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3. The Current State of the Art

The last 40 years have seen a revolution in the study of interstellar dust. This has been a four-fold process. First of all, the observational access to the UV and the IR brought into focus the fact that there had to be a very wide range of particle sizes and types to account for the blocking of the starlight. Secondly, the IR provided a probe of some of the chemical constituents of the dust. Thirdly, laboratory techniques were applied to the properties and evolution of possible grain materials. Fourthly, advances in numerical techniques and the speed and memory of computers have greatly enhanced our modelling capabilities.

Thanks to the successful performances of IUE (International Ultraviolet Explorer), IRAS (Infrared Astronomical Satellite), COBE (Cosmic Background Explorer), HST (Hubble Space Telescope), ISO (Infrared Space Observatory) as well as various ground-based UV, optical and IR instruments, we have witnessed an explosive accumulation of new observational information on interstellar extinction, polarization, scattering, IR continuum emission and spectral features, as well as elemental depletion. Rapid progress on laboratory experiments has also been made. Below we present an overview of our current knowledge of the dust which brings many related astrophysical problems to the fore.