Sunshine

By Eric M. Jones

THE ANCIENTS WERE SURROUNDED by mystery just as we are. But their grandest mystery was the burning ball of sunshine that dominated the daytime sky, that controlled their lives, and drove away the cold and darkness. What made it burn?

The Sun was the god of the ancients—but that is largely forgotten today. All the oldest gods were Sun gods. At its zenith came the summer solstice. When the Sun barely peaked over the horizon and the Earth was cold and dark, it marked the end of one year and the start of another. When the Sun mercifully began the ascent back to the top of its sky vault again—usually with the coaxing of the priest and some incantations—new hope grew in the human heart.

But what made it burn? Certainly it was not a simple fire, for what fire could possibly last so long? Was it getting fuel from some invisible place? What was the fuel? Was it coal or whatever caused incandescent liquid rock—like that which poured from volcanoes? Was it a white hot metal ball like the Greek philosopher Anaxagoras proposed? These causes would all have meant that the Earth had been warm for only a relatively short time. Nothing stayed hot forever, or certainly not without prayers ...

Anaxagoras of Clazomenae (5th century B.C.E.), by the way, was a pre-Socratic philosopher of Athens. Among other wild theories, he proposed that the intellect or mind was the cause of the cosmos. He was quantum-mechanically pretty close. But he also incorrectly proposed that the Sun was a mass of incandescent metal, and that the stars are simply fiery stones.

[“Sphaera mundi,” section at right, is a medieval introduction to astronomy. It is based primarily on the works of Ptolemy, and includes concepts from Islamic astronomy. It was one of the most influential pre-Copernian works of astronomy in Europe.]

In 1650, Archbishop James Ussher of Ireland determined that the creation of the Earth and Sun occurred during the evening of October 22, 4004 B.C.E., using what was called the “Begat” method of counting ... And unto Enoch was born Irad: and Irad begat Mehujael: and Mehujael begat Methusael: and Methusael begat Lamech (Genesis 4:18) ... To be fair, Ussher was not a complete fool, and generation counting was relied upon for building calendars based on royal families and dynasties everywhere. Furthermore, many mideastern civilizations came alive around that time, probably due to climate changes. Still, the Vincas, Hattians, and even the Hittites could count their histories further back than Ussher’s estimate. Farming began in China about 7,000 B.C.E., and some pottery dates back to 18,000 B.C.E.. Humans were around from much earlier times.

For most of human history, the Sun was thought to be much smaller than the Earth and, of course, revolved around it—just outside of the Moon’s orbit, past Mercury, and between Venus and Mars. The Earth was surrounded by air and then by a shell of invisible phlogiston which burned up rocks falling from the heavens. Obviously, the rocks that made it through the fire layer and hit the Earth were burned black and melted (and damned hot!) whereas air blowing on them would have made them cool. (Right? That’s science!)

But if the Sun was so small, this made the source of the Sun’s power even harder to understand. But if the Earth was only a few thousand years old ... maybe it was right.

By the time of Newton, the size of the Sun (Not a simple problem as it turns out!) and the geometry and orbits of the celestial bodies had been meticulously calculated. The general theory of the source of the Sun’s power seemed well worked out too: it was simply the heat of compression of the rocky and probably meteoric (called aerolithic) material that made up the Sun. If you squeeze something, it gets hot. (Early people often carried fire starters consisting of a small piston that when compressed would set fire to tinder. Very slick!

The great German physicist-physician-professor-psychologist-philosopher and discoverer of the Conservation of Energy, and who built the mechanical foundation of thermodynamics, Hermann Ludwig Ferdinand von Helmholtz (whose students, by the way, included Max Planck, Albert A. Michelson, William James, and Heinrich Hertz), worked out the mathematics for the “Meteoric Theory” of gravitational contraction ... in his spare time no doubt.

