ASTRONOMY

Condensed from the book: ASTRONOMY For anyone who wants to learn what science now knows about our universe.

During the past two decades, astronomy has experienced a growth unmatched by any other area of human knowledge. It has even been said that this growth has equaled the total advance of astronomical knowledge in the three centuries since the death of Galileo.

Many discoveries and inventions in other areas of science and engineering have helped in this sensational advance. For example, developments in atomic engineering have confirmed what astronomers had long suspected: the fusion of light atoms into heavier ones could liberate enough energy to account for the heat radiated by the sun and other stars over billions of years. The knowledge about such atomic reactions was so precise that astronomers were able to construct by calculation the internal structure of stars. They could, moreover, determine the course of a star's evolution, from its prenatal state as a contracting cloud of gas and dust through its life as a stable, shining star, to its extinction in a gigantic explosion or in slow decline into a cold, black clinker coursing through interstellar space. At last man, despite his living on a perpetually beclouded planet, could accomplish what Sir Arthur Eddington, the great British astronomer, had suggested several decades earlier: he could take pencil and paper and deduce the stars.

The electronics industry developed increasingly powerful radio transmitters and ever more sensitive receivers. Experts in communication had become aware that the sun, the moon, various planets and galaxies, and a multitude of other celestial objects were the source of radio noise ranging from a smooth purring to bursts of cosmic static. As our radio antennas and receivers were improved, still stranger sources of radio noise came to our attention. These included the blue points of light that came to be known as quasi-stellar objects, or "quasars," because they resemble stars except for the outpouring of radio energy. And then there were the still more mysterious "pulsars," objects that sent out pulses of radio energy so regularly that some astronomers at first thought they might be a form of cosmic radio beacon from beings on some distant planet. Soon, however, astronomers were able to identify the source as a rapidly rotating neutron star of very advanced age that had been compressed to extreme density.

But astronomers were tantalized by the possibility that intelligent beings might exist elsewhere in the universe. For a time, an exploratory program carried out a continuous recording of radio emissions entering our atmosphere from various directions. There was plenty of radio noise and static, but nothing suggesting intelligent messages. And so the program, called "Project Ozma," was discontinued.

The earth's atmosphere, although transparent to certain regions of the radio spectrum and the visible spectrum and slightly into the ultraviolet and infrared, is opaque to immense ranges of the electromagnetic spectrum. Employing the latest rockets, scientists found that they could raise some of their equipment above the earth's atmosphere and study thitherto inaccessible ultraviolet and X-ray regions. The results were indeed astonishing, for they showed that the sun and many other celestial bodies were emitters of ultraviolet radiation of high intensity. And the development of extended space programs, first by the Soviet Union and later by the United States, opened up vast new vistas of astronomical research.

Orbiting telescopes began to record the appearance of the universe in various wavelengths. Such studies revealed X-ray sources in distant realms of the galaxy. And as the space vehicles increased in efficiency so that they could leave the environs of the earth and travel through our solar system, astronomers were able to acquire first-hand knowledge of our moon and the planets. The Russians were the first to photograph the moon's far side which man had never seen. Later, American vehicles photographed the entire lunar surface with a detail impossible from earth-bound telescopes. Probes sent to Venus confirmed what we had already suspected: the surface of this cloud-covered planet is very hot. And probes sent to Mars transmitted detailed photographs of its crater-pitted surface. Finally man achieved the goal of landing on the moon; he explored it briefly and brought back rocks for analysis. All of these space programs required the development of special instrumentation, such as the cameras that took and automatically developed photographs in space, then turned the photographic records into electronic pulses that our great radio receivers could capture. Electronic computers then reconverted these pulses back into high-resolution pictures of the photographed object. High-speed digital computers have revolutionized astronomical calculation. A computer can now perform in a few seconds the calculations that would have taken an early astronomer a full year to carry out. The accuracy of the calculations has also been vastly increased. Without these computing facilities, the space program would have been impossible. Computers check and assess every detail of a space flight from liftoff to splashdown. They have furthermore been vital in the design and performance of multi-stage rockets.

