Astronomy Through the Ages - 0748407480 by Unknown

Author:Unknown
Language: eng
Format: epub
Published: 2016-12-03T05:00:00+00:00

CHAPTER EIGHT

Astronomy in the Early Twentieth Century

The riches of heaven’s pavement, trodden gold

Than aught divine or holy else enjoyed

In vision beautific

Paradise Lost

The developments in nuclear physics in the mid-1930s, outlined in Chapter 7, provided a clue to one of the greatest puzzles in astronomy, one that had baffled astronomers and physicists for well over a century—the source of the Sun’s energy. If the Sun was made of coal, it would burn itself out in about a thousand years, whereas historical records showed that it had hardly changed in ten thousand. This led Hermann von Helmoltz and William Thomson, the Professor of Natural Philosophy in Glasgow University, to propose in the nineteenth century that the source of the Sun’s energy was gravitational in nature. Thomson was later to be made a peer and this is only mentioned here because he became better known by the name he then adopted as Lord Kelvin. Among other things, he established the absolute scale of temperature which bears his name (degrees Kelvin) and which is based on the centigrade scale but with a zero at the absolute zero of temperature (when atoms or molecules have no motion) rather than the freezing point of water which on the Kelvin scale lies at 273 degrees. 1 Kelvin’s theory was that the Sun was still under gravitational contraction, thereby supplying the energy to keep it hot. His estimate of the Sun’s lifetime was several hundred million years, far in excess of historical records, but geological sciences were developing rapidly at that time and, from fossil records, deduced an age of the Earth, and therefore the Sun, of thousands of million years. The geologists were right and it was only when the scale of nuclear energy was revealed in the 1930s that a likely explanation emerged. The largest single step in releasing nuclear energy was afforded by the fusion of hydrogen into helium, when 1 per cent of the matter would be released as pure energy, an immense amount. The Sun was known to be composed mainly of hydrogen (like the giant planets), it was very hot at its surface (about 6000°) and it was known that this meant that it was extremely hot in its interior. What if it was so hot that some of the nuclei of hydrogen (protons) had such large velocities that they could overcome the electrical repulsive forces and fuse together to ultimately build helium and thereby release the enormous energy of the nuclear force? Since the nucleus of helium consists of two protons and two neutrons, whereas four hydrogen nuclei give four protons, this required some transformation of two of those protons into neutrons, perhaps by the absorption of an electron. The actual processes are somewhat complex and had to wait some time for their elaboration, but the amount of energy that would be released by any such fusion process was large enough to be convincing; it would allow the Sun to shine for many thousands of million years.

This possible explanation of the

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