ABSTRACT
The starting point for the formation of all objects in the galaxy is the interstellar medium (ISM). At first sight it appears a very unpromising source of material. The ISM is rather lumpy, less dense and hotter in some regions, denser and cooler in others. A characteristic state is with a density a few times 1021 kgm3
and temperature about 10 000K. It is mostly hydrogen and helium but also contains 1-2% by mass of solid grains that may be ices, silicates or metal. Although it is so diffuse, the ISM actually contains a significant prtoportion of the mass of the galaxy. In the solar neighbourhood the density of stars is about 0.08pc3,† which is equivalent to 3 1021 kgm3, assuming that the mean mass of a star is about 0.5M8.