Computers have also played an important part in other astronomical experiments. Our great radio telescopes send pulses of radio energy into space, where they may be reflected from the surface of the moon or one of the planets. The signal, much weakened, returns to earth, where receivers transmit the data to a computer. This highly sophisticated application of radar has yielded valuable information about distances within our solar system, the size and rotation of planets, and irregularities on their surface. And flashes of light from a laser beam have fixed the distance to the moon to within a foot.

Such studies have also led to great improvement in our measurement of time. Half a century ago, an astronomer was satisfied if he could measure time to a hundredth or even a tenth of a second. Today scientists confidently measure intervals even shorter than a nanosecond (10-9 sec.). And special clocks, using atomic or molecular vibrations to control their movements, are more accurate timekeepers than our earth, which up to recently was the standard measure of time.

Such are some of the explosive developments that have revolutionized astronomy.

The Birth of Astronomy

Let us try to reconstruct man's early struggle to understand the world around him. No matter how limited his curiosity, primitive man certainly noticed the sun and recognized it as the giver of light and warmth. He became aware of its rising and setting. He learned to associate the changing altitude of the sun with the changing seasons, and the departure of summer and the onset of winter with days growing shorter and shorter. The moon, brilliant luminary, doubtless attracted almost as much attention as the sun. Man surely observed its changing shape, from crescent to full and back to crescent again, and wondered what it signified. And he measured time by the moon's circuit of the sky; today we still recognize and use this measurement of the "moonth," or rather, month.

Ancient man probably realized that most of the bright points of light in the sky, which we call stars, did not change their relative positions each night as they traced circles around the sky. He also noted that a few of the bright stars "wandered" with respect to the "fixed" stars in a path roughly parallel to that of the moon. Today we call these special "stars" the planets, a word derived from the Greek planet, meaning "wanderer."

The Universe A Panoramic View

Modern science has revealed to us a universe far moré vast and magnificent than the people of antiquity could have imagined. Year by year scientific inquiry has pushed back the frontiers of knowledge, disclosing new wonders of the cosmos, the stars, planets, and nebulae, and of stellar systems complex beyond belief. Daily our own earth has become more and more insignificant a microscopic speck floating in the illimitable ocean of space. Imagine, if you will, a solar system reduced in size five billion (5 x 10º) times so that a million miles equal one foot. On this model, the sun will be reduced to the size of a basketball, a sphere approximately one foot in diameter. On this scale the earth appears about as large as a grain of wheat, revolving at a distance of 100 feet in a circle about the miniature sun. Within the earth's orbit lie Venus and Mercury, the size of a second grain of wheat and a mustard seed and spaced respectively at about 70 and 40 feet from the basketball sun. Mars, a trifle larger than Mercury, stands 160 feet away. We represent Jupiter, the largest of all the planets, by a sphere the size of a golf ball, and place it 500 feet away from the sun. Saturn, the size of a large cherry, encircled by its famous ring system, lies 1000 feet off. Uranus and Neptune, both as large as good-sized peas, fall 2000 and 3000 feet away respectively. Finally, Pluto, another mustard seed, fits into the picture just about a mile away. We must not forget the asteroids more than two thousand minor planets which are mere specks between the orbits of Mars and Jupiter. The moon is another mustard seed, revolving around the earth in an orbit whose radius is about three inches.

