Among the many mysteries of the life of Italian Renaissance priest Giordano Bruno (1558-1600) was how he made the inference that the sun was a star and that the stars were all suns. Although he was among the few Europeans of his time who believed in the Copernican theory of celestial motions about the sun, he had no instruments for observing nor did he live to benefit from Galileo’s telescopic discoveries. Yet somehow this idea of the sun as a star became so pervasive that Immanuel Kant could write in 1755 in a treatise about the universe’s physical laws that “all the suns in the firmament have orbiting motions, either around one common central point or around many.” But in contrast to this early run of good guesses and characterizations, in 1835 Auguste Comte the French philosopher and originator of Positivist philosophy would make an infamous negative assertion when he claimed that the composition of the stars would never be known.
“On the subject of stars… While we can conceive of the possibility of determining their shapes, their sizes, and their motions, we shall never be able by any means to study their chemical composition or their mineralogical structure … Our knowledge concerning their gaseous envelopes is necessarily limited to their existence, size … and refractive power, we shall not at all be able to determine their chemical composition or even their density… I regard any notion concerning the true mean temperature of the various stars as forever denied to us.”
The intuitions and fates of Bruno and Comte differed markedly. Bruno was burned at the stake by the Inquisition. Comte’s argument, however, was clearly influenced by the immense distance to stars other than the sun; but at the very least he should have by then seen the sun as a star which gave leverage on solving the enigma of stars in general. Measures of the sun’s distance, its width and its mass derived from Newtonian physics gave a value for its density, even if Comte was still at a loss as to how to relate it to the other stars in the firmament. He should also have been made aware of developments related to spectral lines in gaseous media, the experiments and laws of Kirkhoff (begun before his writing) plus the observations later on of Fraunhofer and Jesuit priest Pietro Secchi, who catalogued the spectra of over 4000 stars in the ensuing decades and developed a stellar classification system. The sun’s chemical composition was on the way to being deciphered.
We are all aware of the “rainbow” prism effect on white light. As light passes through a glass, the index of refraction bends light of shorter wavelength (blue more than red) and visible light is spread out over a spectrum of red, orange, yellow, green, blue, violet and beyond. But in the rainbow phenomenon itself we neglect to note any dark lines in the spectrum where the light simply drops out. Yet light sources such as florescent lamps filled with excited gases like hydrogen or helium when refracted through a prism reveal dark lines that can be traced to various specific wavelengths. In sunlight lines of almost any element (and some simple molecules) can be found represented. These are characteristic of electron transitions from one energy level to another. They identify gas elements (and even isotope ratios) distinctly and tend to refute Comte’s assertion quoted above about the stars and the sun.
While such 19th century developments put science on the path to understanding the sun’s nature, it did not resolve the matter entirely. Solar observatories can detect hundreds or even thousands of absorption lines in solar spectra. Some prominent lines attributable to sodium, calcium and iron suggested at first blush that the sun might be made of large amounts of such substances. What was also first observed on the sun were the mysterious spectral lines of a gas identified as helium (from the Greek helios or sun) since it was first observed in sunlight. Later helium was found to exist here on earth – and could be discerned in the light of other stars. Hydrogen would turn out to make up the bulk of the sun’s matter, however.
Among other early realizations about the sun: a few years after Bruno’s death, Galileo and others made observations of how sunspots near the sun’s equator rotated past Earth’s view over 27 days, but spots in higher latitudes took longer. The sun had a fluid nature unlike the cratered moon, though whether it was liquid, gas or charged particles reaching to any depth, it was too early to tell.
The problem of the sun’s size and distance deserves some reflection as well. Kepler’s 3rd law relates planetary orbital periods, radii and primary mass. The moon orbits the Earth within a month and is nearly the same size as the sun in the sky (leaving only a corona about it in a full solar eclipse) and the Earth orbits the sun in a year; but the sun’s distance and width would remain a mystery if this were the only information available. The existence of other orbiting planets helped. Measuring the parallax angle to Mars from two points on Earth (Paris and in Africa) in 1676 provided an initial measurement scale for the solar system, revealing eventually that the sun was about 100 times wider than the Earth and 330,000 more massive. Had there been no planets nearby for measures, Monsieur Comte might have had the last laugh after all.
