A History of the Source
of the Sun’s Energy
by Dave Lengyel
Every culture throughout the world has realized how important the Sun is
to life on Earth. It was worshipped by some as a god and certainly appreciated
by all as a necessary source of heat and light. Without its warmth, cold
and darkness would prevail. But what is the source of this life-giving
heat and light?
Humankind’s explanations as to this source of energy have changed considerably
over the past 2500 years. The models that developed, like all good scientific
models, fit the facts as they were known then. The earliest models to explain
the Sun’s energy might seem a bit silly today, but just as Galileo’s earliest
telescope might seem to be worthless by today’s standards, it was indeed
“state of the art” at the beginning of the 17th century. Similarly, early
explanations of how our star produces huge amounts of energy must be looked
at with respect to what knowledge was available at the time.
Among the earliest people to examine the question of the source of the
Sun’s energy were the ancient Greeks. Anaxagoras, in the 5th century BC
described the Sun as a large mass of hot metal which crossed our sky on a
daily basis. Around 350 BC, Aristotle, in keeping with his ideas of the
heavens being perfect, taught that the Sun was perfect and made of pure light.
This idea of perfection would be challenged as new observations with telescopes
were made about 2000 years later.
William Herschel, in 1794, proposed that the Sun was a dark, solid
body surrounded by glowing clouds and populated by beings apparently able
to live under those rather bizarre conditions. Gaps in the clouds (sunspots)
revealed the dark Sun below.
In 1848, J.R. Mayer examined the then-popular theory of the Sun as being
composed of burning coal. He stated that if the Sun began burning 5000 years
ago, corresponding to the Biblical age of the Earth, then by his calculations,
it already would have burned out.
Mayer’s idea was that the Sun produced and maintained its heat by meteoric
impact. These high-speed impacts, due to the Sun’s large gravitational
field, would produce much more energy, as kinetic energy, than would the
conversion of chemical energy due to burning. It turns out that this process
would still not produce enough energy to maintain the Sun’s heat, unless
the Earth itself were red hot due to a proportionate number of meteoric impacts.
The idea that gravitational contraction could be responsible for the Sun’s
energy was first proposed by Hermann von Helmholtz in 1854. William Thompson,
Lord Kelvin, at first subscribed to Mayer’s meteoric theory, but later realized
that gravitational contraction was a better explanation. According to this
theory, the Sun released energy from its hotter interior regions and this
energy leaked out by radiative diffusion and convection. The interior would
thus lose energy and contract, but this contraction would cause the interior
to heat back up. So, odd as it may seem, as the Sun would lose heat by radiation,
it would actually get hotter. Further, the amount of actual contraction would
be very hard to observe directly from Earth. The Sun would need to shrink
by only about 27 meters per year in order to supply its output of energy,
and this relatively small amount of shrinkage would have been undetectable
a century ago. This process could allow the Sun to produce energy for as
much as 45 million years.
One thing that was becoming obvious in the mid to late 1800s was that the
age of the Earth was important in determining how the Sun produced its energy.
It might be fine for the Sun to be a huge burning ball of coal if the Earth
were only a few thousand years old, but 19th century scientists were beginning
to find out that the Earth was much older. Charles Darwin, in 1859, had estimated,
by looking at rates of erosion, that the Earth was at least 300 million years
old. The Sun, which is the ultimate source of erosional energy, must be at
least that old. Darwin’s dating of the age of the Earth was strongly disputed
by Lord Kelvin. Radioactive dating was not yet available, so the dispute
on the age of the Earth, and thus the Sun, continued.
By the end of the 19th century, research progressing rapidly in areas of
nuclear physics and chemistry had shown that certain atoms undergo spontaneous
changes to their nuclei which release energy. Could this new source of energy
be used to explain the Sun’s heat?
Some hypotheses which incorporated these new findings began to emerge.
