Why the Stars Shine

Original article appears in GuideStar October, 2012.
By Don Selle

“Science is the one human activity that is truly progressive. “ Edwin Powell Hubble

“I can find in my undergraduate classes, bright students who do not know that the stars rise and set at night, or even that the Sun is a star.” -- Carl Sagan

Humans are creatures of habit, it’s built into us. It’s an evolutionary adaptation that enhances our chances of survival in the natural world. We like things to be normal and routine, and for most of us, it takes a lot to change our patterns of behavior and of thought.

But nothing progresses if it is constant or follows the same patterns over and over again. Because our thought processes follow common, established patterns, we often need a challenge to jolt us into finding a “new normal”, a new way of looking at things.

Thomas KuhnT. S. Kuhn, was a physicist, historian and philosopher of science who has been an influential voice in describing the process of the development of science. His book “The Structure of Scientific Revolutions” published in 1962 introduced the concept of a “paradigm shift”, or a shift from one set of standard explanations or theories or body of shared knowledge to another1.

This very influential view of science holds that in an accepted paradigm, the work of science is to solve the puzzles that are presented so that the body of knowledge is increased. In the course of this “normal science” anomalies will turn up which will eventually create a crisis, which will lead ultimately to the stress necessary to produce a new paradigm2. In other words, it is the intellectual stress caused by facts or predictions that we cannot fit into an existing pattern, that cause us to shift our view and adopt a new way of looking at things. The history of our understanding of the stars, why they shine and how they evolve, when viewed in this light provides a good example of Kuhn’s view of how science makes progress.

As human civilization developed, the ability to recognize star patterns would evolve to the lore of the constellations, and recognition that some stars, (the planets) did not stay fixed at all but wandered through the heavens. The human need to exert personal control is probably what lead to the establishment of astrology in ancient Babylon, and which eventually also lead to the astronomical philosophies of the ancient Greeks .

In the astronomy of Aristotle ( the culmination of the work of several philosophers who preceded him) the universe was constructed of a series of nested rotating spheres upon which the Moon. Sun. planets and stars were fixed. These spheres and their rotation were developed as an explanation of how the stars and planets moved in the night sky. It was Aristotle who first proposed that the stars are attached to the inside of the outermost (and therefore fastest moving) of the spheres, and that their glow is due to the friction caused as they flowed through the ether (or perfect fifth element) which filled the void between the spheres.

About 500 years after Aristotle’s death, Claudius Ptolemy added to Aristotle’s astronomy but he did not change it. He could better calculate the positions of the planets, Sun and Moon, but the paradigm of the crystalline spheres and the world view they embodied remained the same throughout middle ages.

The “Copernican Revolution” of the Renaissance is the epitome of a “Khunian” paradigm shift. Nicholas Copernicus challenged the old view of astronomy which held that Earth was the center of all things and replaced it with a sun-centered universe. Copernicus realized that the Ptolemy’s system could result in predictions of planetary positions that were quite inaccurate and set out to find a way to improve these predictions. His answer would create a new way to calculate the movements of the planets. It would not be much more accurate than Ptolemy’s, since like Ptolemy, Copernicus used circular orbits, but it was much easier to use.

Apparent motion of the Sun and planets with Earth at the center. Wikimedia CommonsIt would take the combined work of Tycho Brahe, Johannes Keppler and Isaac Newton to finish the job that Copernicus started. The extensive and accurate observations of planetary positions made by Tycho fueled the mathematical genius of Keppler who deduced the laws of planetary motion. Newton took the laws of Keppler, and showed that they were a manifestation of his own law of gravitation, and made it a “universal” law by showing that the Moon was constantly “falling” towards Earth, under the influence of gravity. The Moon in doing so, obeyed the “planetary” laws in its orbit around Earth. While it did nothing directly to advance our understanding of why stars shine, it opened the door to viewing the heavenly bodies as subject to the same physical forces that were evident here on Earth, and to the further deduction that perhaps they were made of the same stuff. The Sun also began to be understood to be a star like all of the others, just much closer to Earth.

In the 18th century, this lead to speculation that the Sun was powered by earthly processes like combustion. One such idea was that the Sun was made of burning coal. Subsequent calculations showed that if this were so, the Sun would have burned through its fuel in only a couple of thousand years, less than the length of recorded history. The theory was extended to allow the Sun to be bombarded by thousands of coal like meteors. This did not work either, as Earth was not similarly under such assault, and the mass added to the Sun over the millennia would change the orbit of Earth, an effect that was also not observed.

Fast forward to the early 19th century. Here, Joseph Fraunhofer, a talented optician, telescope and instrument maker invented the spectroscope in 1814. With this instrument, he was able to determine that the same bright line that he saw coming from a campfire was present in the light of the Sun. He would go on to catalog 574 fixed dark lines in the solar spectrum. In 1859, these lines were shown by Gustav Kirchhoff and Robert Bunsen to be absorption lines created when colder chemical elements in the Sun’s atmosphere absorb light at very specific wavelengths. The pair were able to determine the spectral lines for many earthly elements, and later identify their distinctive pattern in the dark line absorption spectra. They therefore, proved that the Sun contained the same earthly elements. It was made from the same stuff that we are.

