Until the year 1939, the source of the sun’s energy was not known. Before 1939, several theories were proposed to explain the source of the sun’s energy. In the early days, it was thought that the sun was a huge ball of burning coal. However, this theory has to be rejected in view of astronomical observations. Astronomical observations show that mass of the sun is about 1029 tones. It is quite a large amount of coal; however, scientists showed that this much amount of coal would burn first 5000 years. Thus, if the sun were made up of coal then its age could not have been more than 5000 years. Contrary to this, the sun has been burning for much longer than that. Evidence ion this favor has been obtained from the record of fossils of ancient life forms as old as 600 millions years old. These living bodies could not have existed without the sun’s energy. It is estimated that the sun has been shining for the last seven to eight billion years. If it were made up of coal then it would have been reduced to dead ash by now.
The Technique of Spectrum Analysis
Around 1920, the technique of spectrum analysis came to be known. By this technique, it became possible to identify the elements present in the substance by study of its spectrum. This technique is based on the fact that each element emits light of only definite wavelengths which is characteristic of element. For example, sodium emits light of wavelengths 589.2 nm and 589.6 nm. No other element emits light of these wavelengths.
A substance when excited by some method such as by heating, by passing electric current or by passing electric discharge starts emitting light. This light when passed through a prism undergoes dispersion and splits up into radiations of different wavelengths. After dispersion the radiations get arranged in the order of increasing wavelength. The pattern of radiation obtained after passing the light through prism is called spectrum. From the spectrum it becomes possible to know the wavelengths of the radiations (light) emitted by the substance. Since each elements emits light of only characteristic wavelength which is differ from the light emitted by the order elements, therefore, by identifying the wavelengths of the light emitted by a substance it becomes possible to identify the elements present in the substance.
The sun is made up of Hydrogen
The spectral study of the sunlight indicated that it contained the radiations of the wavelengths corresponding to the spectrum of hydrogen. Thus it was concluded that the sun is largely made up of hydrogen.
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Once it came to be known that the sum is made up of hydrogen. It was proposed that energy emitted by the sun was due to combustion or burning of hydrogen. However this could not be accepted as the correct explanation because combustion of hydrogen could not liberate such an enormous amount of heat energy that we are receiving. Therefore, some alternative explanation was required to account for the energy of the sun.
The correct explanation for the massive amount of energy emitted by the sun was given by the German physicist, HANS BETHE in the year 1939. It was proposed by Bethe that the sun contains in its core, hydrogen nuclei moving at a very high speeds. Whenever this nucleus undergoes fusion to form nucleus of a heavier element such as helium, a large amount of energy is liberated. It is the energy liberated during such nuclear reactions that we are receiving in the form of sunlight.