The Solar System in Miniature The Smallness of Solar Bodies

Mark Twain, in his story Captain Stormfield's Visit to Heaven, related an incident obviously intended to seem absurdly extravagant. Captain Stormfield, having died and started on his way to heaven, was unable to resist the temptation to race with a comet. He ended up, far off course, in a sort of heavenly "missing persons bureau." There an angel went up in a balloon alongside of a map of the universe about as big as the state of Rhode Island to look for our solar system, and came down three days later saying he had perhaps found it, but maybe it was just flyspecks. Mark Twain may not have intended us to take his sense of distance seriously, but nevertheless the map formed a fair model, to scale, of the universe as known in his time. Since then, modern telescopic equipment has greatly extended, and is still extending, the boundaries of the surveyed portion of the universe. Instead of Rhode Island, therefore, let us use our entire earth to represent the volume of the universe as we know it today. For the measurement of interstellar distances, the mile is a ridiculously small unit similar to expressing the distance between New York and San Francisco in terms of the breadth of a human hair. Astronomers often use a light-year (that is, the distance that light, moving at 186,000 miles per second, travels in a year) as their unit of measurement. In a year a flash of light will have moved approximately six million million (6,000,000,000,000) miles into space. On the scale of our new model, with the earth representing the whole universe, we shall let 1/16 inch correspond to six million million miles, or one lightyear. Alternatively, we may say that 1/100 inch equals a million million miles. Now, let us enter this model universe and try to find the solar system.

As we enter the great volume our first impression is that the building is entirely empty. Fortunately, we have brought with us a super-microscope, for on the scale that we have used, no ordinary microscope would be able to render the stars visible. The largest are scarcely one-millionth of an inch across, and the smaller ones are about the size of atoms. When our eyes become adapted to this microscopic vision, we immediately note that the universe shows clear signs of order. Stars are by no means scattered uniformly throughout the enormous volume. They form groups, or clusters, several hundred feet in diameter, something like giant swarms of gnats. The stellar population is concentrated in these regions, with vast realms of nothingness in between. Some of these groupings of stars are irregular in shape. Others are round and flat, like a pie plate. Each group contains hundreds of millions and sometimes as many as a hundred billion stars, with the model stars spaced, on the average, a few tenths of an inch apart. Again, what impresses us most is the emptiness of space and the vast distances between the stars and groups of stars. Stellar traffic lanes are decidedly uncrowded. The chances of two stars accidentally colliding is almost infinitesimal.

There are millions of groups of clustering stars. In many clusters the stars are arranged like the coils of a watch-spring, giving the aggregation a pin-wheel appearance. These are the great spiral nebulae. If we are to search out the sun, we must first find our Milky Way. Like many other galaxies, it has spiral arms. In our model, it is a disk-shaped aggregation of stars approximately 500 feet in diameter and 50 feet in thickness. It contains about one hundred billion stars and the task of searching out our sun from among that vast assemblage, even after we have located the Milky Way, is herculean. The task is infinitely more difficult than finding the proverbial needle in a haystack. A hundred billion small coins, spread over a 100-yard football field, would make a pile nearly 50 feet high. Mark Twain's angel who succeeded in finding the solar system only after several days of looking, had rare luck. Just to count the stars in the Milky Way at the rate of one per second would take about a thousand years. When at last we find our solar system, we discover it to be a very small "speck" indeed. The largest orbit of all, that of the planet Pluto, is invisible to the eye, and our earth is smaller than an atom.

In this universe, the stars are bodies like our own sun, self-luminous spheres of hot gas. Each star is a nuclear power plant, transforming matter into energy by processes akin to those of the H-bomb, except that the huge mass of the star controls the reaction and keeps it from escalating into a vast explosion. In the universe we find all kinds of stars: large and small, hot and cool, young and old. Part of our task will be to interpret the observational evidence in the light of known Until the recent discovery of radio emissions, the universe has given man only one clue in revealing its mysteries . . . light. Some so distant that when seen by man it had left its source long before he existed. Some so faint it can be seen only with the most powerful telescopes.

"LIGHT"

And yet, by refining, polarizing, bending, breaking, reflecting, and amplifying this light, scientists extract information of incredible proportions about a source they cannot see.

As for the stars, we find innumerable varieties of them. We see giant stars with diameters 100 times that of our sun, glowing red because their surfaces are relatively cool. Then we find much hotter, somewhat smaller stars, but giants in luminosity, shining brilliantly white or blue because they are so very hot. At the other extreme we see stars even smaller than Jupiter and relatively cool and faint. These "red-dwarf" stars are among the most numerous of all the celestial suns, but they are so faint that we can see only the nearest ones. Our sun, though much less brilliant than the red or blue giants, is still a star of quite considerable size midway between the brightest and the faintest.

