  Polycyclic aromatic hydrocarbons and polycyclic aromatic nitrogen
heterocycles are thought to be widespread throughout the Universe, because
these classes of molecules are probably responsible for the unidentified
infrared bands, a set of emission features seen in numerous Galactic and
extragalactic sources. Despite their expected ubiquity, astronomical
identification of specific aromatic molecules has proven elusive. We present
the discovery of benzonitrile ($c$-C$_6$H$_5$CN), one of the simplest
nitrogen-bearing aromatic molecules, in the interstellar medium. We observed
hyperfine-resolved transitions of benzonitrile in emission from the molecular
cloud TMC-1. Simple aromatic molecules such as benzonitrile may be precursors
for polycyclic aromatic hydrocarbon formation, providing a chemical link to the
carriers of the unidentified infrared bands.
