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A polycyclic aromatic hydrocarbon (PAH) is a hydrocarbon (i.e., compound of hydrogen and carbon), more specifically, an aromatic hydrocarbon (including a ring), arranged in benzene rings (rings of six joined carbon atoms) interconnected so as to form regular hexagons, like chickenwire (fence material of wire joined forming similar six-sided shapes). There is no limit to the types of PAHs, given any such ring can share a side with up to six additional such rings forming, molecules of arbitrary size.
On Earth, PAHs are pollutants, some carcinogenic. The infrared spectral signatures of PAH emissions have been observed in interstellar space, suggesting theories about how compounds are formed in molecular clouds. It has been suggested that PAHs that reached Earth from such clouds might have been the precursors of life. Possibly they are also a component of interstellar dust.
At least some aromatic hydrocarbons, including at least some PAHs exhibit fluorescence, which can be used to detect them. Such fluorescence might be a usable biosignature usable in in situ investigations, such as on solar system moons and planets.
PAHs can be flat, but can be curved and joined together to form tubes, ovals, or spheres of "chickenwire", termed fullerines. A sphere of sixty carbon atoms (C60) arranged like typical soccer ball's (football's) covering-seams is termed a buckminsterfullerene, or for short, a bucky ball (terms which are also sometimes used for similar carbon molecules, e.g., with more carbon atoms squeezed into the pattern). Bucky balls and other fullerines have been detected in interstellar clouds.