The final stage of the stellar evolution on the AGB is formed by the deeply red shining carbon stars, in most of the
cases also Mira Variables or "Carbon Miras". In the atmosphere of a star, finally moving to the upper part of the AGB, the C/O ratio becomes__WVX > _. This results in a carbon excess which accumulates in
a circumstellar cloud, dominating now impressively the star's spectrum. Thus, in the intermediate class SC, and increasingly in the following C–Class, moderately high resolution spectra show now
predominantly absorptions of diatomic carbon molecules. In addition to CH and CN the so-called Swan bands due to C2 are particularly striking – discovered in 1856 by the Scot William Swan. Further
visible are atomic lines of S–Process products and impressive absorptions of Na I. Angelo Secchi was the first to discover that the intensity gradient of the C2 Swan bands is reversed [ ] in contrast to
other molecular absorptions, such as titaniumand zirconium oxide. For this feature, he created the separate spectral class lV.