Download Lec 25.1- WHY DOES THE SUN SHINE

WHY DOES THE SUN SHINE?
The story begins by asking what at first seems to be a simple and rather harmless question: "Why does
the sun shine?" and the sun has been shining for a very long time indeed, perhaps for billions of years, as shown
by the fossil record on the earth. The question then becomes one of explaining how it is possible for the sun to
keep shining for that length of time. About 100 years ago when the question was first posed, the major source
of fuel was coal, and of course it was thought perhaps that the sun might be made out of coal. Well, if this
were the case, one could calculate for an object the size of the sun how much energy could be expected,
assuming that it was made out of coal. Conclusion: the sun gives us far more energy than is possible if it
were made from coal. The sun is not made out of coal.
Well then, what is the sun made out of, and how is it making its energy? This is the problem which
perplexed astronomers about a hundred years ago, and the one which they were unable to solve at that time.
The answer eventually was to come not from astronomers, but from physicists, particularly the physicists who
would eventually be called the "nuclear physicists". Their story essentially begins in the year 1895 when the
German physicist Wilhelm Roentgen discovered X-rays. This discovery was followed a year later, in 1896, by
the discovery of Henri Bequerel that there were certain rocks, certain minerals which spontaneously gave off
X-rays, and this spontaneous emission of X-rays was called radioactivity. There seemed to be something
distinctly different about the rocks that were radioactive as opposed to rocks that did not give off X-rays. It
became the goal of the researchers at that time to find out what the difference was. (That difference turned
out to be a difference in the structure of the atoms making up these rocks, although that was not known at the
time.) It was found by Bequerel and others that many different types of rocks gave off these X-rays; many
different kinds of rocks were therefore radioactive. And it became the work of Pierre and Marie Curie to
isolate one of the strongest sources of radioactive X-rays, which they were able to do in the year 1901, a
substance which they henceforth called radium. (It is ironic that later on Marie Curie died of Leukemia, a blood
cancer produced by the X-rays released by the radium which was her life's work.) The question still remained,
how was radium and other radioactive rocks different from those which were not?
The next piece to the puzzle came in 1905 when Einstein published his first paper on the Theory of
Relativity. As part of this theory, Einstein predicted that if you could take a certain amount of mass and
completely destroy it, a certain amount of energy would show up in its place, and although it was not known
how mass might be completely destroyed to produce pure energy, according to Einstein's theory, this was
theoretically possible.
By 1915, the Danish scientist Niels Bohr gave us a new model of the atom. In this model we see the
atom as a small nucleus made up of positive charges which also contain some particles without any electrical
charges at all. The positive charges are called protons, and neutral charges are called neutrons. Going around
this very dense, compact nucleus are particles of negative energy called electrons. Investigations showed that
the difference between radio-active and non radio-active atoms was simply a difference in the structure of their
nuclei. When these unstable atoms with their unstable nuclei broke down, what resulted was two smaller
atoms, and some neutrons, which evidently were shot out of the nucleus. The peculiar thing was this, if one
took all the particles which resulted after an atom broke down, and added up the weights of all these particles,
they did not equal the original weight of the original atom. What happened to the missing weight? This is
what Einstein was talking about when he said that if you could take some mass and destroy it, energy would
result. What results is energy which sometimes can be seen in the form of light, and that would explain why
radioactive rocks would glow in the dark. Energy was being emitted as mass was being lost, as the atoms were
spontaneously breaking down and disintegrated.
Unfortunately, however, the rate at which atoms break down in a normal sample of radio-active rock is
very, very small. The amount of energy released therefore is also very small, and it becomes impractical to try
to use this to do some work. The more physicists began to look at this, the more they realized that it might be
possible to take a bunch of radio-active atoms together and utilize the neutrons emitted from some radio-active
atoms when they disintegrate to attack or split and other nearby radio-active atoms, causing them all to break
down in what would be called a "chain reaction".
There also seemed to be another way energy might be gotten out of the atoms. If light atoms, atoms
like hydrogen, are taken and put together under tremendous pressure and temperature these atoms might fuse
into each other to form helium. In doing so, tremendous amounts of energy would be released. Now the sun
is made up of great quantities of hydrogen gas and hydrogen atoms. Could it be that this is what is going on in
the sun; that this is what is taking place to cause the sun to shine? In the late 18\930's the German scientist
Hans Bethe worked out the essential equations which show how the sun was taking four hydrogen atoms and
converting them to one helium atom, and doing this process millions of times over each second, releasing
tremendous amounts of energy thereby. This then, was what caused the sun to shine, and to keep on shining
for billions upon billions of years.
By the end of the '30's then, the astronomers had their answer as to why the sun shined. But there was
something else happening developing in the 1920's and 1930's and this was not in the field of science. It was in
the field of International Politics. These developments which could already be seen toward the end of the
1930's were to eventually erupt into World War II. Many of the scientists involved in the research of the
structure of the atom and the question of the energy of the sun, were living in Europe at the time, and left
Europe, eventually forming the Manhattan Project, a project which the world knew about in 1945 with the
dropping of the two atomic bombs on Japan. The simple question asked about 100 years before, "Why does
the sun shine?" resulted in the instantaneous death of almost 200,000 Japanese, and World War II came to an
end.