The following excerpt is from the “Popular Science,” May 1887, article entitled “The Sun’s Heat,” by Sir William Thomson, OM, GCVO, PC, PRS, PRSE, FRS, also known as the Uber-Brainiac Lord Kelvin. Remember the Second Law of Thermodynamics?

(There simply was no greater scientific authority with the exception of a James Clerk Maxwell ... another Scotsman of course. Maxwell was considered by Einstein to be the smartest man since Newton, and by Feynman as the smartest man in the last 10,000 years. The best British brains are Scottish. But I digress.)

“I suppose I may assume that every person present knows as an established result of scientific inquiry that the sun is not a burning fire, and is merely a fluid mass cooling, with some little accession of fresh energy by meteors occasionally falling in, of very small account in comparison with the whole energy of heat which it gives out from year to year. You are also perfectly familiar with Helmholtz’s form of the meteoric theory, and accept it as having the highest degree of scientific probability that can be assigned to any assumption regarding actions of prehistoric times. You understand, then, that the essential principle of the explanation is this: at some period of time, long past, the sun’s initial heat was generated by the collision of pieces of matter gravitationally attracted together from distant space to build up its present mass; and shrinkage due to cooling gives, through the work done by the mutual gravitation of all parts of the shrinking mass, the vast thermal capacity in virtue of which the cooling has been, and continues to be, so slow. I assume that you have not been misled by any of your teachers who may have told you, or by any of your books in which you may have read, that the sun is becoming hotter because a gaseous mass, shrinking because it is becoming colder, becomes hotter because it shrinks.”

(A cautionary tale: This understanding of what powered the Sun was viewed as the settled scientific orthodoxy of the day. Probably ninety-five percent of scientists believed this theory, or some slight variation of the theory. The other five percent were clearly wackos, deniers or paid shills.)

Thompson (aka Lord Kelvin) goes on to calculate various aspects of the Sun’s power and its diameter and history, which is not difficult if the reader chooses to look it up. Thompson ends his talk with the conclusion that:

“... it would ... be exceedingly rash to assume as probable anything more than twenty million years of the Sun’s light in the past history of the Earth, or to reckon on more than five or six million years of sunlight for time to come.

So there’s the rub! The Earth’s history had to be short for the gravitational compression theory to work. Thompson’s calculations are not wrong. I repeat, not wrong! Well, OK, they are stupidly wrong ... but given what was known at the time, they were as right as rain. In fact: The Kelvin–Helmholtz Contraction Mechanism is still seen as the primary reason Jupiter emits more heat than it receives from the Sun, maybe Saturn too, and brown dwarf stars all over the cosmos exist as very hot but non-nuclear-powered bodies. The Kelvin–Helmholtz Contraction Mechanism is also what makes clouds of matter get to the temperature where they sometimes get hot enough to ignite and become stars.

There were various other earlier calculations of the age of the Sun, but they were usually based on similar conceptions; that the Sun was not very old, (nor perhaps was the Earth), and the Sun’s heat output depended on the gravitational compression of its mass—or as some put it—the conversion of gravitational energy into electromagnetic energy. But Kelvin and Helmholtz did the math and made it “Science.”

Aging the Earth

For those following along at home, you will notice I have assumed the age of the Sun and the age of the Earth to be basically interchangeable. It is hard to imagine otherwise. It is easy to assume one creation made them both together at about the same time. Whether the Sun was at the center of the cosmos, or the Earth, this simultaneous formation seemed transparently obvious. So the real problem was that the Sun’s lifetime had to be short for explanations of what powered it to be realistic, and the Earth had to be a much longer age for evolution, natural selection, erosion and other phenomena to have had enough time to operate.

This was a puzzle. Darwin had looked at the problem earlier and estimated the age of the Earth had to be at least 300 million years, based primarily on studies of geological erosion. But when Darwin published his theory of evolution, “On the Origin of Species,” in 1859, there was nary a mention of the estimated age of the Earth or Sun. Darwin perhaps felt he had enough controversy to deal with as it was.

