Interstellar space is not empty, but contains gaseous and particulate matter that is concentrated into very large regions known as interstellar clouds. In the denser and cooler clouds, the gas is molecular and most of the molecules detected are organic in nature. The gas-phase molecules are synthesised from precursor atoms by rapid exothermic reactions in the gas and on the surfaces of tiny dus...

Identification by the Ulysses spacecraft of interstellar grains inside the planetary system provides a new window for the study of diffuse interstellar matter. Dust particles observed by Ulysses and confirmed by Galileo are more massive (≥ 10−13 g) than the ’classical’ interstellar grains. Even bigger grains (≈ 10−7 g) were observed in form of interstellar meteors. We analyze the consequences o...

The arriving abundances of a variety of cosmic-ray nuclides consist of comparable amounts of primary material produced by stellar nucleosynthesis and secondary matter resulting from fragmentation of heavier nuclei by collisions during interstellar propagation. We discuss a technique for extracting the primary and secondary contributions that takes advantage of energy spectra of individual nucli...

The importance of understanding the current and historical galactic environments of cool stars is discussed. The penetration of interstellar gas into a stellar astrosphere is a function of the interaction of the star with the interstellar cloud surrounding the star, and this factor needs to be understood if an efficient search for life-bearing planets is to be made. For the Sun, both current an...

A new calculation of the Galactic diffuse gamma-ray spectrum from the decay of secondary particles produced by interactions of cosmic-ray protons with interstellar matter is presented. The calculation utilizes the modern Monte Carlo event generators, Hadrin, Fritiof and Pythia, which simulate high-energy proton-proton collisions and are widely used in studies of nuclear and particle physics, in...

In this chapter we describe how elements have been and are still being formed in the galaxy and how they are transformed into the reservoir of materials present in protostellar environments. We discuss the global cycle of matter from stars, where nucleosynthesis produces heavy elements that are ejected through explosions and winds into the interstellar medium (ISM), through the formation and ev...