ABSTRACT
The Milky Way is a rotating stellar system of some 1011 stars that has been created about 15 × 109 yr ago. After an initial and violent phase it has settled into a quasi-stationary state where global parameters, like luminosity, spectral appearance, gas mass or supernova rate change only gradually, on a time scale of ∽1010 yr. The space between the stars is not empty but filled with gas, dust grains, relativistic particles (moving close to the speed of light), magnetic fields and photons. Together these components constitute the interstellar medium. The dust then changes its environment leaving, for example, the warm and tenuous medium and entering a cold and dense phase. This affects the composition and structure of the grains and happens repeatedly during their lifetime. Molecular clouds block the starlight and, therefore, on photographic plates, the near ones appear as holes in the sky that is otherwise littered by myriads of stars.
