Interstellar space abundances relative

H2C0, HCN, HC0+, HCC, C3N, C4H. The high density and temperature of black interstellar clouds facilitates a richer chemistry in which molecules such as dimethyl ether and ethyl alcohol are formed [1]. Figure 1 summarizes the carbon compounds which have been found in interstellar space and their abundances relative to hydrogen. Note that the carbon compounds decrease in abundance with increasing complexity. 

In addition, UV line spectra are useful because they contain the resonance absorption features of most of the common ions expected in stellar atmospheres and provide sensitive means of measuring temperature and relative abundances. Since, as discussed later, starlight can be absorbed or scattered by dust particles in interstellar space, measurements of absorption lines produced in the stellar atmospheres can, in some cases, provide more useful... 

Typical abundances of PAHs are of the order of 10 relative to hydrogen, which makes these species the most abundant polyatomic molecules present in space. Vibrational spectroscopy is eminently suited for detecting the presence of classes of molecules, but it is much more difficult to identify specific molecules within a collection of species. However, while the gross characteristics of these stellar and interstellar spectra are very similar, when examined in detail, they vary from source to source. Analysis of these variations may well provide us with a tool to identify specific molecules within the circumstellar and interstellar PAH family and such efforts, supported by extensive laboratory studies, are now underway. 

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