The Stars

We must classify these stars in such a way as to show their interrelationships. Then, perhaps, we may indeed be able to understand how they are born, how they live, and how they die.

We have no direct way of ascertaining how many of the hundred billion stars of our galaxy possess planetary systems. One faint neighbor, known as Barnard's star, appears to have an invisible companion about the size of Jupiter. We conclude this from the motions of the star itself which, instead of moving precisely in a straight line, pursues a slightly wobbly path, presumably because it is revolving around the center of gravity of both the star and planet.

Many astronomers once believed that the solar system came about through the collision or near-collision of two stars, the planetary material being the wreckage of that encounter. Because interstellar space is so sparsely populated, the probability of so close an approach is almost negligible. In fact, if our solar system was formed in this manner, it might well be unique in the universe. Later we shall discuss the modified version of this theory that astronomers have adopted in recent years.

Our disk-shaped galaxy is so large that light takes about 100,000 years to travel from one rim to the other, and some 10,000 years across its shorter dimension. The galaxy rotates, and stars in the vicinity of the sun revolve around the center of the galaxy in a period of about 230 million years. This is a short time as compared with the probable lifetime of the galaxy, which must be counted in thousands of millions of years.

The Sun, Our Nearest Star

Our sun is a star, bright only because it is so close. If it were as far away as Alpha Centauri, the next nearest star (which is a quarter of a million times more distant) the sun would be indistinguishable from other bright stars. Aside from the fact that the sun is the center of our solar system, it has no special features that distinguish it from a billion or more similar stars in our Milky Way - at least as seen from the vast distances we must view them.

Of course, from the point of view of earth-dwellers, the sun plays a particularly important role: it is the source of the light and heat that permit life to exist on earth. But, vital as the sun may be to man's existence, that existence is of no importance in the cosmos as a whole.

The nearness of the sun does enable us to study features that would otherwise be invisible and that we simply could not have conceived of. We infer that other stars probably possess them too, because our sun can scarcely be unique. These special features include the sunspots, which appear as dark areas on the shining solar disk. Then there is the upperIn solar atmosphere, the chromosphere, above which we see flame-like clouds solar prominences sometimes raining hot gases back upon the solar surface or exploding fiery geysers into space. Finally there is the outermost layer of all, the delicate solar corona, so faint that until about 1930 astronomers had been completely unable to detect it except during total solar eclipses, when the moon obscures the bright solar disk. We further know that the outer parts of this corona are escaping into space in variable amounts and speeds, producing the phenomenon of the solar wind. In a sense, then, our earth is still "inside the sun," since these tenuous streamers extend well beyond the earth's orbit. The early theories of the nature of these outer layers would never have led us to suspect that the sun was emitting X-rays, radio waves, or showers of energetic atoms.

Structure of the Sun

The knowledge of atomic nuclear reactions, detailed in an earlier chapter, has enabled the astrophysicist to construct a satisfactory model of the main body of the sun, from its visi-ble outer surface, often called the photosphere, here, or "sphere of light," down into its core. The layman usually thinks of the sun as a gaseous ball of fire; actually the weight of the outer layers compresses the inner layers until we find, at the center, matter some 350 times denser than water and even denser than lead.

