Our universe is unbounded and limitless, consisting of uncounted billions of stars burning in the dark, unmeasured span of space. On and on it goes-a reach of space and time that seems more than mind can grasp. Among the stars and nebulas, among the many galaxies and clusters of galaxies whirls the spiral galaxy of which we are a part. We see it from within as a faint blur of distant stars that arches in a milky streak across the nighttime sky.
To us this single galaxy, our Milky Way, seems incomprehensibly vast-a hundred billion suns sprinkled in a spiral pattern to form a flat, rotating disk. The spiral is so large that a flash of light from one edge must race a hundred thousand years to reach the other side.
Far out along one spiral arm there gleams the star we call our Sun. It is, we think, an average star of middle age and middle size, much like hundreds of thousands of its distant neighbors. It belongs to a common class of stars called dwarfs, to distinguish them from giant red stars like Antares and Betelgeuse, whose diameters would stretch across 500 suns. The diameter of our Sun is about 1.4 million km, nearly 10 times that of the largest planet, Jupiter, and 100 times that of Earth.
Our small Sun may be unique in one respect: its family of planets; however, they too, may be common features of the universe. We are much too far from other stars to determine directly whether any has Earths or moons or Jupiters.
Like all planets, Earth derives its light and heat from the Sun. It is sometimes surprising to realize how far we are from the Sun; in its warm rays we feel much closer to our star than facts of distance say we are. The Earth travels around the Sun in a nearly circular orbit, never getting closer than about 140 million km, more than 400 times the distance between Earth and Moon-man's furthest excursion into space. A spacecraft sent to the Sun would spend several years en route. Were it possible to fly to the Sun in a jet airliner, it would take nearly 18 years to make the trip at 1000 km/hr.
The Sun has no permanent features and is entirely gaseous-a glowing ball of chiefly incandescent hydrogen. Other elements, all in gaseous state, are present in amounts that seem to fit a universal, cosmic recipe: about 10 parts hydrogen to 1 part helium, with a pinch of each other element from the periodic table, but mostly oxygen, carbon, nitrogen, magnesium, and iron. Although we speak of the "surface" of the Sun, and of specific layers in its atmosphere, the Sun is really all atmosphere. It has no easily defined boundaries or sharp discon....
 ...tinuities like those on Earth that separate air and water and solid ground. The solar corona, chromosphere, and photosphere are distended, rarified regions that blur into each other and have definite form only when seen at great distance, as from Earth. The average density of the Sun, about 1.4 g/cm3, is only about a quarter of that of Earth. The solar photosphere which we see with the naked eye is so diffuse and thin that we would call it vacuum here on Earth, and the layers above it, the chromosphere and corona, are far less dense. The Sun becomes gradually denser as we go inward, but it is not until we have gone a tenth of the way to the center that we find material as dense as the air we breathe on Earth, and not until we are halfway to the center that the Sun is as dense as water.
No one has seen within the Sun. We presume the central temperature is about 15 million kelvins and the central density is about 160 g/cm3, or 10 times that of ordinary metals. These estimates are based on calculations that depend upon our knowledge of the surface temperature and pressure, the mass and size of the Sun, and the physical laws that govern stellar interiors.
The exceedingly high temperatures and densities of the solar interior trigger nuclear reactions that produce the energy that fuels the solar furnace. The exact nuclear process is not known, but the most likely is one by which hydrogen, the Sun's most abundant fuel, is converted to helium. In this process of nuclear fusion, energy is released. In the highly compressed solar interior, the disruptive forces of nuclear reactions are contained and muffled by a press of gravity that is 250 billion times as great as that on Earth.
For each helium atom created in this way, a small amount of energy is emitted in the form of a gamma ray at the invisible, short-wavelength end of the electromagnetic spectrum. The normally piercing gamma rays travel but a short distance through the densely packed material at the center of the Sun before they are absorbed and then reemitted to be absorbed again. The nuclear energy carried in gamma rays finds its way outward through the Sun by a long and tortuous series of repeated absorptions and reemissions, gradually losing energy and changing to longer and longer wavelengths in the process.
When the energy finally reaches the surface of the Sun, most of the gamma rays have been replaced, first by X-rays and then by ultraviolet rays, so that when they at last emerge, they are in the form of the visible and infrared radiation that makes up the sunshine that we feel on Earth. It is a measure of the size and density of the solar core that energy created by nuclear fusion at the center of the Sun takes nearly 50 million years to jostle its way to the surface. There the solar energy easily escapes to space, traveling at the speed of light to reach the orbit of the Earth in but 8 minutes.
