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l Introduction OUR COSMIC HERITAGE Nature offers no greater splendor than the starry sky on a clear, dark night. Silent, timeless, jeweled with the constellations of ancient myth and legend, the night sky has inspired wonder throughout the ages. It is a wonder that leads our imagination far from the confines of Earth and the pace of present day, out into boundless space and cosmic time itself. Astronomy, born in response to that wonder, is sustained by two of the most fundamental traits of human nature: the need to explore and the need to understand. Through the interplay of discovery, the aim of exploration, and analysis, the key to understanding, answers to questions about the Universe have been sought since the earliest times, for astronomy is the oldest of the sciences. Yet it has never been, since its beginnings, more vigorous or exciting than it is today. Through modern astronomy, we now know that we are con- nected to distant space and time not only by our imagination but also through a common cosmic heritage: the chemical elements that make up our bodies were created billions of years ago in the hot interiors of remote and long-vanished stars. Their hydrogen and helium fuel finally spent, these giant stars met death in cataclysmic supernova explosions, scattering afar the atoms of heavy elements synthesized deep within their cores. Eventually 3

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4 ASTRONOMY AND ASTROPHYSICS FOR THE 1980's this material collected into clouds of gas in interstellar space; these, in turn, slowly collapsed to give birth to a new genera- tion of stars. In this way, the Sun and its complement of planets were formed nearly 5 billion years ago. Drawing upon the ma- terial gathered from the debris of its stellar ancestors, the planet Earth provided the conditions that ultimately gave rise to life. Thus, like every object in the solar system, each living creature on Earth embodies atoms from distant corners of our Galaxy and from a past thousands of times more remote than the begin- nings of human evolution. Although ours is the only planetary system we know, others may surround many of the hundreds of billions of stars in our Galaxy. Elsewhere in the Universe, beings with an intelligence surpassing our own may also at this moment gaze in wonder at the night sky, impelled by an even more powerful imagination. If such beings exist-possibly even communicating across the vast expanses of interstellar space they, too, must share our cosmic heritage. This recognition of our cosmic heritage is a relatively recent achievement in astronomy. However, it is but one of many such insights that our generation alone has been privileged to attain. In all of history, there have been only two periods in which our view of the Universe has been revolutionized within a single human lifetime. The first occurred three and a half centuries ago at the time of Galileo; the second is now under way. EXPLORATION AND UNDERSTANDING The discoveries of the past 20 years, made from both ground- based and space observatories, have radically changed our con- cepts of the origin and evolution of stars, galaxies, and the Universe itself. The 1960's saw the discovery of quasars, x-ray sources outside the solar system, the cosmic microwave background radiation, pulsars, high-energy celestial gamma rays, large-scale inhomo- geneities in the solar corona, and polyatomic molecules in inter- stellar clouds. The rapid pace of discovery continued in the 1970's: 1970: Uhuru, the first satellite x-ray observatory, is launched; it reveals that many bright Galactic x-ray sources are neutron stars accreting matter from nearby companion stars and that many

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Introduction 5 clusters of galaxies are pervaded by hot intergalactic gas whose mass rivals that of the galaxies themselves. 1971: Using very-long-baseline interferometry (VUBl), radio as- tronomers find that in a number of quasars, individual radio components appear to move apart from one another at speeds greater than that of light; there is still no definitive explanation for this phenomenon. 1972: The Copernicus satellite is launched to provide high-reso- lution ultraviolet spectroscopic observations of stars and inter- stellar gas; vast regions of the interstellar medium are shown to be heated to hundreds of thousands of degrees by the shock waves from supernova explosions. 1972: The Gamma Ray Explorer satellite is launched, provid- ing the first detailed picture of the Galactic plane in gamma rays, as well as measurements of the spectrum of the diffuse extraga- lactic gamma radiation; two pulsars are also detected. 1973: Ground-based observations reveal a cloud of sodium gas surrounding Jupiter's satellite lo; later a torus of gas injected by lo into orbit around Jupiter is found, presaging the discovery of Io's vigorous volcanic activity by the Voyager 1 spacecraft. 1973: Observations by the Skylab satellite confirm earlier indi- cations that high-velocity solar-wind streams flow from "coronal holes," solar regions of much reduced x-ray and ultraviolet emission; these streams are later recognized as the source of many geomagnetic disturbances on the Earth. 1974: Radio observations of the pulsar PSR 1913 + 16 show that it orbits a companion star every 6 hours; precise measurements of this binary pulsar later confirm the prediction of the General Theory of Relativity that a binary-star system radiates energy in the form of gravitational waves. 1975: Observations confirm theoretical predictions that the so- lar surface undergoes 5-minute oscillations as a result of waves in the solar interior; detailed measurements of these oscillations later provide seismic probes of solar structure that reach nearly to the Sun's core. 1976: An experiment to measure the flux of neutrinos from the center of the Sun, in progress since 1970, shows that the flux is no more than one third of that predicted by current theory; this finding prompts a searching re-examination of the theory of generation of nuclear energy in stars. 1977: In the first such discovery since the rings of Saturn were discovered in the seventeenth century, rings are found around

