Interstellar Medium (MFA) - the substance and the fields fill interstellar space within galaxies. Material: interstellar gas and dust (1% of the mass of gas), interstellar magnetic fields, cosmic rays, as well as the invisible dark matter. The chemical composition of the interstellar medium - a product of primary nucleosynthesis and nuclear fusion in stars. Throughout his life, the stars emit stellar winds, which returns on Wednesday elements from the atmosphere stars. And at the end of life of a star with a shell is discharged, enriching the interstellar medium the products of nuclear fusion. Spatial distribution of interstellar medium not trivial. In addition to pan-galactic structures, such as a jumper (bar) and the spiral arms of galaxies, there is a separate cold and warm clouds, surrounded by a hot gas. The main feature of FHI - its very low density - 0,1 .. 1000 atoms in a cubic centimeter.