Diffuse Interstellar Bands — the 75^th Anniversary

Abstract

This paper reviews the 75 year long history of observations of diffuse interstellar bands (DIBs), the puzzling features which appear in the spectral range from the near UV to near IR, in spectra of reddend stars. They apparently form absorption spectra of dark, HI clouds, together with other spectroscopic features such as atomic and molecular lines or continuous extinction and polarization. DIB intensity ratios vary evidently from cloud to cloud in the interstellar matter (ISM), strongly suggesting that several carriers must be identified to account for the whole DIB spectrum. The recent surveys discovered hundreds of very weak interstellar features; the pattern of them is clearly related to the well-known, strong DIBs and thus they may facilitate the identification of the DIB carriers. These unidentified species are most likely big molecules or very small dust grains which can produce only one strong and several very weak features each. In single cloud cases, the DIB intensity ratios clearly split into several groups, most probably characterizing different cloud environments. The paper discusses different types of HI interstellar clouds and gives a suggestion of how to classify them and put into an evolutionary sequence.