diffuse interstellar bands (DIBs)
Spectrum showing around 100 diffuse interstellar bands.
Diffuse interstellar bands are a series of absorption bands, of interstellar origin, first recorded on photographic plates of the spectra of distant stars in the early 1900s. Well over 100 such bands have been identified in the ultraviolet, visible, and near infrared regions of the spectrum, between wavelengths of 440 to 685 nanometers. According to some estimates, up to 10% of cosmic carbon may be in the molecules that cause these features.
Identifying the carriers of diffuse interstellar bands has become one of the classic astrophysical spectroscopic problems. Recent work suggests they are caused by polycyclic aromatic hydrocarbons (PAHs), or, most likely, their cations, since PAH ions of all sizes absorb in the visible and near infrared, and such molecules are expected to be ionized by the intense ultraviolet field present in much of the interstellar medium.
1. Herbig, G. H. "The Diffuse Interstellar Bands," Annual Review of Astronomy and Astrophysics, 33, 359 (1995).