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6 ASTRONOMY AND ASTROPHYSICS FOR THE 1980's Uranus. In 1979, Voyager 1 detects the first known ring around Jupiter and in 1980 reveals complex structure in the rings of Saturn. 1978: The Einstein Observatory, carrying an imaging x-ray tele- scope, is launched to acquire the first high-resolution images and detailed spectra of the x-ray sky; these observations provide im- portant new insights into the properties of nearly every kind of astronomical object. 1978: Infrared spectroscopy reveals the expansion of a shell of dust and gas away from a protostellar condensation in the Or- ion nebula; if the expansion is due to the pressure of radiation from a newborn star within the shell, this star must be only 2000 years old, and thus the youngest ever observed. 1978: Radio and x-ray emissions from the Galactic object SS 433 prompt optical studies that reveal that it emits jets of matter at about one fourth of the speed of light; SS 433 may be a small- scale stellar counterpart of the mysterious objects that produce energetic jets in some quasars. 1978: The International Ultraviolet Explorer (lUE) satellite is launched; observations establish temperature scales for hot stars, reveal resemblances between quasars and Seyfert galaxies, and show that mass loss from stars at rates high enough to affect stellar evolution is nearly universal. 1979: Isotopic analysis of cosmic rays shows conclusively that they constitute a sample of matter with a nucleosynthetic his- tory different from that of the Sun. 1979: The most intense gamma-ray burst ever recorded is ob- served by an international network of spacecraft; it is shown to have come from the same direction as that of a supernova rem- nant in the Large Magellanic Cloud. 1979: A multiple quasar is detected and shown to be the re- sult of the splitting of light beams from a single, distant quasar by the "gravitational lens" effect of an intervening galaxy; a second example is found in 1980. These and other key discoveries of the past 20 years must certainly be ranked in significance with those in the decades following Galileo's telescopic observations. They are the direct results of the imaginative application of new technologies for obtaining and recording astronomical information. Indeed, the pace of discovery has been set largely by the rate of advance of technology.

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Introduction 7 During the 1970's, for example, the maturing of space tech- nology and instrumentation opened up the far-ultraviolet, x-ray, and gamma-ray regions of the spectrum to extensive new obser- vations. Infrared astronomy from the ground and from aircraft and balloons made major strides, with orbiting telescopes sched- uled for the decade ahead. The development of highly efficient detectors for optical astronomy permitted the study of extremely faint and previously inaccessible objects with large telescopes and enabled smaller facilities to become major research tools. New technologies for telescope construction employing thin mirrors or mirror segments, ultrashort focal lengths, lightweight support structures, and innovative designs for domes or housings-made it both technically and economically feasible to build much larger telescopes for optical and infrared studies. Computers evolved into workaday tools in both the laboratory and the observatory so that it is now feasible to apply them to data reduction, image pro- cessing, and theoretical studies in research facilities across the nation. The 1980's will almost certainly witness further discoveries to rank with those of recent decades; however, new discoveries do not necessarily lead immediately to a deeper understanding of the Universe. Such understanding, the ultimate goal of astronomy, requires the careful analysis of observations to test their validity and to assess their relationship to currently accepted knowledge. In contrast to observational discoveries, major advances in our understanding of the Universe seldom burst suddenly upon the scientific world, bearing key dates that will ring through his- tory. Rather, they usually require the systematic measurement, classification, interpretation, and study of many objects and phenomena, whose common properties and unifying features may not become apparent until years of effort have been devoted to the task. Our empirical knowledge of the Universe is gained al- most entirely from the laborious, time-consuming collection of the feeble electromagnetic radiations that reach us from very distant objects. Instruments and facilities to follow up and analyze ini- tial discoveries must therefore be more powerful, more versatile, and longer lived than those intended primarily for discovery. They must also be able to return and process new information at more rapid rates, if advances in our understanding are not to lag behind the pace of initial exploration. The gathering of observational data is, however, only the first step in the quest for understanding; these data must then be

