Why does the sun shine?

Until the 19th century, there was no attempt to scientifically explain why the sun shines. The sun as a celestial body was only dealt with by astrologers, mythologists, theologians and other dogmatic "intelligence". The sun as the source of life was only discussed in 1833 in the works of the famous astronomer John Herschel: "The sun's rays are the basic cause of practically every movement that occurs on the earth's surface. Thanks to its reviving power, plants arise from inorganic matter, without which there are neither animals, nor people, nor the enormous energy made available to people in the form of coal deposits".

photo: wikipedia.org

How old is the Sun? And why does it shine? Herschel was unable to define any specific answer to these two eternal questions, but he was the first to realize how interconnected they are. The amount of energy emitted by the Sun is relatively easy to determine by measuring the intensity of sunlight on the Earth's surface. If we know how old the Sun is, we can also determine how much energy it has emitted into the cosmos in total. The older the Sun, the more powerful energy sources are needed to maintain its constant brightness for longer and longer periods of time.

Thanks to the enthusiasm of a German professor of psychology, and later a great physicist, Hermann von Helmholtz, in the middle of the 19th century, the understanding that the Sun's radiation is a consequence of the compression of its matter under the influence of gravitational forces became established. However, the main impetus to the early study of the nature of solar energy was given, quite unexpectedly, by Charles Darwin, the author of the idea of ​​the evolutionary origin and development of the living world on Earth. Darwin was aware that evolution takes a very long time, so in his famous work On the origin of species by natural selection tried to determine some concrete figures. Observing the rate of erosion of a valley in southern England, Darwin calculated that it would take nature at least 300 million years to completely wipe it off the face of the earth, which is slow enough even for the slowest evolutionary processes.

Darwin's estimate caused great discomfort among astronomers: if the age of the Earth is really measured in hundreds of millions of years, the Sun must be even older. If the Sun is really that old, then it is even more difficult to explain what causes its enormous power. Lord Kelvin, the famous physicist who formulated the second law of thermodynamics and formed the absolute temperature scale, joined the discussion. Kelvin was a great opponent of Darwin and immediately challenged his estimate of the speed of geological processes. Kelvin, however, admits that no chemical reaction can be the source of sunlight, given that "even the most intense reaction of substances in an amount equal to the mass of the Sun would not last longer than 3000 years". As a source of solar energy, Kelvin, like Helmholtz, initially emphasized the gravitational energy of the solar mass. However, pressured by astronomical evidence from which it follows that the observed gravitational energy would not last long enough, Kelvin modified his theory in 1862 and with great persuasiveness and eloquence claimed that solar energy is created as a result of the impact of countless meteorites on the surface of the sun. Given that the cosmos is a virtually inexhaustible resource of meteorites, Kelvin believed that this way he explained the source of the Sun's radiation for approximately 20 million years. "Who are we," Kelvin asks, "to claim, like Darwin, that something can last 300 million years?"

After the discovery of radioactivity, Darwin tried to fight back by claiming that the Sun owes its energy to radioactive processes taking place in its interior. It was supported by many physicists, such as Ernst Radeford, who was the first to measure the energy of alpha particles. However, when astronomers turned their instruments back to the Sun, they found that there was very little radioactive material there. The sun, like the entire cosmos, is mainly made of light, stable elements: hydrogen and helium. There was no progress until 1905 when Einstein showed that mass and energy are just two different forms of existence of matter. Another key breakthrough was made by FV Aston, who determined through a precise experiment that the mass of four hydrogen atoms is slightly greater than the mass of one helium atom.

These two facts were connected into one whole by the brilliant English astrophysicist Arthur Eddington in 1920. According to him, the source of the Sun's energy could be the process of fusion (joining) of four hydrogen atoms into one helium atom. Given that the mass of matter at the beginning of the fusion process is greater than the mass at the end, the missing mass would manifest, according to Einstein's relation, as the energy of the Sun's rays. According to Eddington, the conversion of only 0,7 percent of the Sun's mass into energy is enough to ensure its undiminished brightness for at least 100 billion years. Eddington was the first to realize the tremendous potential hidden in fusion energy and predicted that, provided he was right, this energy could be used "for the benefit of humanity, but also for its suicide." When Edward Teller constructed the first fusion (hydrogen) bomb, Eddington's warnings only gained weight.

A key breakthrough came when Hans Bethe, a young and talented German scientist, fleeing the Nazis because of his Jewish ancestry, moved to America in 1935. He got a job at Cornell University, which at the time had the ambition to create a team of top nuclear physicists. At first, Bette was not very interested in the question of the source of solar energy, but, intrigued by the partial results that were presented at the Fourth Annual Conference of Physicists in 1938, he immediately set to work, and by the end of the conference he already had a draft of a complete theory that he would later publish in two revolutionary scientific papers. In these papers, Bethe mathematically proved that nuclear fusion in the Sun's core is indeed the source of its energy. He was the first to estimate the conditions in the Sun's core: today we know that the temperature in it is 15,7 million Kelvin, and that the pressure is even 100 billion times higher than that on the Earth's surface.

Bethe's theory was later experimentally confirmed, when solar neutrinos emitted during the fusion process were detected on Earth (their number and nature were first explained by Raymond Davis and Masatoshi Koshiba, who will also receive the Nobel Prize in 2002). As calculations show, every second the Sun converts 600 million tons of hydrogen into helium, with about four million tons "disappearing", i.e. is converted into energy, according to the famous Einstein equation. The sun is not a particularly efficient generator: one cubic meter of the sun barely provides enough energy to run a standard light bulb. But when you add to that the fact that the volume of the Sun is about a million times greater than the volume of the Earth, it is easy to calculate that the Sun is a colossal source of energy. When we take into account the available amount of hydrogen in the Sun, it is certain that it will shine as it has so far for at least another 4-5 billion years. And then, we'll see...