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Introduction and Summary The past 60 years have brought about revolutionary changes in our understanding of the universe. For the first time we have been able to measure its size and age, to Took back toward its birth in the very distant past, and to understand its present appearance and past evolution in terms of the laws of physics and chemistry discovered on Earth. The intellectual impact of astronomy, in union with physics, continues today. Recently, we have found that our improved the- oretical understanding of elementary particles, the fundamental constituents of matter, allows us to ask deep questions about the nature of the universe that we could not even formulate a short time ago. Discovery of the underlying relationship between the large-scare properties of the universe and the microscopic laws of physics would be a new triumph of human thought, compara- ble in impact with the Newtonian synthesis or Einstein's theory of general relativity. The extraordinary richness of astronomical phenomena ensures that the age of discovery is not behind us and that "there are more things in Heaven and Earth, Horatio, than are dreamt of in your philosophy." Even though the details of astronomical knowledge change as new discoveries are made, there are three major themes that have guided the astronomer's quest for knowledge for several decades. 1

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2 The same themes, in the judgment of the task group, will continue to express the aims of astrophysics for the foreseeable future. These are to understand the following: the origin of the universe; the laws of physics governing the universe; and . the birth of stars and planets and the advent of life. These goals are vast in scope, challenging in their complexity, and profound in their implications. The experience of the past 30 years clearly shows that obser- vations across the electromagnetic spectrum from gamma rays to radio waves, as well as detection of cosmic-ray particles, are re- quired to study the enormous variety of physical conditions found in the universe. The use of space observations has already revealed the existence of matter in previously unknown forms. Matter at densities one million billion times the density of water has been found to exist in stars at the end of their lives when they contract to become white dwarfs and neutron stars. Similarly, gases at temperatures of tens of millions of degrees fill the space between galaxies and. despite far lower densities ~ont.ain a~ milch m: that in all stars and galaxies together. The next 30 years can be expected to present exciting new opportunities for fundamental discoveries through space-based ob- servatories operating at all wavelengths. The size of these facilities will be comparable with the present scale of ground-based obser- vatories. Major new instruments will peanut us to observe at sen- sitivities that are orders of magnitude greater than any currently available. The program of new initiatives for the era 1995 to 2015 fo- cuses on improvements in capabilities in two areas: higher angular resolution and greater collecting area. Over the past century, astronomy has forged increasingly strong links to all other branches of science. There are clear con- nections between the scientific objectives of the terrestrial, plan- etary, solar, and astrophysics programs. The search for life in other solar systems is a common goal; the prospects of detect- ing gravitational radiation have importance for physics as well as astrophysics; the study of the birth and chemical evolution of stars and planetary systems has a direct connection not only to the origins of life but also to the fundamental state of matter as , _ ~_ i_ ~ ,,~.v^_~, _~.,^ ~ - A,4~ - ,~ I ~O

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3 it existed in the first few moments after the universe began its . exp oslve expansion. Astrophysics, like most of the basic sciences, is international by nature. The United States has held a leading position in this field, innovating in using new technology to construct highly sen- sitive instruments both on the ground and in space. In turn, the success of our scientists has inspired the best scientific and engi- neering talents to join the space effort. Cooperation with scientists of other nations has also been a considerable source of strength. The task group's plan for the era 1995 to 2015 recognizes that in- ternational cooperation will be crucial for optimizing the scientific return. THE IMPORTANCE OF ADVANCED INSTRUMENTATION IN ASTROPHYSICAL PROGRESS Modern astronomy traces its beginnings to 1609 when Galileo first looked at the heavens with a spyglass-an instrument that had been invented only the year before. That one invention drasti- cally changed both the direction and the pace at which astronomy was to be conducted. Increasingly, astronomers came to utilize ad- vanced observing techniques in all accessible parts of the spectrum. The most striking recent advances have depended on instruments designed to detect radio waves, infrared emissions, and x rays as well as the even more energetic gamma rays. These instruments have permitted the discovery of quasars, pulsars, neutron stars, whole galaxies that emit the bulk of their radiation as x rays, and other galaxies that emit most powerfully at infrared wavelengths. They have even let us "hear" the ubiquitous radio background ra- diation reaching us from the most distant portions of the universe ever observed attesting to an intensely hot, explosive origin of the cosmos. When we examine the history of technological advances for monitoring all wavelengths of radiation, we see a rapidly expand- ing observational capability in the post-WorId War II era. In the wake of this expanding front of technical competence we see new discoveries crystallizing with amazing rapidity. Astronomers first discovered quasars at radio wavelengths around 1963 through the use of instrumentation unavailable just four years earlier. X- ray stars and galaxies, discovered in 1962 and 1966, respectively,

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4 were first detected with instrumentation not yet invented in 1959. Novel, more powerful instruments were crucial to these discoveries. Recognizing the importance of new and more powerful ob- serving capabilities, the National Aeronautics and Space Admin- istration has planned a farn~ly of observatories, to be launched in the new few years. Under this Great Observatories in Space program, NASA expects to launch the Hubble Space Telescope (HST), the Gamma-Ray Observatory (GRO), the Advanced X-ray Astrophysics Facility (AXAF), and the Space Infrared Telescope Facility (STRTF). These Tong-lived, orbiting observatories should be active throughout the 1990s. The extensive wavelength cover- age that they will provide spans most of the spectrum accessible solely from space. Their sensitivity will be far higher than any available to date. Their spectral resolving power will advance in- strumental capabilities into realms unreachable before. With such powerful technological innovations, this family of great observato- ries should continue to uncover new phenomena whose significance will match the greatest discoveries of past decades. Experience shows that we have been most successful in ex- plaining complex cosmic phenomena when we can observe across the entire wavelength range. Only in that way can a variety of dif- ferent processes underlying a given phenomenon be distinguished from each other. Each process typically has distinctive signatures at different wavelengths: When we observe the Milky Way at optical wavelengths, we see a steady stream of light from stars. Infrared telescopes sense the heat emitted by interstellar dust and permit us to trace the distribution of dust clouds throughout our galaxy. X-ray telescopes, in contrast, register x-ray pulses emitted by compact binary stars, while radio telescopes present us with a map showing the distribution of hydrogen clouds pervading the interstellar spaces. Finally, a gamma-ray telescope scanning the Milky Way would observe occasional powerful bursts of gamma rays from unpredictable directions at intervals of a few weeks. We do not yet understand the cause of these bursts, which last only a few seconds. ASTROPHYSICAL GOALS The time scale of the universe is best thought of as being governed by a peculiar kind of clock the logarithm clock. Like a stopwatch, its face is divided by tick marks into 60 intervals. But,