If the sun were cool throughout, the outer layers would collapse upon the inner regions to form a body smaller than our earth, with a density millions of times greater than that of water. Under such enormous pressures, the negative electrons would combine with the positive protons to form neutrons. Astronomers term such an object a neutron star. Neutron stars do exist, but, judging from its huge size, our sun cannot be one of them. To prevent its collapse, however, the sun needs high temperatures as much as 16,000,000° K - in its core. Despite its high densities, the sun is gaseous right down to its center. By "gaseous," we mean that it expands or contracts with changes in pressure, unlike a solid, which resists compression. The central pressure is more than 400 billion times greater than that of the atmosphere at the surface of the earth. These figures are not precise and vary somewhat as we change certain assumptions about the sun's interior, but they do indicate the extreme conditions that may be encountered near the center of the sun. Our calculations show that such densities and temperatures are high enough to support the successive captures of protons the proton-proton reaction that builds hydrogen into helium in a series of steps and releases the energy necessary to sustain solar radiation for billions of years.

Sunspots

Sunspots are the most conspicuous feature of the sun. Long before the invention of the telescope, the Chinese Annals recorded many observations of such dark areas on the solar disk, seen presumably when the sun was dimmed by clouds or haze, especially near sunset. The Chinese often referred to them as "flying birds." Galileo and other astronomers of his time, using telescopes, independently discovered the spots. They thereby provoked the criticism of contemporary philosophers, for Aristotle himself had declared that the sun was pure fire; and since nothing that was pure could possibly possess blemishes, sunspots could not, according to medieval logic, exist. Many people refused to look through that wicked instrument, the telescope.

Occasionally they appear as a single spot; most frequently the spots occur in pairs or in groups. Groups of multiple spots can be large enough to be visible to the unaided eye (but no one should stare directly at the sun). Individual spots have a black center, usually somewhat irregular in shape, known as the umbra. The gray penumbra outlines this area, radiating fromthe center like the petals of an aster. The terms umbra and penumbra do not imply that the spot phenomenon has anything to do with shadows. In simplest terms, spots appear dark because they are cooler than their surroundings. Sunspots are not, however, completely black. If a magician could cause the sun to vanish, leaving just a single large sun spot, the earth would not immediately be plunged into darkness. That single spot would continue to radiate about as much light as we would receive from a hundred full moons. The darkness of a spot, therefore, is simply a matter of contrast.Observations of a spot or group from day to day disclose that spots drift across the solar surface from east to west as we view the sun's near side. This means that the sun rotates like the earth and in the same direction west to east. Technically we say that the rotation is direct rather than retrograde. The rotation enables us to determine the positions of the poles and equator. We find that the sun's axis is tilted 7° with respect to the plane of the earth's orbit. But the sun is not a solid body and does not rotate like one. A series of spots lined up on a given meridian of the sun would, after a single rotation, form a curve. Similarly, spots near the solar equator complete their revolution in a shorter time than those at higher latitudes. Thus we know that the rotation period of the sun varies from some 25 days at the equator to about 29 days near the poles.

As observations accumulated over the centuries, astronomers realized that sun spots did not occur at random but according to a well-defined cycle. A single cycle that is, from one minimum to the next takes slightly more than eleven years on the average, the exact duration varying from as few as nine years to as many as sixteen. And the average maximum spottedness also varies, seemingly in random fashion, though some astronomers have suggested that every other cycle is higher. Spots occur most frequently between the sun's equator and latitudes 35° and almost never at latitudes higher than 40°, north or south.

Meanwhile, astronomers have built a huge, international network of solar observatories. Most of them have telescopes fitted with monochromatic filters. In the United States, the National Aeronautics and Space Administration, the National Science Foundation and the Air Force have supported the construction of such a network for the primary purpose of keeping close watch on solar activity. The safety of astronauts from the dangerous solar fallout is the prime consideration here. Of great importance also is the fact that many space experiments depend directly or indirectly on solar activity. So scientists keep the sun under almost continual surveillance; when some stations are beclouded, others can take up the observation. The occurrence of major flares is reported to NASA through a worldwide network. The Environmental Sciences Service Administration (ESSA) also employs the solar data to issue regular forecasts of ionospheric conditions, especially those that could affect radio communication.

