The Warm, Cold and Very Cold Dusty Universe
  We are living in a dusty universe: dust is ubiquitously seen in a wide variety of astrophysical environments, ranging from circumstellar envelopes around cool red giants to supernova ejecta, from diffuse and dense interstellar clouds and star-forming regions to debris disks around main-sequence stars, from comets to interplanetary space to distant galaxies and quasars. In this review I focus on interstellar grains, with particular emphasis on the extinction (absorption plus scattering) and emission properties of cold submicron-sized ``classical'' grains which, in thermal equilibrium with the ambient interstellar radiation field, obtain a steady-state temperature of \~15--25K, warm nano-sized (or smaller) "ultrasmall" grains which are, upon absorption of an energetic photon, transiently heated to temperatures as high as a few hundred to over 1000K, and the possible existence of a population of very cold (< 10K) dust. Whether dust grains can really get down to "temperature" less than the 2.7K cosmic microwave background radiation temperature will also be discussed. The robustness of the silicate-graphite-PAHs interstellar dust model is demonstrated by showing that the infrared emission predicted from this model closely matches that observed for the Milky Way, the Small Magellanic Cloud, and the ringed Sb galaxy NGC7331. 
Aigen Li