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5 instead of going around at a steady pace, the pointer runs 10 times slower at each successive tick mark that it passes. It starts off at a fantastic whirl. After passing the first 2 tick marks, it has already slowed by a factor of 100. By the time 10 tick marks have been passed, it is moving 10 billion times more slowly. But its initial speed is so great that 43 tick marks have to be passed before the hand moves one tick in 1 second. The next tick mark thereafter is reached 10 seconds later, the following tick takes a 100 seconds, the next 1000 seconds, and so on. The formation of the Earth and life as we know it dates to the fifty-ninth tick mark. The pointer today points somewhere between the fifty-ninth and sixtieth marks. It will take tens of billions of years before the sixtieth tick mark is reached. This clock can help explain the importance of the earliest instant in the life of the universe. As our clock advances, the universe expands. With every sec- ond tick mark passed, the universe expands its dimensions by a factor of 10. Passage of 10 ticks of time on the clock increases the dimensions of the universe one hundred-thousandfold. Simul- taneously, the temperature of the universe the energy of the particles that it contains drops a similar amount, one hundred- thousandfold for every 10 ticks of the clock. The importance of this temperature drop is that the nature of physical interactions depends strongly on the energy of the interacting radiation or matter. A change in particle energy by a factor of 10 can lead to dramatic changes in the types of particles that the universe contains, in the interactions that these particles exhibit, and in the laws of physics that such matter obeys. To understand how the universe today evolved from its initial state, we must trace back to these earliest times. In part, we can do this by using powerful telescopes that look far out into the universe. Optical and infrared observatories in space, like HST and STRTF, will look back in time to the era when the galaxies first formed. They will be able to analyze, with their powerful spectrometers, what the chemical constituents of the universe were at that time- whether hydrogen and helium were the only constituents present or whether some lithium and deuterium also existed. Measuring the abundances of these rare substances at those early times would provide us with a sensitive determination of the density of ordinary matter in the universe at the time when helium nuclei were formed. By inference, it would also determine the nuclear matter density in the universe today.

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6 Determining the present density of nuclear matter is impor- tant for understanding the fate of the universe. The masses com- puted for galaxies from observations of their rotations and mutual attractions systematically exceed the total mass of stars, gas, and dust that we can observe in these galaxies. So striking is this dis- crepancy that we now talk of "dark matter" an unknown form of matter perceived only through its gravitational attraction, oth . erwlse invest ~ e. With AXAF we will be able to probe the universe to an even greater depth. The distribution of gases at temperatures of millions of degrees in the halos that surround some galaxies can be used as a sensitive probe of the distribution of the gravitational force field and thereby also of the distribution of mass. HST and STRTF will also be able to examine this mass distribution. They will look, respectively, at the gravitational pull exerted on faint populations of stars in the outer reaches of galaxies and at the populations of Jupiter-sized objects evident only through the infrared radiation they emit. AXAF and GRO may also be able to discern the x-ray and gamma-ray emission from the earliest quasars formed. We may be able to learn whether the first large condensations to take shape were quasars, individual stars, or galaxies. THE NEXT GENERATION OF POWERFUL OBSERVATORIES IN SPACE While the family of great observatories will undoubtedly pro- vide substantial advances in our understanding of the universe, the task group foresees still further instrumental advances to help resolve a number of fundamental problems. We need to understand the formation of the solar system and of the planets. We need to learn how prevalent planetary systems are in the galaxy. To date, we know of no other system that much resembles ours. But recent infrared observations by the In- frared Astronomical Satellite (IRAS a joint U.S.-Dutch-British project) and from ground-based observatories have provided evi- dence of some stars surrounded by disks of dust and of others with companions not much more massive than the planet Jupiter. While STRTF and HST will provide us with information on preplanetary disks surrounding nearby stars, and will even be able to detect massive planets surrounding them, large interferometers,

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7 or an extremely large single mirror, placed in earth orbit toward the end of the century, could allow detailed investigations of such planets. These observations could open the way to a search for life on other planets. Such interferometers-high-spatial-resolution instruments- would also permit the probing of the central engines of quasars, the most luminous objects so far discovered in the universe. Quasars eject masses of material at velocities that at first sight appear to exceed the speed of light. Their brightness can vary suddenly from hour to hour, indicating an enormously compact central en- ergy supply that powers these enigmatic sources. Both optical and infrared interferometers of the future could achieve angular resolu- tions measured in microarcseconds a resolution that would allow someone on Earth to distinguish the features of a man standing on the Moon. We will also be able to launch massive cosmic-ray detectors, for a detailed analysis of this most energetic form of galactic matter, possibly even searching for antinucleons that would herald the possibility of a symmetric universe in which galaxies of ordinary matter and antigalaxies consisting entirely of antimatter could coexist.