As we have already noted, the earth's atmosphere absorbs strongly, especially in the ultraviolet region of the spectrum. Hence observations of the sun and other celestial bodies from rockets and satellites are playing an important role in our understanding of the sun's outer corona. One special program of the series of the Orbiting Solar Observatories (OSO), under Leo Goldberg of Harvard, has sent back telecommunications records of the solar ultraviolet spectrum. It has also transmitted maps of the sun in selected atomic lines of the far-ultraviolet region. These records refer mainly to the solar corona, and enable astronomers to probe its radial structure. Herbert Friedman, of the United States Naval Research Laboratory, has obtained spectacular records of active solar regions, in the X-ray region of the spectrum, from rockets that go far above the earth's atmosphere.

The outer corona merges, at some distance from the sun, into that faint glow extending parallel to the ecliptic in regions near the sun. This phenomenon, called the zodiacal light, is ordinarily attributed to a zone of fine dust or other particles orbiting the sun inside the orbit of Mercury. But the outer corona and the zodiacal glow are so faint that ordinary methods of observing them near the sun have unfortunately failed. Richard S. Tousey and his colleagues of the Naval Research Laboratory succeeded in photographing these external regions from rockets. These records show outer corona streamers extending outward from the sun by 8 or 10 diameters as they merge into the structureless glow of the zodiacal light. To prove that the light so photographed comes from regions near the sun, Tousey took records at new moon, so that the dark lunar disk could clearly be seen eclipsing the background corona.

The sun is important to scientists because it is the only star whose surface and behavior they can study in detail. It is particularly interesting because of the great range of its temperatures and densities from its core to its outer corona. It provides, in fact, a huge laboratory where they can observe the effects of high temperatures on matter. And it challenges scientists with many mysteries such as the origin of magnetic fields and the sunspot cycle.

Other Galaxies

To complete this survey we must go beyond our own Milky Way system to the vast numbers of galaxies that lie beyond. They take many forms. Like our own, some are spiral. Others are amorphous blobs, with perhaps a slight concentration of density toward the center, similar to that of a globular star cluster. Others are so irregular that they have no specific outline or shape. These objects appear to be similar to our galaxy, each consisting of billions of stars, gaseous nebulae, and extended clouds of dark dust. These galaxies are islands of light and activity, sparsely strewn through a universe that is otherwise nearly empty. The distance between galaxies is tremendous, light taking between 100,000 and 1,000,000 years to go from one galaxy to the next. The objects nearest us are two galaxies, known as the Clouds of Magellan, which look to the naked eye like segments torn loose from the Milky Way. They are approximately 100,000 light-years away from us. In effect they are dwarf companions of our galaxy. Organization in the universe does not cease with the individual galaxies. Here and there we find galaxies themselves tending to cluster together. Groups of five, dozens, or even hundreds of galaxies appear to form families or related sets. Even clusters of clusters occur.

Where does it all end? And how? Let us return to our model, where the scale is so small that an entire light-year occupies but a sixteenth of an inch. Our galaxy seems to be somewhere near the center of this system. As we look out toward the "edges" of the universe we discover the other galaxies receding from us at higher and higher velocities. This "dynamic" universe seems to be expanding rapidly. What the fundamental significance of this expansion is we shall see when we take up the complexities of Einstein's theory of relativity. In thus depicting the universe in panoramic form, I have painted rapidly, with a broad brush. My aim has been to picture the universe as a complex structure governed by physical laws. In later chapters I shall discuss the various units and their relationship to one another and to the universe as a whole. We cannot proceed, however, without first discussing the basic blocks that make up the universe the atoms. Further, we shall have to discuss how we have gained knowledge of the universe through observation of the various kinds of radiations emitted by heavenly bodies. Since our earth is one of the planets of the solar system, we can also learn a great deal about other bodies by studying its composition and structure.

Astronomers can determine the temperature of stars, their velocity and direction as they hurtle through space, their mass, age, volume, distance, the composition of their atmospheres and interiors, including metal abundances. The solving of age-old mysteries of the universe and the constant revealing of the new has been accomplished through this clue, this light, and a quality inherent not only in men of science but in all men . . . an insatiable sense of curiosity.