The sun has a layered structure which is suggested by the results of helioseismic soundings, measurement of neutrino flux, continuous monitoring of X-rays and gamma rays emanating from the sun's interior and inferred from the standard theoretical model. The layered structure is shown in Fig. 7.1.
Some details about the layers in the interior of the sun are as follows:
7.3.1 The Core - Nuclear Reactions
The Core is the central region of the sun. Its boundary extends from the center to a distance of about 25% of the sun's radius. Its volume is only 1.6% of that of the sun, but, because of its high density, its mass is almost 50% of that of the sun. The constituents of the core region are predominantly hydrogen (92%) and helium (7.8%).
Under the intense pressure and temperature prevailing in the core, the atoms of these elements are completely ionized with their nuclei and electrons existing in a state of plasma. In such a state, hydrogen (H1) gets converted into helium (He4) (where the superscripts 1 and 4 denote respective atomic mass number) by thermonuclear fusion and a resulting mass defect which, according to Einstein's special theory of relativity (E = mc2, where E is the energy, m the mass defect and c the velocity of light), leads to release of an enormous amount of energy from each conversion by the following process (see McGraw-Hill Encyclopaedia of Science and Technology, vol. 17, 9th edition, 2002):
H1 + H1 = D2 + e+ + v + 1.44 Mev D2 + H1 = He3 + y + 5.49 Mev He3 + He3 = He4 + H1 + H1 + 12.85 Mev where e+ denotes a positron, v a neutrino, y a Gamma ray, D2 a deuteron and the superscript denotes the atomic mass number.
The above proton-proton chain reaction which shows how hydrogen ions which are in great abundance in the interior of the sun get converted into helium accounts for nearly 98.5% of the total solar energy. The remainder is made up by another chain reaction known as the carbon-nitrogen chain reaction in which hydrogen ions get converted into helium through a chain reaction with carbon and nitrogen ions to form helium, as shown below(details are taken from Saha and Srivastava (1931):
126.96.36.199 Carbon-Nitrogen Chain Reaction
N14 + H1 = O15 + y O15 = N15 + e+ + v N15 + H1 = C12 + He
Energy evolved 2.0 Mev 0.5 Mev 8.2 Mev 7.5 Mev 0.7 Mev 5.2 Mev
Total 24.1 Mev
Half-life 10.5 min.
Thus, the core is virtually a nuclear furnace and serves as the source of all of sun's energy. Of the source of energy of stars in general (remember that the sun is also a star), Prof. H.N.Russell writes: "Hydrogen forms the fuel and helium forms the ashes of the process of combustion which keeps the stars shining through ages". The combustion also produces a large flux of neutrinos, particles with no mass or charge, at several points in the above chain reactions. About the neutrinos, Saha and Srivastava (1931) write: "It is assumed that 7% of the gross amount of energy evolved is carried away by the neutrinos, but even this may also ultimately be converted to energy by some process not yet known to us". Since the theory of nuclear fusion in the Sun was first formulated, the search for neutrinos on earth has continued (see further in sect. 7.7).
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Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.