Interstellar Grains

Our current understanding of interstellar dust is summarized at an introductory level. Submicron-sized interstellar dust grains absorb and scatter light, and reradiate the absorbed energy in the infrared. The grain population spans a range of sizes, from molecules containing only tens of atoms, to particles containing 10^{10} atoms. Most of the grain mass appears to be due to two types of solid, in approximately equal amounts: amorphous silicate mineral, and carbonaceous material. Approximately 2/3 of the interstellar carbon in diffuse clouds is in solid form. Interstellar grains are important for many reasons: they obscure our view (in a wavelength-dependent manner); the infrared emission from dust provides a valuable probe of dense regions; and because dust grains play a central role in interstellar chemistry, gas dynamics, and heating and cooling of interstellar gas.