Fusion in stars
In this particular article Fusion in stars and different cycles, we are going to discuss Fusion in stars, different cycles like Carbon-nitrogen cycle and Proton-proton cycle in the easiest way possible.
On the basis of scientific calculation, it is observed that nearest star from us i.e. sun is emitting radiation 4×10 J/s for million years, even when no change is found in temperature of the sun. From chemical reactions, it is not possible to produce such a huge amount of energy. Because if the sun is made up of carbon, then by burning of carbon we can get the energy up to thousand years only.
According to Helmholtz another possible source of energy of Sun may be gravitational energy. But due to the contraction of the sun against the gravitational field, the energy produced cannot be greater than 1%. That of the energy actually emitting by the sun. Because the presence of heavy elements in sun is negligible. Mostly sun is having hydrogen and helium.
From the study of all exothermic reactions existing in the earth, it can be said the source of the tremendous amount of energy in the stars is due to the fusion reactions of light elements. Because in the most of the stars about 90% of total matter is hydrogen and helium and rest about 10% is other elements. For fusion of two or more light nuclei, it is necessary that the kinetic energy of the nuclei must be greater than the potential energy of the light nuclei. When they brought very near to each other.
For this energy of light, nuclei must be greater than about 0.1MeV and this is possible only when the temperature is off the order of 10^7-10^8 K. Since the temperature of the internal core of the sun is about 1.5*10^7 K, the fusion reaction is, therefore, continuously taking place in the sun. This is the energy of the sun and stars may be attributed to the fusion of hydrogen nuclei into helium nuclei.
According to Bethe, the fusion is taking place under the following two different cycles.
- Carbon-nitrogen cycle.
- Proton-proton cycle.
1. Carbon-nitrogen cycle
American scientist Bethe in 1939 suggested that the production of stellar energy is by the self-sustaining thermonuclear reactions in which for protons (Hydrogen nuclei). Fuse together to foam helium nuclei with the help of nuclear catalyst carbon and nitrogen. This reaction completes in a cycle form. This cycle is known as carbon-Nitrogen cycle and it is completed in successive six reactions as shown:
Thus in this carbon-nitrogen cycle, four protons fuse together to form a helium nucleus. And two positrons and two neutrinos are emitted with 24.68 MeV energy. These two positrons annihilate with two electrons. And emit 2.04 MeV energy. Thus in this fusion cycle, total energy 26.72 MeV is released.
It is observed from the nuclear data that this carbon-nitrogen cycle completes only at very high temperatures. But being the temperature of our sunless than that of other stars. The possibility of completing this cycle is less in the sun. There is another fusion cycle known as the proton-proton cycle. Its possibility is high in the sun.
2. Proton-proton cycle-
In this cycle also four protons fuse together to form the helium nucleus. But the resultant fusion reaction comes out to be the same as in the case of the carbon-nitrogen cycle. It is completed in successive reactions as shown in fig.
The temperature of the internal core of the sun is 1.5* 10^7 K and pressure is 10^6 atmospheric pressure. At this temperature and pressure hydrogen converts into helium insufficient amount. As a result, the sun radiates energy at the rate of about 4 * 10^26 J/s. To radiate this amount of energy sun is losing mass at the rate of about 4* 10^9 kg/s. This loss of mass is 5 * 10^20 times lesser than the mass of the sun. From calculation, it can be shown that the at this rate of emission entire hydrogen of the sun would be converted into helium in 3*10^10 years. I.e. in the next 10^10 years sun would be radiating energy at the present rate.