The temperature of the Sun decreases outward from the core, and at the visible surface of the photosphere it has fallen to about 6000 K. This white-hot surface radiates energy at a rate of about 3.8 x 1023 kW, which flows out equally in all directions. Earth-a tiny target at a great distance-intercepts but a billionth part of the prodigious solar output of heat and light. But even this small share deposits on Earth a continuous supply  of about 8 x 1014 kW, which is vastly more than the full capacity of all the power plants ever envisioned on Earth. Each square meter of Earth receives about 1.5 kW of solar energy, which, if harnessed efficiently, could heat and light a small room.
In producing this energy, the Sun's nuclear furnace consumes about 5 x 109 kg of hydrogen each passing second, burning outward in successive shells and gradually transforming the solar interior to a predominantly helium core. We estimate that hydrogen within the Sun has been consumed at this fantastic rate for the past 4 or 5 billion years. Yet the Sun is so large that there is still hydrogen within it to continue the process at the present rate for another 100 billion years, or 20 times as long as it has burned in the past. Long before the hydrogen is depleted, however, helium will probably begin to serve as fuel, in other fusion reactions to create other elements, which will later take their turn as solar fuel.
The Sun. Ruler, Fire, Light and Life of the Planetary System was Richard Proctor's title for his book on solar physics written in 1871. Indeed, all life on Earth depends upon the Sun. "Solar energy," which we talk about today as new, has served Earth with constancy for about 5 billion years.
The Sun is the ultimate source of almost all the energy we use, because wood, coal, and oil are fossil fuels that store the energy of sunlight from eons past, and in harnessing wind and river we are but harvesting the solar heat that drives the circulation of the air and lifts the moisture from the sea. Our food supply is totally dependent on the Sun, directly or indirectly, through solar photosynthesis in green plants and aquatic forms. Photosynthesis also releases to the atmosphere the oxygen essential to all animal life.
The same sunlight that feeds and gives us fuel and oxygen warms us as well. Without it the salty seas would freeze and the very atmosphere would condense and solidify. Without the Sun there would be no rain, no snow, no rivers or lakes, no winds or clouds or blue of the sky, no rainbow, no moonlight, and no vestige of life upon our planet. Little wonder that early man on every continent recognized his total dependence on the solar star and fell to worship it.
Gradually the early worship of the Sun grew into a more practical desire to know and understand it. Was it, as the Greek Anaxagoras said, a mass of fiery stone, 56 km across? Or, as dictated by medieval dogma, a perfect fire? How far away was it? How large? How hot? How constant? How long would it last? What were the spots seen repeatedly on its surface by Chinese astronomers 200 years before Christ, and by Galileo with a telescope  in 1611? Were there changes on the Sun that might explain the cyclic change of weather and climate on Earth? The ice ages? The recurrent droughts and their implications for world concerns of the 1970's?
Today, about 20 percent of the world's astronomers, including over 200 in the United States alone, are engaged in the full-time study of the Sun. They work in universities, observatories, in Government laboratories, and in private industry. A large number of amateurs make solar astronomy their specialty.
The Sun is studied by astrophysicists as a star- the only one that can be seen in detail. It is studied by physicists as a laboratory where unique conditions of temperature, density, fluid motions, and magnetic fields exist. It is studied by atmospheric physicists, aeronomers, and climatologists for its important terrestrial effects.
Solar research is pursued in almost all the nations of the world, and, by need as much as by tradition, it has enjoyed a vigorous spirit of international cooperation. In the last decades, through the coldest winters of political and idealogical war, solar data and research results flowed freely between solar observatories of East and West.
In these traditions Skylab gave much of its budget of time and resources to an intensive, cooperative study of our Sun, to achieve what now seems to be the greatest step ever made in the long history of man's study of the Sun.
Observations of the Sun from Skylab have changed the course of modern solar physics and have built a lasting bank of solar data that will long serve astronomers of all the world. The power of Skylab was its ability to keep a continuous watch on the Sun simultaneously in all wavelengths, including the crucial ultraviolet and X-ray radiation normally blocked by our atmosphere. This power was brought to bear by the force of the most sophisticated array of telescopes ever turned on the Sun, the advantages of trained observers in space, and the support of the most extensive world effort ever organized to wrest the secrets from the Sun.
The results exceeded all expectations.
The aim of this book is to explain the major findings of Skylab's solar effort, and to show the new and colorful Sun that Skylab brought to view. To answer the inevitable "What do we know now that we did not know before?" we must first explain, in broad terms, what we knew of the Sun before Skylab, and how we came to know it.