry; (v) as an efficient catalyst for the formation of H2 and other simple molecules as well as complex organic molecules (and as a protector by shielding them from photodissociating ultraviolet [UV] photons) in the interstellar medium (ISM); (vi) as an efficient agent for heating the interstellar gas by providing photoelectrons; (vii) as an important coolant in dense regions by radiating infrared (IR) photons (which is particularly important for the process of star formation in dense clouds by removing the gravitational energy of collapsing clouds and allowing star formation to take place); (viii) as an active participant in interstellar gas dynamics by communicating radiation pressure from starlight to the gas, and providing coupling of the magnetic field to the gas in regions of low fractional ionization; (ix) as a building block in the formation of stars and planetary bodies and finally, (x) as a diagnosis of the physical conditions (e.g. gas density, temperature, radiation intensity, electron density, magnetic field) of the regions where dust is seen.

The dust in the space between stars - interstellar dust - is the most extensively studied cosmic dust type, with circumstellar dust ("stardust"), cometary dust and IDPs coming second. Interstellar dust is an important constituent of the Milky Way and external galaxies. The presence of dust in galaxies limits our ability to interpret the local and distant Universe because dust extinction dims and reddens the galaxy light in the UV-optical-near-IR windows, where the vast majority of the astronomical data have been obtained. In order to infer the stellar content of a galaxy, or the history of star formation in the Universe, it is essential to correct for the effects of interstellar extinction. Dust absorbs starlight and reradiates at longer wavelengths. Nearly half of the bolometric luminosity of the local Universe is reprocessed by dust into the mid- and far-IR.

Stardust, cometary dust and IDPs are directly or indirectly related to interstellar dust: stardust, condensed in the cool atmospheres of evolved stars or supernova ejecta and subsequently injected into the ISM, is considered as a major source of interstellar dust, although the bulk of interstellar dust is not really stardust but must have recondensed in the ISM [11]. Comets, because of their cold formation and cold storage, are considered as the most primitive objects in the solar system and best preserve the composition of the presolar molecular cloud among all solar system bodies. Greenberg [20] argued that comets are made of unaltered pristine interstellar materials with only the most volatile components partially evaporated, although it has also been proposed that cometary materials have been subjected to evaporation, recondensation and other reprocessing in the protosolar nebula and therefore have lost all the records of the presolar molecular cloud out of which they have formed. Genuine presolar grains have been identified in IDPs and primitive meteorites based on their isotopic anomalies [8], indicating that stardust can survive journeys from its birth in stellar outflows and supernova explosions, through the ISM, the formation of the solar system, and its ultimate incorporation into asteroids and comets.

Except in a few cases cosmic dust can be studied in situ (e.g. cometary dust [32], dust in the local interstellar cloud entering our solar system [22]) or in terrestrial laboratories (e.g. IDPs and meteorites [8], cometary dust [7]), our knowledge about cosmic dust is mainly derived from its interaction with electromagnetic radiation: extinction (scattering, absorption), polarization, and emission. Dust reveals its presence and physical and chemical properties and provides clues about the environment where it is found by scattering and absorbing starlight (or photons from other objects) and reradiating the absorbed energy at longer wavelengths.

This chapter deals with the optical properties of dust, i.e., how light is absorbed, scattered, and reradiated by cosmic dust. The subject of light scattering by small particles is a vast, fast-developing field. In this chapter I restrict myself to astrophysically-relevant topics. In Section 2 I present a brief summary of the underlying physics of light scattering. The basic scattering terms are defined in Section 3. In Section 4 I discuss the physical basis of the dielectric functions of dust materials. In Section 5 and Section 6 I respectively summarize the analytic and numerical solutions for calculating the absorption and scattering parameters of dust.

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