The formation and evolution of interstellar dust: Observational evidence

Interstellar dust can be observed in different environments in the Galaxy by absorption and scattering of the optical and ultraviolet starlight and emission at infrared and submillimeter wavelengths; its composition can be inferred from these observations, from the absence of much of the expected heavy element content from the gas phase, and from direct study of meteoritic inclusions. The dust properties vary with environment, demonstrating their evolution in the interstellar medium. Observations show directly that dust is destroyed in shocked gas and is created in a variety of ejecta from evolving stars. This review considers the timescales for creation, destruction and movement of the interstellar medium among its phases, and compares the observational properties of dust in regions where it is formed with those of dust in the interstellar medium. The tentative conclusion is that dust grains are largely created in the winds of red giants and evolve by coagulation and shattering/evaporation in the interstellar medium, but there are many uncertainties at every stage.