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The 1984 issue of Astronomy and Astrophysics Abstracts lists 9361 papers written by 10,863 authors covering 2,200 major topics in astronomy. Under the simple heading of 'Stars', for example, you would find forty-five sub-topics ranging from 'Star Catalogs' to 'Stellar Winds' and spanning some 1189 individual papers. The activity implied by all of these papers is quite enormous. Each paper represents months or years of work based, in part, on the findings of still earlier research programs that have probably appeared in the literature over the years. A single paper may have thirty or more references to data or ideas presented by other researchers.

What is ultimately the result of all this activity? As astronomers, we hope that it will eventually culminate in a thorough, detailed understanding of all the major collections of matter in the universe, their properties at a single instant in time, as well as their evolution from birth to death. These 'major collections' include: interstellar molecules, dust grains, asteroids, planets, solar systems, stars, nebulae, star clusters, galaxies, clusters of galaxies, and the universe itself. This represents a range in length scale of 10³⁶ and a mass range of 10⁵²! Not only would we like to know, by recourse to basic physical law, the answers to general questions like "How do stars with 5.5 times the mass of the sun evolve?" but, we would also like to be able to explain specific objects in our universe such as, "What mechanisms are producing the jets in SS 433 and 3C 273?"

As a chronicle of the progress in our knowledge over the centuries, let's consider the subject of stellar structure and evolution. It is safe to say that for the last century, more articles have been written on this subject and related issues than any other. That means that more time and effort has gone into understanding stellar structure and evolution than any other field in astronomy. It is, what you might call, a mature discipline whose basic theoretical and observational ingredients are reasonably well understood at the present time. That we can know so much about objects that are so far away is a testament to the power of the scientific method and human technological inventiveness.

For thousands of years, stars were simply lights burning silently in the depths of the heavens; any discussions about what they were revealed more about the human imagination than about nature. It wasn't until Joseph Fraunhofer invented the spectroscope, and began to examine the light from the sun, the planets, and several bright stars, that the first step was taken towards answering the age old question, "What is a star?". Just as for the sun, each bright star that was examined with the spectroscope revealed a rainbow of colors crossed by a pattern of dark lines. It was quickly discovered that the lines could be matched by a number of commonly known elements available in the laboratory. By 1864 Father Angelo Secchi at the Vatican Observatory began a program of systematically classifying the spectra of 4000 stars. Sir William Huggins and William Miller carefully studied the light from Sirius, Aldebaran and Beta Pegasi, identifying the elements hydrogen, sodium and magnesium from the dozens of spectral lines detected.

The first issues of The Astrophysical Journal, published in 1895, covered new developments in spectroscopy, both the theoretical principles on which it was based, and improvements leading to the design of even more powerful and sensitive spectroscopes. By this time, thanks to the pioneering efforts of Gustav Kirchoff and Robert Bunsen, the solar spectrum had been resolved into twenty thousand spectral lines corresponding to thirty-nine elements. The state of the art of understanding the sun was candidly summarized by E.J. Wilczynski from the University of Berlin: "Almost every student of solar physics has his own theory, and usually he himself is the only one that believes in it." Although much of the work in solar physics had involved a careful study of sunspots and the spectroscopic features of the sun's surface, the internal structure of the sun was also becoming a lively topic of discussion. By the later decades of the 19th century, older ideas about the solar interior, involving a 'liquid body with clouds suspended over its surface', were being quickly replaced by the more modern view of a fully incandescent, gaseous ball which rotated at an increasing rate as you moved from its poles to its equator. It might amuse you to know that among the early speculations about the solar interior, Sir John Hershell announced that the surface of the sun was covered by living, luminescent organisms thousands of miles long!

The origin of the sun's tremendous energy supply also made its appearance into the arena of acceptable inquiry. Hermann von Helmholtz in 1854, had proposed that the gravitational energy lost by the sun during its slow contraction, will show up as a comparable quantity of heat energy which could provide the 'missing energy source' for the sun. To produce the measured solar power of 400 trillion trillion watts, the sun's radius would only need to decrease by about thirty meters each year. Not much of a change when you consider that the radius of the sun is 700 million meters. But, this means that about 30 million years ago, the sun was twice its present size and that in another 30 million years, it will be a burned-out red cinder, incapable of supporting life on earth. Between 1878 and 1883, Helmholtz's idea remained popular and was even refined to obtain an age of 4.3 million years. Fortunately, for us, this cartoon sketch does not represent the real world.

In 1906, Karl Schwarzschild published a fundamental paper in astronomy, describing the appearance of an incandescent, stable ball of gas in considerable detail, using basic principles in physics. Not only did he show that the sun's limb should be darkened to the precise degree observed, but went on to prove that the distribution of matter within the sun could be determined once you could specify the exact dependency of the gas pressure on its temperature and density. He also discovered that, under certain conditions, energy would be transported from the center of the star outwards, either by the convective boiling motion of matter, or by the streaming of radiation from the core to the surface. Sir Arthur Eddington continued this work by including the affects of radiation pressure, showing that stars that are mechanically stable are only possible for certain combinations of mass and luminosity. An amazing discovery indeed! Even for the creation of stars, nature followed a set of very specific rules favoring certain stellar properties over others.

Between 1913 and 1917, Henry Norris Russel and Ejnar Hertzsprung claimed from their study of star sizes, that blue stars were the hottest as well as the largest, while red dwarf stars were the smallest. They proposed that a star began its life as a hot blue star and, by contaction, wound up as a dull red dwarf. Eddington, mentioned above, further discovered that the core temperatures of all the 'main sequence' dwarf stars that Hertzsprung and Russel had been studying, were actually very similar, about 20 to 30 million degrees, and that this temperature didn't depend on the star's mass or size. Instead of evolving from blue to red as they cooled, Eddington proposed that gas clouds would contract until their central temperatures reached about 20 million degrees at which time they would stop contracting and become a stable star. This explanation re-ignited interest in two older questions, "What was the process that stopped the contraction of the star at this temperature?", and "Where did the energy come from if not from Helmholtz's mechanism of gravitational contraction?" The answers could not emerge from the physical principles understood at that time, but had to wait for the 20th century discovery of nuclear dissintigration and fusion.

The British astronomer R. d'E. Atkinson was the first to suggest, in 1931, that the capture of a proton by an atom could liberate enough energy to light the sun. … to be continued