Over the past 20 years, ground- and space-based observations have revealed that the universe is filled with molecules. Astronomers have identified nearly 200 types of molecules in the interstellar medium (ISM) of our galaxy and in the atmospheres of planets; for the full list, see www.astrochymist.org. Molecules are abundant and pervasive, and they control the temperature of interstellar gas (see the box on page 40).1 Not surprisingly, they directly influence such key macroscopic processes as star formation and the evolution of galaxies.

Interstellar molecules attest to the chemical evolution of the universe—from the atoms formed in stellar interiors to the dust and planets of solar systems. Besides controlling the birth and evolution of stars and galaxies, the molecules can be used as unique probes of conditions in the ISM and as tracers of processes therein. Among those molecules are polycyclic aromatic hydrocarbons (PAHs)—essentially fused benzene rings joined together in flat sheets whose edges are decorated with hydrogen atoms. The multiple benzene-
ringconfiguration delocalizes the electrons in the rings and stabilizes the PAHs.

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