The Sun emits electromagnetic energy at the rate of about 4X(10^23) ergs/sec and astronomical and geological evidence shows that the sun has been radiating energy at about its present rate for several billion years.
Q. Why chemical reactions cannot be the source of solar energy?
Ans. Because even if sun consisted of pure carbon its complete combustion would supply enough energy to maintain the radiations for only a few thousand years.
Q. Why conversion of gravitational energy into heat energy cannot be the possible source of solar energy?
Ans. Because it has been shown that if contraction were taking place it could supply not more than 1% of the total energy output needed, and if it were the only source, the sun could be not more than 20 million years old.
The inability to account for the energy emitted by the sun on the basis of ordinary energy sources led to the suggestion that subatomic or nuclear processes provide the necessary energy.
Stars can be classified according to their luminosity, or brightness, and their radiation spectra. The spectral properties of star are related to the effective temperature at which the star radiates energy; the effective temperature is the convenient way of describing the total radiation per unit area. The sun is an example of 1 class of stars, the so called main sequence stars. In addition to the main sequence stars there are white dwarfs, red giants, variables, novae and supernovae. The effective surface temperature of the sun is about 6000 degree Kelvin and the internal temperature may be as high as 2X(10^7) degree Kelvin. The reactions that occur at such high stellar temperatures (10-20 million degrees) are called thermonuclear reactions.
Q. Why fission of heavy elements cannot be the source of solar energy?
Ans. Because the abundance of these elements in the sun is much too small to account for the rate of emission and for the lifetime of the sun.
Hydrogen and Helium together form about 90% by weight of the sun's matter, with approximately equal fractions.
Two sets of thermonuclear reactions have been proposed as sources of energy in the sun and other stars of the main sequence.
1. One set, sometimes called the proton-proton chain, consists of the reactions,
with a total energy release of 26.7 MeV. When the kinetic energy of the neutrinos is subtracted the energy is 26.2 MeV. The positrons are annihilated by free electrons with the production of gamma rays.
Another proton-proton chain consists of the reactions,
It is thought that the former chain is important at lower temperatures, corresponding to those in the sun when it was first formed, and that the latter chain is more important in the present state of the sun, with its higher central temperature and larger He-4 (Helium with mass no. 4) concentration.
The production of 1 helium nucleus from 4 protons is an example of process called fusion in which a heavier element is built from 1 or more lighter elements. Under appropriate conditions, reactions such as those discussed can liberate vast amounts of energy, amounts much greater than that released in an atom-bomb explosion, and thermonuclear reactions form the basis of the so-called thermonuclear or hydrogen bomb.
2. Another set of reactions, the well known carbon-nitrogen cycle was proposed by Bethe to account for energy production in the sun and other stars of the main sequence.
The reactions involving carbon and nitrogen are found to have the remarkable property that they could be formed into a cycle in which the carbon and nitrogen nuclei are not used up, but are regenerated. These nuclei act as catalysts in a series of reactions in which 4 protons are converted into a helium nucleus and about 26 MeV of energy is liberated. The sequence of reactions is,
For some years it was thought that the C-N cycle was responsible for nearly all the solar energy production but, because of recently obtained nuclear data, the proton chain is considered to be more important in the sun than the carbon cycle. It is thought that carbon cycle produces more energy in the main sequence stars which are much more luminous than the sun and whose central temperatures are higher, while the proton chain is more important for main sequence stars less luminous than the sun.
(Source: Nuclear Physics by Irving Kaplan)