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8 ASTRONOMY AND ASTROPHYSICS FOR THE 1980's analyzed and interpreted with the aid of astrophysical theory. The task of theory is to develop physical models for the mecha- nisms that underlie the observed phenomena, to calculate the properties of the models, to test these properties through com- parisons with the observations, and to predict the results of new observations. When such comparisons are favorable and the pre- dictions accurate, our understanding has advanced. The con- struction of astrophysical theories is based on the results from mathematics and the other sciences, particularly physics and chemistry. Laboratory studies are essential to provide the atomic, molecular, and nuclear data needed in theoretical analysis. All of these activities essential to the search for understanding are nurtured in our universities and research institutions, where the astronomical knowledge and skills gained in the past are refined by new research results and transmitted to those who will fol- low. The past decade has seen impressive progress toward under- standing many of the more significant and challenging problems of contemporary astronomy. For example, in the decade be- tween the launch of the Uhuru satellite in 1970 and the imaging and spectroscopic work of the Einstein Observatory, - x-ray as- tronomy progressed from a low-resolution survey of the x-ray sky that revealed a few hundred mostly unidentified sources, to the detailed study of x-ray emission from thousands of astronomical objects of all classes, ranging from normal stars to quasars and clusters of galaxies. The model of x-ray stars based on the the .. . .. . . . . . . . ory ot accretion ot matter onto compact objects such as neutron stars and black holes has provided a possible model for also understanding the vastly more powerful energy sources of ac- tive galactic nuclei and quasars. Another example of impressive progress recorded during the 1970's is our increased understanding of the structure and en- ergy balance of the interstellar medium. Ultraviolet spectroscopy provided by the Copernicus and JUE satellite observatories, cou- pled with x-ray observations from rockets and satellites, has re- vealed that large regions of interstellar space are filled with gas heated to hundreds of thousands of degrees by shock waves from supernova explosions; other ultraviolet studies have shown that mass loss is a feature of normal stellar evolution. These investi- gations, powerfully aided by theoretical and laboratory studies of atomic and molecular properties, have led to a new understand

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Introduction 9 ing of the ways in which stars are coupled to their surrounding environment. In some important areas of astronomy, however, understand- ing has progressed less rapidly, often because critical observa- tions needed to distinguish between alternative theoretical possi- bilities are lacking. One such area is star formation, now known to be occurring in certain regions of the disks of many galaxies, including our own. The phenomena involved are extremely complex. Stars are born in clouds of gas and dust that form at low temperatures and radiate predominantly in the infrared and millimeter wavelength regions of the spectrum. Despite the great improvements in detectors and ground-based instrumentation in these wavelength regions during the 1970's, a new generation of observational facilities both in space and on the ground will be required to furnish the increases in spatial and spectral resolu- tion needed for detailed comparison of regions of star formation with increasingly sophisticated theoretical models. The evolution of galaxies is another area in which our under- standing has been impeded by the lack of observations of suffi- cient numbers of objects at or beyond the limits of present fa- cilities, although the 1970's provided fascinating hints of what we will eventually find. The 1960's furnished clear evidence that we live in an evolving Universe: the detection of the cosmic micro- wave background radiation, believed to be a relic of the big bang, and the discovery that the densities of radio sources and qua- sars increase with distance as one looks back to earlier stages in the history of the Universe. Recent observations suggest that distant galaxies are bluer than nearby ones, providing the first direct evidence for the evolution of galaxies. A better under- standing of the evolution of the Universe will require measure- ments of much more distant and hence much fainter galaxies than are currently observable with our largest telescopes equipped with the best detectors. Only through such measurements will we be able to determine the general properties of galaxies at early stages in their evolution, when these properties were noticeably differ- ent from those of galaxies closer to us. A DECADE OF OPPORTUNITY The Astronomy Survey Committee believes that the programs recommended in this report are those that will most effectively