The radioactive decay of large nuclei into smaller nuclei might be producing
energy in the Sun. Or perhaps the annihilation of protons by electrons
could be at work, or the process of building larger nuclei from smaller
one might be the answer. It soon became clear that only the last of these
hypotheses were probable. Observational evidence indicated that the Sun
did not contain very much high atomic mass radioactive material. The collisions
of protons and electrons would make the Sun very unstable. More and more,
the answer seemed to lie in the combination of small nuclei to form larger
ones. In other words, nuclear fusion.
A key breakthrough in understanding how nuclear fusion processes might
be at work inside the Sun came in the 1920 experiments by F.W. Aston, who
showed that the mass of four hydrogen nuclei combined are less than the mass
of a helium nuclei formed by the combination of the four hydrogens. What
becomes of the lost mass?
Einstein’s famous equation, E=mc2, indicates that even a tiny
bit of mass can be converted into an enormous amount of energy, since c
represents the speed of light, 3 X 108 meters per second, and
this amount is then squared. This led Sir Arthur Eddington to propose in
1920 that this difference in mass in the fusion of hydrogen into helium,
could account for the Sun’s energy output, when changed into energy. If
the Sun was indeed fusing hydrogen into helium, the mass difference between
the four hydrogen nuclei and the helium nucleus was about 0.7% of the mass
of the helium nucleus. The conversion of this relatively small amount of
mass into energy would theoretically allow the Sun to shine for billions
of years.
In the 1930s some sequences of nuclear reactions which may occur inside
the Sun, and inside other stars, were provided by Hans Bethe. Stars like
the Sun were proposed to follow the proton-proton chain, in which six hydrogen
nuclei undergo nuclear change to produce one helium nucleus and two hydrogen
nuclei, for an overall net change of four hydrogens into one helium. This
process takes place in a series of steps. First, two ordinary hydrogen (hydrogen-1)
nuclei, which are actually just single protons, fuse to form an isotope
of hydrogen called deuterium (hydrogen-2), which contains one proton and
one neutron. A positron (a positively charged electron, a form of antimatter)
and a neutrino ( an neutral particle which travels nearly at the speed of
light and has, perhaps, almost no mass) are also produced. The positron
is very quickly annihilated in the collision with an electron and the neutrino
travels right out of the Sun. The newly-formed deuterium fuses with another
regular hydrogen to form an isotope of helium, helium-3 containing two protons
and one neutron. Next, two of the helium-3 nuclei fuse to form a helium-4
nucleus and two hydrogen-1 nuclei. Energy as gamma rays are produced in
each step. Another series of reactions may occur in stars hotter than the
Sun. This is the carbon-nitrogen cycle, in which a carbon nucleus is involved
in the first step. Here again, the overall result is to combine four hydrogen
nuclei to form one helium nucleus. Other cycles, involving nitrogen and
oxygen, have been proposed for explaining the fusion process in even hotter
stars.
If fusion does indeed occur in stars, then the neutrinos produced in the
process should be detectable. Several recent experiments have detected neutrinos
indicating that fusion is the source of the Sun’s energy.
As humans have used better instruments to study the Sun and other stars,
and as more information has come into play from geologists and biologists,
the theories for how the Sun produces its vast amounts of energy have evolved.
Our current nuclear model of the Sun seems to be in agreement with what
we currently know about the Sun, but more questions will arise as our knowledge
increases in the future.
References
Bowen, Eliza A., 1890, Astronomy by Observation, Appleton and Co.,
New York
Abetti, Georgio, 1938, The Sun, D. Van Nostrand Co. Inc., New York
Doig, Peter, 1950, A Concise History of Astronomy, Chapman and Hall
Ltd., London
Giovanelli, Ronald G., 1984, Secrets of the Sun, Cambridge University
Press, Cambridge
Noyes, Robert W., 1982, The Sun, Our Star, Harvard University Press,
Cambridge
Bahcall, John N., 2000, How the Sun Shines, Nobel e-Museum web site,
http://www.nobel.se/physics/articles/fusion/index.html