WHAT ARE THE STAR?

As already mentioned(https://forastrophysicist.blogspot.com/2019/08/the-revolution-that-was-unfolding-at.html?m=1), Fraunhofer`s discovery of dark lines in the spectrum of the Sun enabled the physicists to conclude that the Sun`s outer layers were gaseous. By 1870, the Sun and the stars had been modelled as spheres, held together by their own gravity. The outstanding question at the turn of the nineteenth century was the following: what is the source of energy that makes the star shine?
In 1920, Sir Arthur Eddington at Cambridge University in England made the extraordinary suggestion that the source of energy was the transmutation of hydrogen into helium at the center of the stars. He went on to construct a detailed theory of the stars based upon the simple principle that the inward-directed force due to self-Gravity was balanced by the combined pressure of the gas and radiation, both of which are directed outwards. Despite its simplicity, many predictions of this theory were in remarkable agreement with observations; yet, a detailed understanding of the source of energy radiated by the stars had to wait till the emerging discipline of Nuclear physics had come of age. In 1938, Hans Bethe worked out all the details of the fusion reactions. His result for the energy released per second by the Sun by fusing hydrogen into helium agreed extremely well with the energy radiated by the Sun per unit time, but there were no direct evidence of such fusion reactions taking place in the center of the Sun. such evidence could only be provided by the other by products of the reactions. It was therefore imperative to detect the neutrinos produced when hydrogen is transmuted into helium. If Eddington and Bethe were correct, then the Sun should be emitting approximately 1038 neutrinos every second. Since a fair number of these neutrinos would reach the Earth, it would appear to be a simple task to detect them. Unfortunately, neutrinos interact incredibly weakly with matter. But physicist were undeterred. The neutrinos of the Sun were finally detected in 1968. However, there was a interacting problem. The number of neutrinos detected was only one-third of what the theory had predicted. This puzzle challenged the physicist for three decades and was solved only in 2001. So the mystery of the source of energy radiated by the stars was finally solved.

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