Red giant phase

The evolution of a. star after it leaves the red-giant phase depends to some extent on its mass. If it is not more than about 1.4 M it may contract appreciably again and then enter an oscillatory phase of its life before becoming a white dwarf (p. 7). When core contraction following helium and carbon depletion raises the temperature above I0 K the y-ray.s in the stellar assembly become sufficiently energetic to promote the (endothermic) reaction Ne(y,a) 0. The a-paiticle released can penetrate the coulomb barrier of other neon nuclei to form " Mg in a strongly exothermic reaction ... 

During the red giant phase of stellar evolution, free neutrons are generated by reactions such as C(a,n) and Ne(a,n) Mg. (The (ot,n) notation signifies a nuclear reaction where an alpha particle combines with the first nucleus and a neutron is ejected to form the second nucleus.) The neutrons, having no charge, can interact with nuclei of any mass at the existing temperatures and can in principle build up the elements to Bi, the heaviest stable element. The steady source of neutrons in the interiors of stable, evolved stars produces what is known as the "s process," the buildup of heavy elements by the slow interaction with a low flux of neutrons. The more rapid "r process" occurs in... 

In the red giant phase, the gravitational force ruthlessly pursues its task of crushing the core. The temperature and density increase in symphony, for compressed gases automatically heat up. When the temperature reaches about 100 million K, corresponding to a density of 1(X)0(X) g cm, a new type of nuclear reaction begins to occur in the centre of stars. 

The chances of such three-body colhsions are very slight because the mediating Be is so ephemeral. This guarantees the red giant phase an enviable longevity of several million years even this collision probabihty is amplified by a perfectly chance resonance, to which we shall return shortly. 

The role of such convective dredge-up of matter in the red-giant phase of evolution of 1-IOMq stars is now understood to be an extremely complex process (Busso et al., 1999). On the first ascent of the giant branch (prior to helium ignition), convection can bring the products of CNO cycle burning (e.g., and... 

A less el3 dent, but more reliable source of interstellar oxygen originates in the red-giant phase of stars, which occurs after the useable hydrogen has fused to helium in the core (see chapter 2). In this stage, the so-called triple-alpha process produces but a side effect is that the carbon can interact with the available helium nuclei, so that... 

The most abundant elements (Fig. 2.2) up to Fe are multiples of He ( C, O, Mg, Si, etc.). During the red giant phase of stellar evolution, free neutrons are generated which can interact with all nuclei and build up all the heavy elements up to Bi all nuclides with the atomic number > 84 are radioactive. Recently (2003) it has been found that even ° Bi decays, but extremely slowly (Ti/2 = 1.9 10 yr). The build-up of elements of every known stable isotope depends on different conditions of density and temperature. Thus, the production process required cycles of star formation, element formation in stellar cores, and ejection of matter to produce a gas enriched with heavy elements from which new generations of stars could form. The synthesis of material and subsequent mixing of dust and gas between stars produced the solar mix of elements in the proportions that are called cosmic abundance (Fig. 2.2 and cf Table 2.13). 

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