19840014449.Pdf

I TH ADVANCED X-RAY ASTROPHYSICS FACILITY// 'Observing _ the Universe inX·Ra~s-_------- ;1J IT 5 f7 -£P - I r I JOHN F. KENNEDY SPACE CE TER_L,'~~ LOCUMENTS DEPART E NI\S/\ REf ERE E~ -Y National Aeronautics and Space Administration By Dr. VaLerie NeaL in collaboration with the Marshall Space FLight Center, the Office ofSpace Science and AppLications and the Smithson ian AstrophysicaL Observatory. AXAF For sale by the Superintendent ofDocuments U.S. Government Printing Office, Washington, D.C. 20402 EP-199 Foreword s the last decade astronomy came of age, rich the heavens. Advanced optical ended, an inno with promise for improved and radio observatories (Space vative orbiting understanding of the universe. Telescope and the Very Large telescope was Now X-ray astronomers are Array) are already being devel focusing X-rays digesting data collected a few oped. The highest priority new Afrom distant celestial objects years ago, but little new infor astronomy program is the into images of unprecedented mation is being gathered. Our Advanced X-Ray Astrophysics clarity and detail. Suddenly we view of the invisible X-ray uni Facility (AXAF), an orbital could see more X-ray sources verse is fixed until a new and observatory for detailed, long than we ever imagined to exist, keener eye is opened. term study of X-ray emissions and we saw the universe as a The national strategy for and the phenomena that produce stage for dramatic high-energy astronomy research during the them. The AXAF is an essential events that cannot be witnessed rest of this century is a coordi step in our continuing quest to by ordinary astronomy. X-ray nated, broad spectrum study of understand the universe. 0 In this sixteenth-century engraving, the mythicaL figure AtLas bears a modeL ofthe universe as it was then known. The four eLements - earth, air, fire, and water - are sur rounded by the moon, sun, pLanets, I~;-....o_ Ii~~~ and heavens. The circles bounding Hie canet errante Lunam>Soliftf; /abores the universe represent the ecliptic, .,.,lrEluruq;,pluuiaJq; byad.gtinifq;.trioes equator, tropic and poLar zones, and the zodiac. The Universe Imagined e are stargazers. ogy and philosophy. As we Since ancient found ways to improve our view times, we have of the sky and to look deeper studied the heav into space, our images of the ens, cataloguing universe changed. By the early the stars, noting seasonal varia seventeenth century, mathemat tions in their positions, record ics and the telescope forced us ing phases of the Moon, to abandon our view of an watching the Sun, devising cal Earth-centered universe and endars, and navigating by points acknowledge the Sun as central. in the sky. We have sought to We later realized, however, that understand the mysteries of our our Sun was but one star in a universe, its organization, ori vast galaxy of stars, and that our gin, and fate. From our observa Galaxy was not the only one in tions, we have constructed space. We found ourselves in an models of the universe and of inconceivably vast universe of our place in it. galaxies, each one an "island Early concepts of the universe universe. " blended fact and myth; astron We recently realized that the omy was inseparable from theol- universe is not a static, stable place. Modem scientists show us a dynamic, expanding, cataclys mic universe that spans unima ginable time and distance. The age-old image of a quiet, unchanging universe has been supplanted by the concept of a cosmos filled with turmoil and violence on a grand scale. Virtually every advance in astronomical theory or tech nique stimulates a burst of dis covery and subsequent revision of the reigning model of the uni verse. New instruments, such as telescopes and spacecraft, inev itably lead to discovery. New techniques of detection enable us to see and understand some thing that was hitherto invisible. The history of astronomy, and more recently of astrophysics, is the history of bringing the uni verse into ever-sharper focus. Since spaceflight became possi ble a mere twenty-five years ago, we have seen farther into space and learned more about the universe than we did in all of human history. NASA has played a major role in developing new instru ments for use in space and opening new windows to the ~ universe. The results of our , recent discoveries are a host of I' unanswered questions and a , continual revision of our theo- ' According to Aristotle and Ptolemy, the center ofthe universe was the ries and models. The universe ~ Earth, orbited by the moon, sun, and five planets. The ancient Greeks .l looks even vaster, more com- j D. imagined the universe as a series ofconcentric, transparent spheres plex, and more mysterious . ," enclosing the planets and stars. The outermost "primum mobile" con than we ever imagined. 