Thermonuclear reactions in stars

Essentially all deuterium in the Universe is believed to come from BBNS, because thermonuclear reactions in stars only cause net destruction of D and it is vastly... 

PROTON-PROTON REACTION. A thermonuclear reaction in which two protons collide at very high velocities and combine to form a deuteron. The resultant deuteron may capture another proton to form tritium and the latter may undergo proton capture to form helium. The proton-proton reaction is now believed to be the principal source of energy within the sun and other stars of its dass. A temperature of the order of five million degrees Kelvin and high hydrogen (proton) concentrations are required for this reaction to proceed at rates compatible with energy emission by such stius. 

Stellar nucleosynthesis No 21B is produced by thermonuclear reactions in stellar interiors. It is instead destroyed if heated above several million degrees. This means that the only boron that exists in normal stars is that inherited when the star formed from the interstellar gas, and, moreover, that stellar boron is limited to the outer layers of the atmospheres of the stars. So its natural source must be found elsewhere. 

In stars of small mass (<0.1 times the mass of the sun) the energy liberated by gravitational contraction is not sufficient to reach the temperature necessary to start thermonuclear reactions. These stars are directly entering the stage of black dwarfs (black holes). 

This article is organized essentially in the same sequence that a massive star burns successively higher atomic number elements in its core, until it collapses and explodes in a supernova. The introductory part discusses how the rates of thermonuclear reactions in (massive) stars are calculated, what the different classes of reactions are and how the stars (usually) manage to burn their fuels so slowly6. The middle part describes the nuclear physics during the collapse phase of the massive star. The last part describes a few typical examples of what can be learned by optical, IR and X-ray studies about nucleosynthesis and dynamics of explosion in supernovae and supernova remnants such as Cassiopeia A, SN 1987A etc. Only core-collapse supernovae are discussed in these lectures, those that arise from massive stars (e.g. stars more massive than 8Mq with typical solar metallicity at the time they start... 

For the (n,y) jS case the upper horizontal row of Figure 15.2 rqrresoits the successive formation of higher isotopes of the target element (the constant Z-chain) and the vertical rows the isobaric decay chains of each of these isotopes (the constant A-chains). The first of these two rows is indicated by heavy arrows. Chains which involve both induced transformations and radioactive decay play a central role in theories about the formation of the elements in the universe, in the thermonuclear reactions in the stars (Ch. 17), and in the synthesis of transuranium elements (Ch. 16). 

The helium immediately fuses and the intense heat developed spreads as a heat shock, which passes to the cooler outer shells of hydrogen and helium, initiating new thermonuclear reactions in the mantle. As a result the whole star explodes as a supernova. While the outer layers expand into space, the core contracts to a black hole. Other mechanisms producing supemovae are also known. 

Fig. 2.1 Scheme of thermonuclear formation of chemical elements (fusions reaction in stars). 

There is a very low cosmic abundance of boron, but its occurrence at all is surprising for two reasons. First, boron s isotopes are not involved in a star s normal chain of thermonuclear reactions, and second, boron should not survive a star s extreme thermal condition. The formation of boron has been proposed to arise predominantly from cosmic ray bombardment of interstellar gas in a process called spallation (1). 

If an external body is engulfed, it can enrich the star with the original interstellar medium abundances of 6Li, 7Li, 9Be and 10,11B (written here in increasing order of hardness to be destroyed by thermonuclear reactions). This mechanism is then supposed to produce stellar enrichment of these elements up to the maximum meteoritic value. Also, the engulfing star will suffer a rotational increase due to the gain of the planet momentum and a thermal expansion phenomenon due to the penetration of the body provoking mass loss phenomena (Siess Livio 1999). An extension to this scenario has been proposed by Denissenkov Weiss (2000) in order to explain supermeteoritic Li abundance values, via a combination of stellar rotation and activation of the 7Be mechanism at the base of the convective layer produced by the penetration of the external body. 

Stars of mass greater than 1.4 solar masses have thermonuclear reactions that generate heavier elements (see Table 4.3) and ultimately stars of approximately 20 solar masses are capable of generating the most stable nucleus by fusion processes, Fe. The formation of Fe terminates all fusion processes within the star. Heavier elements must be formed in other processes, usually by neutron capture. The ejection of neutrons during a supernova allows neutron capture events to increase the number of neutrons in an atomic nucleus. Two variations on this process result in the production of all elements above Fe. A summary of nucleosynthesis processes is summarised in Table 4.4. Slow neutron capture - the s-process - occurs during the collapse of the Fe core of heavy stars and produces some higher mass elements, however fast or rapid neutron capture - the r-process - occurs during the supernova event and is responsible for the production of the majority of heavy nuclei. 

The nuclear reactions in which tighter nuclei fuse together to form a heavier nuclear are called nuclear fusion reactions. Such reactions, occur at very high temperature (of the order of > 10 K) which exist only in the sun or interior of stars therefore, such reactions are also called thermonuclear reactions. 

Bob s description of the energy-producing fusion of hydrogen to produce helium is correct, but a simplification of the actual process. The proton-proton reaction is actually a chain of thermonuclear reactions that are the main energy sources for the Sun and cool, Main Sequence stars.13 In the first step of the reaction, two hydrogen nuclei H (shown as two protons, each symbolized by + in the rectangle of figure 7.10) combine to form deuterium 2H. One of the... 

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. 

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 ... 

If the temperature in the core of the stars becomes sufficiently high, thermonuclear reactions give rise to new phases of nucleogenesis, and the energy produced by these reactions leads to further emission of radiation, including visible light. 

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