Since parts of this puzzle were not solved all at once or disseminated to everyone, Auguste Comte might not have been informed of all earlier or on-going developments. The refinements of the gravitational constant connecting Earth’s mass with that of the sun was undertaken by Henry Cavendish in 1798 via small laboratory proof masses through delicate measurements. A formulation of radiative heat and temperature relations was not arrived at until the 1880s with the thermodynamic research of Stefan and Boltzmann establishing heat flux as proportional to the fourth power of absolute temperature at a surface, dropping off with the inverse square of distance from the source. Extrapolations back from the Earth gave the sun a surface temperature of 11,000 degrees Fahrenheit or 5,800 Kelvin (Celsius absolute).This posed two further questions:
Where was all this heat coming from in the sun’s interior? How long had this been going on?
Comte would have been doubly astounded by the nature of the answers as well as to the fact that they were actually found. Answers required looking at the Earth’s biological and geological record and examining as well the statistical data provided about stars from star clusters of various ages. The answer to the second question came more readily than the first. Terrestrial organisms can be traced back hundreds of millions of years. Via geochemistry the Earth itself can be aged at over four billion years by examining decay products of radioactively decaying isotopes ( e.g., ratios of uranium-232 to lead 206 in uranium bearing ores ). Even the exposed geology of the Grand Canyon argues for a steady source of energy from the sun over eons. Astronomy revealed that more massive stars burned more brightly and much more quickly ran through their fuels and that stars in clusters of various ages eventually flared up – and burned out ( or else exploded) – evidently because they eventually ran out of fuel entirely.
In the late 19th century physicists and astronomers examined suspected methods to keep the sun lit. It was noted that the equilibrium of hydrostatic pressure due to gravity could be a source of internal heat in the sun ( and even planets such as Jupiter) since compressing gas causes it to become hot. But this mechanism was not enough to sustain the sun long enough to explain all the Earth’s fossil remains. In the 1850s the German physicist Helmholtz estimated that this supply of heat would last for 20 million years. Since chemical combustion was insufficient as well, it was safe to say that the sun was not an oil lamp. This truly posed an enormous challenge to 19th century science.
While identification of elements responsible for solar absorption lines had been a breakthrough in understanding starting in 1860, the easiest to observe lines such as sodium, iron, calcium and others led to the erroneous conclusions (by some) such as a molten iron sun. It was not until the 1920s that line strengths and element abundances could be better sorted out – and the prevalence of hydrogen could be established. But all the additional elements and isotopes identified had a story to tell as well. Some of them turned out to be important tracers. Relative depletion of lithium and deuterium were clues about hidden nuclear processes.
At the end of the 19th century physicists turned their attention to the nucleus of the atom and concluded that there were orders of magnitude more energy involved in nuclear transformations than in chemical transitions involving atoms and molecules. When Niels Bohr, Ernest Rutherford and Henri Becquerel posed their theories and did their experiments, no one knew initially where the trail might lead, but British astronomer Sir Arthur Eddington around 1920 was one of the first to suggest that fusion of hydrogen into helium could provide sufficient energy to keep the sun luminous for billions of years. The temperatures and pressures deep within the interior of the sun were high enough to induce nuclear fusion reactions. Although four atoms of hydrogen had the same integer atomic number as one atom of helium, the fractional difference represented matter converted into energy in Einstein’s equation e=mc^2. Like chemical reactions, nuclear reactions involved a chain of reactions to explain the steps of how four atoms could merge into one. Contributors to the development of this theory in the 1930s and 1940s included Hans Bethe, Carl von Weiszacker who identified two principle fusion reaction as likely sources of the sun’s heat: the proton-proton chain and the carbon-nitrogen cycle.
In other words, the sun is a nuclear thermal power plant running on nuclear fusion deep in its interior. In the course of this energy generation a number of other nuclear processes are underway and as a star ages and runs out of its basic hydrogen fuel, the cycles shift and these other processes come to be more prominent. Beside the German researchers above, in the 1950s an American and British team ( William Fowler, Geoffrey and Margaret Burbidge, Fred Hoyle and others) developed a detailed theory of stellar energy processes which described the production of elements through the periodic table up to iron. For besides generating heat and illumination, stars, including the sun, are sources of the other elements beside helium and hydrogen in the universe, released from their surfaces in a number of processes; in the sun’s solar wind and flares, for example. In the case of larger stars, these elements will be released in a supernova explosion, but in the sun’s case several billions of years from now it is likely to be in the form of a less violent “nova” flare up of the sun’s outer layers exploding out into space. Were it not for previous such explosive events connected with the death of earlier stars, the Earth as a planet would have been bereft of building blocks for much of anything – including the organic chemistry needed for life.
Of course, this review of what the sun is and what it is made of has not even mentioned the sun as a crop-grower, the origin of weather and climate or the illumination at sunrise or sunset. In the end, identifying the sun as a star composed of elements sounds incomplete, but it also supplements the connection we have with it every day of our lives.
