The Source of the Sun’s Energy

M. Colleen Gino

Introduction

The Sun, being the brightest thing in the sky, has commanded the attention of all intelligent beings throughout time. Thus it is natural that every culture has had some ideas, speculations or theories about the origin of the Sun and its source of energy. In this essay the ideas and theories proposed by various ancient civilizations through modern cultures will be introduced and discussed.

Mythological Sun

To ancient cultures the Sun was identified as being a god, literally. The Egyptians saw the Sun as the eye of Ra, the falcon-headed god, creator of light and all things. To the Aztecs, the Sun was the result of Huitzilopochtli, the god of war, jumping into a fire as commanded by the four gods of creation. The Sumerians called the Sun Shamash, god of justice because as the sun he could see all things ⁽¹⁾. Apollo was the Greeks’ Sun god, responsible not only for illuminating the Earth with light, but with illuminating the human mind with understanding ⁽²⁾. It is doubtful that the Greek referral to the Sun as a god was more than a symbolic one, however, as they believed the Sun to be a ball of fire that traveled through the sky in the daytime, then passed through the Earth in underground caves at night ⁽³⁾.

Early Greek Theories

In the 6^th century BC, Anaximander described the Earth as being at the center of the universe surrounded by several moving rings or wheels. The Sun was a was a circle 28 times larger than the Earth, and resembled a wheel with a hollow rim full of fire. Philolaus of Crotona believed there to be an central fire at center of universe which supplied the Sun with light and heat.

One of the first theories based on scientific evidence rather than mythology was put forth by Anaxagoras in the 5^th century BC, who witnessed a wagon-sized meteorite strike the earth. Because of its fiery nature, he assumed that it had broken off from the Sun. Since the meteorite was iron, he deduced that the Sun must be a huge ball of burning hot iron 35 miles in diameter 4000 miles away.

Modern Theories Fall Short

There was no question about the energy source of the Sun until the mid19^th century. Up until that time it was commonly believed that the Sun was composed of some special material that had the ability to shine and would do so eternally ⁽⁴⁾. This idea was in direct opposition to newly developed theory of thermodynamics, however, which stated that things simply "wear out". Scientists were faced with the fact that the Sun could not go on shining forever.

John Waterston calculated that the Sun would only stay hot for about 20,000 if powered by a chemical process, such as combustion. Since by this time geologists had shown that the Earth must be at least millions of years old, Waterston concluded that the Sun must be powered by some other source, and the only other source of energy known at that time was gravity. He suggested that gravity could be transformed into heat energy by scores of meteors falling into the Sun. As the Sun attracted meteors through gravity, the meteors fell toward the Sun; thus gravity was turned into kinetic energy (energy of motion). When the meteors hit the Sun, the kinetic energy was transferred to the Sun as heat energy. Although his theory was sound, Waterston realized that there simply weren’t enough meteors to keep the process going for any reasonable length of time.

In 1854, Hermann Helmholtz asserted that it was the mass of the Sun itself that provided the gravitational energy to heat it up. He suggested that at the time of its formation the Sun was composed of tiny meteor-sized chunks which collapsed inwards, releasing a burst of energy comparable to 20 million years of solar radiation⁽⁵⁾. Once again a workable theory, but this burst of energy did not explain the continuing energy output of the sun.

In 1887 Kelvin improved on Helmholtz’s theory. He suggested that the Sun began as a huge cloud of gas and dust which began shrinking under its own weight. As the cloud collapsed it got continually hotter. Once reaching the size of the present-day Sun, the core temperature would be millions of degrees, with the internal heat creating outward pressure ceasing the gravitational collapse. Under these conditions, the star would take a long time to cool, and once it did, the in-falling matter of the resulting contraction would once again produce energy resulting in heat. Kelvin calculated that the Sun only had to shrink about 50 meters per century to stay hot for 100 million years. This theory, based on what is referred to today as Kelvin-Helmholtz gravitational contraction, held into the early 20^th century, despite the protestations of geologists who had fossil evidence that the Earth was more than a billion years old.

Seeing the Light

It was not until Albert Einstein introduced his theory of general relativity stating that mass could be converted into energy that a reasonable hypothesis could be developed. In 1926 Arthur Stanley Eddington put forth the assertion that the Sun was converting hydrogen into helium in a process he called the "transmutation of elements".

Based on Einstein’s famous equation, e=mc², he demonstrated that if one could somehow force the protons present in the nuclei of four pounds of hydrogen together to form helium nuclei, the resulting mass of helium would weigh only 3.97 pounds. The difference in mass would be converted to energy, exhibiting itself as heat and light ⁽⁶⁾. While many of Eddington’s contemporaries thought his theories were nonsense, he was eventually found to be correct. What he called the transmutation of elements, we call nuclear fusion, which we now know to be the energy source of the Sun.

Conclusion

Clearly, the vast majority of cultures throughout history who applied themselves to the problem of explaining the energy output of the Sun had not advanced to a sufficient technological level to appreciate the nuclear processes required to describe the energy source of the Sun. It has taken mankind more than 3000 years to achieve this level of understanding. Hopefully the next 3000 years will reveal equally remarkable progress.

References:

(2) Greek Mythology, http://www.vacation.net.gr/p/mithos.html

(3) Davis, J., 1992, Journey to the Center of Our Galaxy, Contemporary Books, Chicago

(4), (6) Editors, The Sun, Time-Life Books, Alexandria, Virginia

(5) Gribbin, J., 1991, Blinded by the Light, Harmony Books, New York