CHEMISTRY IN THE INTERSTELLAR-MEDIUM

Over the past 25 years, astronomers have discovered a large number of gas-phase molecules in space, ranging in size up to more than 10 atoms . The molecules, which are mainly organic in nature and comprise both normal and abnormal species, are located between stars, in regions kno wn as interstellar clouds, which contain both gaseous material and mat erial in the form of dust particles. The gas is well characterized by high-resolution spetroscopy, whereas the dust is less well characteriz ed by low-resolution infrared spectroscopy and the scattering of visib le radiation. The gaseous molecules are synthesized in situ from precu rsor atomic material, which derives from the mass loss of previous gen erations of stars. The chemical reactions involved in this synthesis a re discussed in some detail as are the models that seek to reproduce t he observed abundances of molecules.