It has been common practice to separate the study of interstellar matter from that of stellar evolution. However, infrared astronomy deals mainly with observations of stars forming and stars dying. Interstellar matter represents a phase intermediate between these two stages, part of a cyclic process (Figure 1).

We find molecules in interstellar space and want to know how they came to be there. Molecules form most easily at high densities and moderately high temperatures. These conditions prevail both in envelopes around forming stars and also around evolved red giants. However, the matter going into star formation is mainly leaving interstellar space, while that from the red giants is going into space. Therefore the evolved red giants are potentially the source of interstellar molecules.

This paper will propose first that the chief interstellar solid molecules were formed in the atmospheres of red giant stars. Volatile solids that might condense in space do not seem to be a major constituent of the grains. Secondly it will be demonstrated that a major part of interstellar matter has probably come from the outer layers of red giant stars. Since the solids formed in these envelopes, it appears possible that less complicated gaseous molecules have also been ejected into space. Whether they are in fact the same molecules that are now observed is a question that cannot yet be answered.