0 I '" trolled the motions ofall the spheres. This model prevailed in Western A ': ,.11~..,j' ~r' thought from antiquity until the Renaissance. 2 L-..--------------~QJi__iiilliiIii_. Left: When the teLescope reveaLed the muLtitude ofstars and the vast ness ofthe heavens, the universe couLd be imagined to consist of many systems simiLar to our own soLar system. In this iLLustration, pubLished in 1750, the universe no Longer appears bounded. Left: In the Copernican modeL ofthe universe, the centraL sun was sur rounded by the pLanets and the reaLm offixed stars. Right: Tycho Brahe's sixteenth-century modeL showed an immobile Earth at the center ofthe universe while aLL the other pLanets orbited both the Earth and the Sun within a boundary sphere offixed stars. 3 The Universe in X-Rays e learn about the nature of our universe. The uni Right: Observations in different universe by study verse as we know it now is regions ofthe electromagnetic spec ing the quality punctuated by explosive phe trum yield quite different pictures of and quantity of nomena associated with the celestial objects. By looking energy emitted by birth and death of stars. When through more than one of these Wcelestial objects. For most of cosmic explosions occur, gases "windows," we gain more history, astronomy has been the are heated to millions of degrees complete information about the uni study of visible starlight. Visible and matter is accelerated to ex verse. Because X-rays do not pene light, however, is only a very tremely high velocities. Much of trate the Earth's atmosphere, X-ray small fraction of the electromag this explosive energy is released images must be obtainedfrom tele netic energy emitted by celestial as high-energy radiation, X-rays. scopes in space. sources. The rest consists of Because a tremendous radiowaves, infrared and ultravi amount of energy is required to olet radiation, X-rays, and create X-rays, we know that gamma rays. wherever they are observed The Earth's atmosphere is a something violent has hap veil that absorbs most of this pened. X-ray emissions carry radiation before it can reach detailed information about the telescopes and detectors on the temperature, density, age, and ground. The atmosphere is trans other physical conditions of the parent only to visible light and celestial objects that produce some radio emissions. Higher them. The study of these emis energy radiation, such as X-rays, sions (X-ray astronomy and does not penetrate and thus can astrophysics) provides insight not be detected from the ground. into many of the fundamental The space program has revo physical processes that govern lutionized astronomy by placing the universe. observational instruments out The sources of copious side the filtering atmosphere. X-rays are some of the oddest During the past three decades, and most fascinating objects in we have been able to look at the the sky. They include spinning universe through these new neutron stars so dense that a "windows." The X-ray region drop of neutron material can of the spectrum has proved to be weigh as much as the combined a very rich source of informa weight of all the people on tion, full of surprises about the Earth. Through X-ray observa- A map ofthe sky in galactic coordinates shows approximate locations ofabout 325 X-ray sources known in 1978. In the 2'/2 years ofHEAO operations. the number of identified X ray sources grew dramatically to almost 10,000. Virgo Super Cluster? NGC 6624 4 tions, astronomers have detected the sky. the universe open or closed, for and studied what may be one of Results from X-ray surveys to ever expanding or not? the remains of stellar explosions date have been spectacular. We Stunning as recent X-ray the black hole, an object that have located nearly 10,000 X-ray observations have been, they has collapsed, leaving a "hole" sources in our Galaxy and else amount to a mere glimpse of the in space from which nothing, where, as well as a background universe compared to astronom not even light, can escape. glow of mysterious origin. We ical observations in other Other X-ray emitters are galax have obtained suggestive evi regions of the spectrum. There ies and the mysterious quasars, dence for the existence of black are probably many more sources the most powerful energy holes. We have recognized the of X-ray radiation than we have sources in the universe. variable, fluctuating character yet had the opportunity to see, Since the first X-ray observa of phenomena in our universe. and no doubt they are rich with tion in 1962, a five-minute The X-ray sky is an intricate new information about the uni glimpse from the tip of a rocket, and everchanging mural.

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