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Stellar Energy Generation and Nucleosynthesis

  1. Nov 4, 2011 #1
    Introduction: The evolution of stellar objects is inherently caused by the chemical composition of the star. Internally, thermonuclear reactions leading to the formation of complex atoms lead to a change in chemical composition which in turn will affect the evolutionary position of a star.

    Energy Generation: Energy generation is made possible by proton-proton reactions. In a proton-proton chain, two protons colliding with a minimum energy of 8 X 106 K will lead to the formation of a heavy hydrogen nucleus that will consist of a proton and neutron. Furthermore, a positron (the electron's anti-particle) and a neutrino will break away. The positron will collide with an electron and the two particles will annihilate one another (as particle physics dictates) to form two gamma rays. A hydrogen atom will then crash into another proton to form helium and a gamma ray.

    Another form of energy generation in stars is from the carbon-nitrogen-oxygen cycle (mercifully shortened to CNO cycle). The CNO cycle is vital and predominate in more massive starts of one solar mass or greater. The minimum energy for a greater CNO output compared to proton-proton output is roughly 20 X 106 K. The CNO cycle is initiated by the conversion of a radioactive nitrogen isotope which is formed by the collision of a proton and carbon nucleus. The nitrogen becomes a carbon isotope by emitting a positron and a neutron. Proton bombardment will lead to the gamma ray emission and the conversion into a stable nucleus of nitrogen. Further proton bombardment will convert the nitrogen nucleus into radioactive oxygen, which will decay into a nitrogen nucleus, a positron and a neutron. The nitrogen isotope will then split into a C-12 and a He-4 after a collision with a proton. The carbon in a CNO cycle serves as the catalyst since it remains unchanged in the CNO cycle.

    It is important to note that the proton-proton and CNO cycles will only take place in the core of a stellar object since it contains the minimum amount of energy to maintain the reactions. Once the stellar core has used up its limited amount of fuel (hydrogen), it becomes almost entirely composed of helium and the CNO and proton-proton cycles cease.

    Nucleosynthesis: Once the cycles cease, the core will contract and the temperature of the core will significantly increase. Once the temperature has reached a temperature of roughly 100 X 106 K, three helium nuclei will fuse together to form one carbon nuclei in a process known as the triple-alpha reaction. After the helium is used up, the core contracts and heats up once again and if the temperature can get up to around 600 X 106 K, the carbon nuclei will fuse together to form heavier elements. This process of building heavier elements will continue on till it reaches iron (the most stable of the nuclei). In order to process elements heavier than iron, the fusion are reactions are required to be endothermic. This atomic altering is known as nucleosynthesis and is the reason that we (the Universe) have heavier elements in a Universe that would only contain the lighter elements from the Big Bang era.

    -Philip Mathew
  2. jcsd
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