Many calculations of the Earth’s age were made using the sedimentation rate of material in the rivers, as well as the increase in salt concentration of the seas. These estimates all depended on how much sediment or salt was returned to the land or lay hidden in the sea. Nobody knew. John Joly in 1899, using salinity calculations, estimated that the Earth had consolidated and that the oceans had been created between 80 and 90 million years ago.

Layers of sedimentation have been studied worldwide for centuries in an attempt to determine the age of the Earth. But in this, as in other methods of dating, there was no absolute-age framework for the time scale. The layering could be used to estimate time scales of some centuries, but the variabilities introduced by ice ages, droughts, climate changes, the cycles of the Earth’s orbital eccentricity, obliquity, tilt, and precession (now known as Milankovitch Cycles), variations in solar output, continental drift, magnetic pole drift, magnetic reversals, igneous processes and a hundred other factors made these counts all severely problematic.

[Layers of the Sun, left, according to a “Popular Mechanics” article of December, 1927.]

Assuming somewhat steady annual growth patterns, “dendrochronology” could be used to count back the number of summers a tree had been growing. This was reasonably good data but only for counting a few thousand years. (Tree rings in Californian bristlecone pines have been used to count back as far as 8,200 years. California redwoods have very long histories, but the climate they grow in is too stable to provide definite year-markers).

The problem was soon recognized that methods of calculating longer time periods all had tremendous uncertainty. So the search was on for some method of figuring out how long the Earth and Sun had been around without recourse to tree rings, layering or erosion.

Lord Kelvin himself did remarkable work on the age of the Earth using thermodynamics (much of which he, of course, invented) to calculate how quickly a liquid ball of rock like the Earth could cool off to grow a solid crust. Nobody knew how thick the Earth’s crust was, but the thicker it got the slower the process became, so an estimate could nevertheless be made. From his initial age estimate (in 1862) of 100 million years, he later jiggered the numbers downward, probably in accordance with Helmholtz’s estimated age of the Sun. Lord Kelvin’s calculation was based on the assumption that the Earth was inert and thus wasn’t producing new heat. He was again right as rain based on what was known at the time.

Enter Radioactivity

But as the 19th century came to a close, the French physicist Henri Becquerel discovered radioactivity while experimenting with the effect of x-rays on uranium salts. His colleagues, Marie and Pierre Curie, rapidly pushed the science forward by discovering radium and polonium in uranium ores such as pitchblende (uraninite). Their lab notebooks are still highly radioactive.

By the way, polonium-210 has very few uses and is perhaps is best known for its lethality when used for assassinations. Many early radiochemists and physicists died from their experiments.

Soon it was discovered that radioactive materials generated heat in roughly inverse proportion to their half-life. It also became apparent that naturally occurring radioactive elements in minerals common in the Earth’s crust account for all observed heat flow. The Earth was, after its initial fiery formation, not cooling.

Still, the second half of the 19th century was a hopeful time for science. Man could see the future. Hydrogen airships cruised the skies. Swift steam trains spanned the continents, Jules Verne took us to the Moon and 20,000 leagues under the sea. Telegraphs sent messages at nearly the speed of light. Giant telescopes spied distant stars in our cozy island universe.

Telescopes clearly showed the Sun was a fluid ball, bringing up hotter material from its core leaving convective holes called sunspots and showing prominences in its corona, seen during total eclipses. All was well. Lord Kelvin, the physics brainbox know-it-all, had laid down the law.

The second half of the 1800’s was the “Second Industrial Revolution.” Electricity was not only in the air, but wired to home lighting, too. Edison was showing his magic. As was Tesla. The automobile was bumping along on urban streets. A giant bridge whose towers were the tallest structures in America connected Brooklyn with Manhattan.

