Carbon cycle stars

The carbon cycle stars are likely to be second generation stars because is needed in the core for the carbon cycle to start. The same star may pass through several novae explosions whereby it loses large amounts of the lighter elements from the outer mantle in each explosion. The chemical composition of a star thus not only indicates its age but also tells us to which generation of stars it belongs. 

It has been detected spectroscopically in great abundance, especially in the hotter stars, and it is an important component in both the proton-proton reaction and the carbon cycle, which account for the energy of the sun and stars. 

The carbon cycle is another sequence of thermonuclear reactions that provides much of the energy released by hotter stars. The net result of the carbon cycle is also the fusion of four protons in a helium nucleus. 

Figure 2.3 Dust production and gas mass return rate by different stellar types in solar masses per year and kpc-2 in the galaxy at the solar cycle. Stars produce mainly silicate or carbon dust only in some cases is a different kind of dust material formed, probably iron or some iron alloy (peculiar dust). Many additional dust components with much smaller abundance are formed in most cases (Data from Tielens 1999 Zhukovska el al. 2008). Abbreviations of stellar types AGB = asymptotic giant branch stars of spectral types M, S, or C OB = massive stars of spectral types O and B on or close to the main sequence RGB = massive stars on the red giant branch LBV = luminous blue variables WCL = Wolf-Rayet stars from the lower temperature range Novae = mass ejecta from novae SN = mass ejecta from supemovae.

If these values are typical, even a young cloud should contain appreciable amounts of carbon cycled through solar nebulae. Abundances of interstellar molecules relative to CO are at least 2 orders of magnitude lower than yields in FTT syntheses (Gammon, 1978). It appears that only a moderate degree of star formation and CO processing would suffice to account for the interstellar molecules. 

It is evident that fusion reactions become possible only at very high temperatures, and they are therefore called thermonuclear reactions. It is assumed that the deuterium cycle (a) prevails in the sun and in relatively cold stars, whereas the carbon cycle (b) dominates in hot stars. In the centre of stars densities of the order of lO g/cm and temperatures of the order of 10 K may exist, and under these conditions other thermonuclear reactions become possible ... 

Hydrogen burning and helium production. Hydrogen burns in the core of a star to form 4He through either the proton-proton chain reaction, which takes place at 5 X 106 K or at higher temperatures (> 20 X 106 K) through the carbon cycle (the C-N-O cycle) in which carbon acts as a nuclear catalyst in the production of He. This process is also known as the quiescent burning phase of a star and is a slow process which takes billions of years and covers much of the life of a star. Our sun is currently in this phase. 

The energy of the sun and stars can be attributed at least in part to the well-known carbon-nitrogen cycle. 

It is interesting to note that all stars without exception are Li rich in the initial part of this cycle, characterizing an initial Li enrichment for all stars (see Figs. 7a and 7b in Drake et al. 2002). An important test of this scenario could be the observation of separated CS around RGB giants as is the case of the more evolved carbon asymptotic giant stars. 

Carbon-nitrogen-oxygen cycle CNO fusion cycle in stars greater than 1 M(.) that produces odd-mass nuclei 13C, 150 and 15N. 

Figure 27. Evolution of the IR integrated absorption of the C—H stretching modes involving sp carbon atoms. The corresponding absorption band is indicated by the star in the IR spectra reported in the inset during a compression (full dots)-decompression (empty dots) cycle.

CARBON-NITROGEN CYCLE (second- and later-generation stars)... 

The emission of a helium nucleus in the final stage regenerates the initial carbon-12. The latter thus plays the role of a catalyst. The overall result is the fusion of four protons into a helium nucleus. At high temperatures, this cycle dominates over the proton-proton chain. Indeed thermal agitation facilitates penetration of the relatively high electrical barrier between proton and carbon nucleus. Whatever hydrogen fusion mechanism is prevalent, the star s mass determines the rate at which it consumes its nuclear fuel, and hence also its lifetime. The higher its mass, the more quickly it bums. 

Hydrogen fusion via either the proton-proton chain or the CNO cycle in the centre of stars comes to an end when most of the hydrogen has been transformed into helium. Helium fusion produces two elements essential to life, namely carbon and oxygen. In fact, carbon constitutes 18% of our bodies, and oxygen 65%, whilst the fractions of these same elements in solar material are just 0.39% and 0.85%, respectively. Only hydrogen and helium are more abundant in the Sun. 

The (,P+ v) reactions are essentially instantaneous and so are much faster than the other reactions at the temperatures in low- and intermediate-mass stars. The fastest proton reaction in this series is 15N(p,a)12C, while the slowest reaction is 14N(p,y)150. As a result, extensive CN cycling converts much of the 15N, 12C, and 13C into 14N. In the CN cycle, 12C is destroyed more rapidly than 13C by about a factor of three. In the solar composition,12C is 89 times more abundant than 13C, so initially more 13C is produced from the destruction of 12C than is destroyed by proton reactions. The 12C/13C ratio decreases until it reaches an equilibrium value equal to the inverse of the reaction rates, where as much 13C is being destroyed as is being produced. From this point on the 12C/13C ratio remains the same as carbon is gradually converted to 14N. 

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