n6 SIMPLER ORGANIC SUBSTANCES 



with the presence o£ carbon. Any heavenly body having a 

 mass greater than one-twentieth of that of the Sun is very 

 likely to have such a cycle occurring within it, in which case 

 it will be a self-luminous formation, in fact a star. 



It is of particular interest to us to enquire as to the form 

 in which carbon exists on stars of different spectral types. 

 On stars of type O, which have a very high temperature on 

 their surfaces, J. Plaskett*^ found that carbon was present 

 mainly in the singly or doubly ionised form (c+ or C++). On 

 these stars the temperature is so high that there can be no 

 question of the presence of any sort of chemical combination 

 of carbon. The carbon atoms themselves are substantially 

 altered in that they have lost some of their outer electrons. 



On stars of type B, which are cooler, F. Henroteau and 

 J. Henderson^" also demonstrated the presence of carbon, 

 though only in the neutral form. However, no carbon com- 

 pounds could exist on these either. Signs of such compounds 

 appear in the spectra of stars belonging to type A. Traces 

 of g-bands (A, 4,314 A) were discovered in the spectra of 

 such stars quite a long while ago,^^ indicating the possibility 

 of the development there of the most primitive carbon com- 

 pounds — the hydrocarbons (methyn, ch). In the spectra of 

 other types of stars the hydrocarbon bands show up more 

 and more clearly as the temperature of the surface of the 

 star decreases, reaching a maximum clearness in the spectra 

 of types M and R. These spectra also reveal the presence of 

 compounds of carbon and nitrogen (cyan) in the atmospheres 

 of the stars. 



In the spectra of the sim-spots, and even more so in the 

 spectra of stars of types N and R, there have also been demons- 

 trated the so-called Swan's bands which indicate the presence 

 of molecules consisting of two carbon atoms combined to- 

 gether (Ca, dicarbon).^^ 



The investigations of these bands by G. Shain" and later 

 workers have shown that the carbon in the atmosphere of 

 some so-called carbon stars is ten times richer in the heavy 

 isotope ^^c than the carbon in terrestrial objects. It follows 

 that the evolution of the nuclear material itself has followed 

 a somewhat different course on these stars from that which 

 it has followed within the solar system. Nevertheless, hydro- 



