spectroscopy. pahs possess very characteristic uv absorbance spectra. these often possess many absorbance bands and are unique for each ring structure. thus, for a set of isomers, each isomer has a different uv absorbance spectrum than the others. this is particularly useful in the identification of pahs. most pahs are also fluorescent, emitting characteristic wavelengths of light when they are excited (when the molecules absorb light). the extended pi-electron electronic structures of pahs lead to these spectra, as well as to certain large pahs also exhibiting semi-conducting and other behaviors. origins of life the cat's paw nebula lies inside the milky way galaxy and is located in the constellation scorpius.green areas show regions where radiation from hot stars collided with large molecules and small dust grains called "polycyclic aromatic hydrocarbons" (pahs), causing them to fluoresce.(spitzer space telescope, 2018) pahs may be abundant in the universe. they seem to have been formed as early as a couple of billion years after the big bang, and are associated with new stars and exoplanets. more than 20% of the carbon in the universe may be associated with pahs. pahs are considered possible starting material for the earliest forms of life. light emitted by the red rectangle nebula possesses spectral signatures that suggest the presence of anthracene and pyrene. this report was considered a controversial hypothesis that as nebulae of the same type as the red rectangle approach the ends of their lives, convection currents cause carbon and hydrogen in the nebulae's cores to get caught in stellar winds, and radiate outward. as they cool, the atoms supposedly bond to each other in various ways and eventually form particles of a million or more atoms. adolf witt and his team inferred that pahs—which may have been vital in the formation of early life on earth—can only originate in nebulae. two extremely bright stars illuminate a mist of pahs in this spitzer space telescope image. pahs, subjected to interstellar medium (ism) conditions, are transformed, through hydrogenation, oxygenation, and hydroxylation, to more complex organic compounds—"a step along the path toward amino acids and nucleotides, the raw materials of proteins and dna, respectively". further, as a result of these transformations, the pahs lose their spectroscopic signature which could be one of the reasons "for the lack of pah detection in interstellar ice grains, particularly the outer regions of cold, dense clouds or the upper molecular layers of protoplanetary disks." low-temperature chemical pathways from simple organic compounds to complex pahs are of interest. such chemical pathways may help explain the presence of pahs in the low-temperature atmosphere of saturn's moon titan, and may be significant pathways, in terms of the pah world hypothesis, in producing precursors to biochemicals related to life as we know it.