At the same time, considerable progress was being made in the science of heat transfer and thermodynamics. This was the basis for powering the Industrial Revolution. One of the key contributors to this knowledge was William Thomson, later to be elevated to the peerage with the title of Lord Kelvin. Both he and his contemporary, Herman Helmhotz, realized that the gravitational attraction of masses of gas could be converted into kinetic energy, the energy of motion. They also realized that kinetic energy in a gas was the same as heat.

They theorized that a spherical body of gas (such as the Sun) would contract due to gravity. As this contraction occurred, the gravitational potential energy, would be converted into kinetic energy or heat, and radiate away into the surrounding space3, hitting Earth and making life on this planet possible. The heating of the gas would increase the pressure and balance the contraction.

Wiilam Thomson, Lord Kelvin
Wikipedia CommonsAs energy radiated away from the Sun, it would continue to contract, but the contraction in a single year would be on the order of a few hundred miles – a pittance compared to the diameter of the Sun. Thomson used this approach to determine that the Sun would continue to shine for about 18 million years, an eternity when compared to the age of Earth which was held to be about 6,000 years at that time.

Unfortunately, the “Kelvin Luminosity” as it came to be known, was the victim of another paradigm shift, but one from outside of the field of astronomy. This one came in the fields of geology and geophysics. During the end of the 19th century geological evidence such as estimates of the rate of sedimentation were used to estimate the age of Earth. These estimates, initially in the range of several hundred million years, were used as evidence to support Charles Darwin’s theory of evolution by natural selection which would require Earth to be at least this old for the theory to work. They had the opposite effect on Thomson’s estimate of the Sun’s age, and hence its source of energy, for it was inconceivable that Earth could be vastly older than the star it orbits.

This controversy was finally settled in 1907 with the development of the technique of dating rocks by measuring the relative amounts of the radioactive decay products of uranium. This lead to the consensus that Earth, and hence the Sun, were at least several billion years old. It was also recognized that this process released heat, and while it was released in sufficient quantities to support the temperature of Earth, it was not at all enough to be the source of energy of the Sun.

Jean Perrin
Wikipedia CommonsOne of the final clues to puzzle was however at hand. Einstein’s publication in 1905 of a paper asking the question “does the inertia of a body depend on its energy content?” included the now famous equation E=mc2. Clearly this was a potential source for the Sun’s energy, as the conversion of only small amounts of matter would yield huge quantities of energy, due to the fact that the speed of light (c) is so huge.

This was also the beginning of a new paradigm in physics which would lead to Einstein’s relativistic universe, composed not just of three dimensional space but of four dimensional space time. As powerful as Special Relativity would be in predicting the behavior of the universe, it alone could not solve the problem of why stars shine.

In 1919 the French physicist Jean Perrin took on the problem. He had previously helped to establish that atoms were a reality through his experiments, and would later win the Nobel Prize for his work proving the existence of molecules. Perrin noted that the atomic weight of helium weighed slightly less (about 1% less) than would four hydrogen atoms which he envisioned could be combined to form Helium4. While the difference in mass was very small, the energy released would be enormous. Perrin had found the right answer but did not have the knowledge necessary to describe the process in detail. The repulsive electrical charge would keep the four protons of hydrogen from merging to create helium and setting free the energy equivalent to the lost mass.

Famed Astrophysicist Arthur Eddington was convinced. He had taken Kelvin’s calculations of stellar density and temperature much further and found the temperatures and pressures at the center of the stars would be hellish, reaching as high as 10 million degrees K. Though, like Perrin, he did not have a physical model for how four hydrogen atoms could collide with such energy that they would fuse into Helium, he was convinced that this occurred due to the fact that Helium existed in stars. In fact his belief in this was so strong, he challenged his critics to “go find a hotter place” than the interior of a star.

Discovery of the mechanism for the creation of most of a stars energy would have to await the new paradigm of quantum mechanics. In 1928, Russian American particle physicist George Gamow, would realize that it was statistically possible for one proton to “tunnel” through the repulsive electric charge of another proton to fuse together.

Finally in 1938, physicist Hans Bethe5 would work out in detail the series of nuclear reactions that could occur in the center of a star which leads to the formation of Helium and release of energy. He and others would also establish another series of reactions that power stars much larger than our Sun, and ultimately produce all of the elements, including those we are made of. Truly we are stardust.

So after looking at the stars for millennia, and studying them scientifically for about 400 years, mankind finally can answer the question of why the stars shine, and in doing so, we come full circle. The answer is not a simple answer but it is profound for it leads to the realization that without the stars, we would not exist to look at them, and without the stars shining, we might never have asked the question nor been challenged to find the answer.

Notes

1. Wikipedia.org – Article on T.S.Kuhn
2. J. Bernard Cohen – “Revolution in Science” 1985, The Belknap Press of the Harvard University Press, pgs 26-27
3. Marcia Bartusiak – Archives of the Universe, Vintage Books, pg 349
4. Ibid, pg 350
5. Ibid, pg 351-2

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