But science in other fields was beginning to cast doubts on the energy that powered the Sun. It was becoming increasingly clear that the Earth was far older than 20 million years. Darwin’s Theory of Evolution was becoming widely accepted. The discovery of magnificent dinosaur fossils made a much longer lifetime for Earth seem more and more likely. But the age of various fossils was fundamentally unknowable until the work on dating of minerals using their radioactive decay by Bertram Borden Boltwood in 1907. Ernest Rutherford, who contributed greatly to the understanding of the atom and nuclear processes, should be given some credit for the initial idea. But it was radiochemist Boltwood who actually did it, and should be celebrated for it. But he committed suicide at age 57, while Rutherford outlived him.

Radioactive dating was subsequently employed in its many variations to show that the Earth was billions of years old. 4.54 billion years is the present estimate, based mostly on the ages of meteorites. The gradual accumulation of evidence and the radioactive dating of fossils and prebiological layers led to the acceptance of the theory of a very old Earth.

In the first few decades of the 20th century, atomic physics defined the atom and named its parts. It also began the complicated work of deducing how the decay of heavy radioactive atoms into other lighter atoms subsequently gave off enormous amounts of energy. Of course, this all leads to the wrong conclusion: that the Sun’s heat is a result of radioactive decay, where perhaps a core of uranium—or something similar—radioactively disintegrates into lighter and lighter elements, giving off energy in the process, every step of the way. Early notions of this involved the Sun burning off the final hydrogen flames at its surface. Just outside of the flaming calcium. After all, even the spectroscopes said the Sun’s outer surface was hydrogen.

Solar physicists and stellar spectroscopists studied the Sun, expecting to find some hints of a giant core of uranium that was decaying into lighter elements step-by-step. But they found only the tiniest trace of uranium—less than one part in a million. They did find that the Sun was ninety-eight percent hydrogen and helium, and contained minute traces of most of the elements up to iron, but almost nothing heavier. Even if heavier elements were buried in its core, some trace should be seen.

It was soon suspected that radioactive “transmutation” played some important role in the Sun’s energy output, but what was happening was unclear.

After the close of WWI, the great physicist Arthur Eddington looked at the available information and stewed over it—The Helmholz-Kelvin contraction hypothesis (many said “Law”) was crumbling as new evidence from astronomy, paleontology, and physics wasn’t even paying attention to the Gravitational Contraction notions. Albert Einstein had written his famous paper on E=mc2. (Eddington was a great promoter of Einstein’s work ... even when German physicists weren’t particularly popular). Rutherford was fusing atoms in the Cavendish Labs. It became clearer every day that even if the Gravitational Contraction idea had some merit, the core of the Sun was millions of degrees hot and bizarre nuclear chemistry was not only possible, but inevitable. So Eddington proposed that the Atomic Disintegration process in these environments was working backwards; that hydrogen atoms were being fused into helium in the core of the Sun, and this was the basic source of the Sun’s heat output after all. Fusion not fission powered the Sun.

It was not all quite so simple. It would be another eighteen years before the details were worked out by physicist Hans Bethe. See “Energy Production in Stars;” if you read only one paper on nuclear physics in your entire life, this should be the one. Our Sun fuses hydrogen to helium and continues to make heavier elements until, in stars similar to ours, a growing core of iron stops fusion processes. Spectroscopic studies of other stars gave support to theories of other stellar fusion recipes, but they all stop at iron as the last stable element. Heavier elements come from catastrophic explosions of big stars in so-called nova, supernova and hypernova sizes. Our Sun and its solar system was formed from the leftover detritus of earlier stars. Some of which might have had planets and civilizations of their own, with creatures looking up into the sky wondering what made their Sun burn. 

Further Reading

“The Sun’s Heat,” May 1887, “Popular Science.”
“Fusion of Stars,” by Arthur Eddington.
“Energy Production in Stars,” by Hans Bethe.
Kelvin-Helmholtz Mechanism.
Early Attempts at Calculating the Age of the Earth.

Eric M. Jones is the Associate Editor of “Perihelion.” He is an engineer, designer, consultant, and entrepreneur, working in the experimental aircraft community, NASA, space transportation companies, and the International Space Station.