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Somewhat earlier, in the s, J. Mayer another German physician and J. Waterson had also suggested that the origin of solar radiation is the conversion of gravitational energy into heat. Biologists and geologists considered the effects of solar radiation, while physicists concentrated on the origin of the radiated energy.
In , Charles Darwin, in the first edition of On The Origin of the Species by Natural Selection , made a crude calculation of the age of the earth by estimating how long it would take erosion occurring at the current observed rate to wash away the Weald, a great valley that stretches between the North and South Downs across the south of England. Firmly opposed to Darwinian natural selection, William Thompson, later Lord Kelvin, was a professor at the University of Glasgow and one of the great physicists of the nineteenth century. In addition to his many contributions to applied science and to engineering, Thompson formulated the second law of thermodynamics and set up the absolute temperature scale, which was subsequently named the Kelvin scale in his honor.
The second law of thermodynamics states that heat naturally flows from a hotter to a colder body, not the opposite. Thompson therefore realized that the sun and the earth must get colder unless there is an external energy source and that eventually the earth will become too cold to support life.
Kelvin was forced by astronomical evidence to modify his hypothesis and he then argued that the primary source of the energy available to the sun was the gravitational energy of the primordial meteors from which it was formed. That some form of the meteoric theory is certainly the true and complete explanation of solar heat can scarcely be doubted, when the following reasons are considered: 1 No other natural explanation, except by chemical action, can be conceived.
Believing Darwin was wrong in his estimate of the age of the earth, Kelvin also believed that Darwin was wrong about the time available for natural selection to operate. Lord Kelvin estimated the lifetime of the sun, and by implication the earth, as follows.
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This calculation yielded a lifetime of only 30 million years. The corresponding estimate for the lifetime sustainable by chemical energy was much smaller because chemical processes release very little energy. As we have just seen, in the nineteenth century you could get very different estimates for the age of the sun, depending upon whom you asked. Prominent theoretical physicists argued, based upon the sources of energy that were known at that time, that the sun was at most a few tens of million years old. Many geologists and biologists concluded that the sun must have been shining for at least several hundreds of millions of years in order to account for geological changes and the evolution of living things, both of which depend critically upon energy from the sun.
Thus the age of the sun, and the origin of solar energy, were important questions not only for physics and astronomy, but also for geology and biology. He wrote in to Alfred Russel Wallace, the codiscoverer of natural selection, complaining about Lord Kelvin:. Today we know that Lord Kelvin was wrong and the geologists and evolutionary biologists were right. Radioactive dating of meteorites shows that the sun is 4. An analogy may help. Suppose a friend observed you using your computer and tried to figure out how long the computer had been operating.
A plausible estimate might be no more than a few hours, since that is the maximum length of time over which a battery could supply the required amount of power. The flaw in this analysis is the assumption that your computer is necessarily powered by a battery. The estimate of a few hours could be wrong if you computer were operated from an electrical power outlet in the wall. Since nineteenth century theoretical physicists did not know about the possibility of transforming nuclear mass into energy, they calculated a maximum age for the sun that was too short.
Nevertheless, Kelvin and his colleagues made a lasting contribution to the sciences of astronomy, geology, and biology by insisting on the principle that valid inferences in all fields of research must be consistent with the fundamental laws of physics.
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We will now discuss some of the landmark developments in the understanding of how nuclear mass is used as the fuel for stars. The turning point in the battle between theoretical physicists and empirical geologists and biologists occurred in On developing the photographic plates, he found to his surprise strong images of his uranium crystals.
He had discovered natural radioactivity, due to nuclear transformations of uranium.
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The most extraordinary aspect of this new discovery was that radium radiated heat without cooling down to the temperature of its surroundings. The radiation from radium revealed a previously unknown source of energy. The young prince of experimental physics, Ernest Rutherford , then a professor of physics at McGill University in Montreal, discovered the enormous energy released by alpha particle radiation from radioactive substances. In , he announced:.
The discovery of the radio-active elements, which in their disintegration liberate enormous amounts of energy, thus increases the possible limit of the duration of life on this planet, and allows the time claimed by the geologist and biologist for the process of evolution. The discovery of radioactivity opened up the possibility that nuclear energy might be the origin of solar radiation. This development freed theorists from relying in their calculations on gravitational energy.
However, subsequent astronomical observations showed that the sun does not contain a lot of radioactive materials, but instead is mostly hydrogen in gaseous form. Something other than radioactivity is required to release nuclear energy within a star. In the next sections, we shall trace the steps that led to what we now believe is the correct understanding of how stars shine.
The next fundamental advance came once again from an unexpected direction. His relation generalized and extended the nineteenth century law of conservation of energy of von Helmholtz and Mayer to include the conversion of mass into energy. The answer was not obvious. Astronomers did their part by defining the constraints that observations of stars imposed on possible explanations of stellar energy generation.
In , Henry Norris Russell, the leading theoretical astronomer in the United States, summarized in a concise form the astronomical hints on the nature of the stellar energy source. Russell stressed that the most important clue was the high temperature in the interiors of stars.
Aston discovered in the key experimental element in the puzzle. He made precise measurements of the masses of many different atoms, among them hydrogen and helium. Aston found that four hydrogen nuclei were heavier than a helium nucleus. This was not the principal goal of the experiments he performed, which were motivated in large part by looking for isotopes of neon. In principle, this could allow the sun to shine for about a billion years. In a frighteningly prescient insight, Eddington went on to remark about the connection between stellar energy generation and the future of humanity:.
If, indeed, the sub-atomic energy in the stars is being freely used to maintain their great furnaces, it seems to bring a little nearer to fulfillment our dream of controlling this latent power for the well-being of the human race—or for its suicide. The next major step in understanding how stars produce energy from nuclear burning, resulted from applying quantum mechanics to the explanation of nuclear radioactivity. This application was made without any reference to what happens in stars. Classically, the probability that two positively charged particles get very close together is zero.
But, some things that cannot happen in classical physics can occur in the real world which is described on a microscopic scale by quantum mechanics. In , George Gamow, the great Russian-American theoretical physicist, derived a quantum-mechanical formula that gave a non-zero probability of two charged particles overcoming their mutual electrostatic repulsion and coming very close together.
In , C. He discovered a nuclear cycle, now known as the carbon-nitrogen-oxygen CNO cycle, in which hydrogen nuclei could be burned using carbon as a catalyst.
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For stars heavier than the sun, theoretical models show that the CNO carbon-nitrogen-oxygen cycle of nuclear fusion is the dominant source of energy generation. Ordinary carbon, 12 C, serves as a catalyst in this set of reactions and is regenerated. Only relatively low energy neutrinos are produced in this cycle. The figure is adapted from J. By April , it was almost as if the scientific stage had been intentionally set for the entry of Hans Bethe , the acknowledged master of nuclear physics.
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Professor Bethe had just completed a classic set of three papers in which he reviewed and analyzed all that was then known about nuclear physics. In the course of the next six months or so, Bethe worked out the basic nuclear processes by which hydrogen is burned fused into helium in stellar interiors. Hydrogen is the most abundant constituent of the sun and similar stars, and indeed the most abundant element in the universe.
He authoritatively analyzed the different possibilities for reactions that burn nuclei and selected as most important the two processes that we now believe are responsible for sunshine. One process, the so-called p-p chain, builds helium out of hydrogen and is the dominant energy source in stars like the sun and less massive stars. Distinguished physicists and astrophysicists, especially A. Cameron, W. Fowler , F.