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10 ASTRONOMY AND ASTROPHYSICS FOR THE 1980's advance both exploration and understanding during the coming decade. These programs address the most significant questions that confront contemporary astronomy. For example: What is the large- scale structure of the Universe? How do galaxies evolve? What role do violent events play in the evolution of the Universe? How are stars and planets formed? What causes activity on the sur- faces of the Sun and other stars? How widespread are life and intelligence in the Universe? And do the connections between astronomy and the nature of the fundamental forces hold the key to a unified understanding of all cosmic processes? Chapter 3 presents a detailed examination of these questions and a discus- sion of how they may be addressed through implementation of the Committee recommendations. The technological developments of the 1970's permit these problems to be attacked now, with a high degree of effective- ness, and at reasonable cost. The maturing of space astronomy, advances in detectors, reduction in cost of computers, and new technologies for constructing telescopes have already been men- tioned; to these developments must be added more specialized advances, such as the refinement of radio Vim; short-wave- length antenna design and fabrication; new approaches to the detection of cosmic rays, neutrinos, and gravitational waves; and infrared interferometry. New facilities incorporating these devel- opments will undoubtedly yield advances in our understanding of the Universe and, like those of the recent past, will produce additional new discoveries of their own. One should also recognize the importance of astronomical re- search as an essential contribution to the scientific and techno- logical vigor of the nation. The entire field has grown explo- sively during the past two decades, and the research activity of trained astronomers has become a significant component of the U. S. scientific enterprise. Maintaining U. S. leadership in astro- nomical research will be an increasing challenge during the 1980's, as other nations continue to develop their scientific capabilities. Research in astronomy has furthermore often had a surprising impact on technology. It was through attempts to understand the orbiting of the Moon and planets that Newton discovered the laws of motion that are the basis of modern engineering. The theory of radiative transfer, the development of which was stimulated by the need of astrophysicists to understand the es- cape of radiation from the outer layers of a star, was later ap

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Introduction 11 plied by engineers to studies of the escape of neutrons from nuclear reactors. In attempting to explain the source of stellar energy, astrophysicists helped to advance our understanding of thermonuclear fusion; astrophysical studies of plasmas in space have contributed to the theory of magnetic plasma confinement, which is applied to the control of thermonuclear fusion for practical purposes. New concepts emerge as astronomy attempts to understand the Universe. Picked up and developed by other scientists, they be- come part of the knowledge that underlies modern technology. Today astronomers are again confronted by puzzling phenomena in space, such as relativistic beams of matter, interstellar ma- sers, and superdense matter. The new concepts that will be re- quired to understand such phenomena will surely have an im- pact on the technology of the future. The programs recommended are those that the Committee be- iieves are most likely to promote scientific knowledge in the coming decade. However, no projections for a decade of re- search can be exhaustive, particularly for an age of discovery like the one in which we live. A different and still more wonderful view of the cosmos will almost surely be revealed to us in the years ahead. The discoveries of our generation have brought us to the threshold of a revolution in physical thought, in which the properties of elementary particles may hold the key to under- standing the early history of the Universe and in which the quantum properties of gravitation, unrecognized in the theories of Newton and Einstein, may play a central role in understand- ing cosmic evolution. Flexibility and openness to new opportu- nities, as well as implementation of the programs recommended here, will be needed to respond effectively to discoveries and developments that cannot now be foreseen.

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A 2-,um infrared image of the center of the Galaxy, hidden by 30 magnitudes of optical extinction. (Photo courtesy of G. Neugebauer and E. Becklin, California Institute of TechnologyJ