Astronomy and Astrophysics for the 1970s: Volume 1: Report of the Astronomy Survey Committee (1972)

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CHAPTER THR EE Astrophysical Frontiers COSMOLOGY For centuries man has struggled to gain a broader and deeper under- standing of the world around him. Studying forms of life. he has come to appreciate. on the one hand, the fantastic diversity of living things and, on the other, the extraordinary similarity of the minute cells that constitute all organisms. Digging into the eanh. he has discovered layer after layer of rock, laid down by unseen processes in the remote past. Peering through the telescope, he has probed far beyond the planets to find billions of stars just like the sun, clustered into vast galaxies, which themselves stretch without number into the depths of space. The earth and living things upon it evolve. Each year there are small changes in the landscape as sea, rain, and wind reshape the earth. Each year there are imperceptible changes in the species as genetic mutations propagate into new generations. Thus change and evolu tion are the basic themes of geology and biology. The early astronomers perceived an opposite tendency--the apparently unchanging quality of the stellar universe. But in the twentieth century when astronomers began to apply the laws of physics to the stars, they re- alized that the prodigious energy stars emit cannot be sustained in- definitely. Even with the most efficient nuclear power. based on the fusion of hydrogen into helium nuclei, stars like the sun can shine at most about 10 billion years. In spite of the apparent steadiness of a star's light. evolu- tion must be occurring as the structure of a star responds to the loss of its energy supply. So astronomy has followed biology and geology in perceiv- 12 

Astrophysical Frontiers 13 ing the importance of evolution. Even the time scales-billions of years- suggested by the astronomers and geologists for the evolution of the earth and stars are similar. The great optical telescopes of the Western United States have shown that space is filled as far as they can see with galaxies rather like our own. Too faint to be seen with the naked eye, the most distant ones at 5 billion light years take many hours to register on film even with telescopes 10 mil· lion times as sensitive as the eye. The galaxies are cities of stars, crowded together in nearly empty intergalactic space. They appear to contain most of the matter and to be the building blocks of the universe. But we do not really know, yet, how much matter may be found between them or whether galaxies are still being formed. When Slipher studied the spectra of galaxies in the 1920's, he was astounded to see that the spectral lines of most galaxies were shifted to the red, indicating that these galaxies seemed to be moving away from us. Why would the galaxies recede from us? From the time of Copernicus. man had learned to suspect the explanation of any phenomenon that re- quired the earth to be the center of things. Unknown to the astronomers working on this problem in the 1920's, a young Russian mathematician, Friedmann, was constructing a model of the universe based on Einstein's theory of general relativity. The model, he reasoned, should be uniform, with equal numbers of galaxies everywhere. Otherwise, there would be a center, contrary to the Copernican principle. But when he applied Ein- stein's equations, he found, as did Einstein, that his model universes could not sit still. Friedmann's models expanded, either indefinitely or for a finite time, after which they collapsed to a point. The reason for this behavior is gravitation-the basic force in Einstein's theory-which continually tries to pull the galaxies back on themselves. The only way this tendency can be withstood is to give the universe a mighty kick outward at the beginning. Whether the universe expands forever (open models) or eventually collapses (closed models) depends on the degree of gravitational pull and. hence, on the mean density of matter. Friedmann predicted that all the galaxies should be observed to be moving systematically either toward or away from us (contraction or ex- pansion, depending on the time elapsed). Furthermore, he pointed out that to preserve uniformity, the more distant galaxies must move faster with respect to us. Lemaitre, Robertson, and Tolman, among others, had developed various predictions of general relativity. Following Slipher's pioneering efforts, the extensive new data on velocities and distances of galaxies as- sembled by Hubble at Mt. Wilson supported the theoreticians of general relativity. Not only are all the galaxies moving away from us, but they obey 

14 A STR ONOM Y A N D A ST R O PHY SI CS FO R Til E 197 o ·s .· . . .. • - -· ••• • •• •• eo• • -· «• ·. • • 't I • .• r • ·= •• . .. ... -· • '"«6 • A ..... .· ' ·:,~. ........,.... . .. •• • .~· . .. ., ·.> ,..... ·····- ...... • ·-· .. . -------------------------------------~--~ • • • • • • • Undentaoclinc obJerntiom ortc:n requires numerous c:alculationt. Here rwo views or a computtr~nr.ratcd model or an encounter berw«n rwo pluies are c:ompued with a photo~ph of NCC 4676. (l'ltoto courttty ofNASA l•stltutt o{Spot< Studia •ltd Hal~ ObJttn•torlr.t. ) 

Astrophysical Frontien 15 Friedmann's predicted proponionality between distance and red shift. For many years Hubble and his collaborator, Humason, extended these measurements with many instruments and techniques. Successors verified this relationship with great accuracy out to enormous distances (5 billion light-years) and red shifts (about 0.5, corresponding to near ly half the speed of light). The velocities are more easily determined than the dis- tances : with the present scale, front the proportionality constant in the Hubble law, one can calculate an age of 10 billion to 20 billion years. If no other forces were acting, this would be just the time for the observed gal- axies to attain their current distance at their present speeds. Actually, be- cause gravitation continually slows them down, the age of the universe is somewhat less. In any case, the time scale of biUions of years is com par· able with that of the age oflife, ofthe earth, and of the stars. Much of what we now know about the galactic universe is consistent with Friedmann's pred ictions. The galaxies are distributed uniformly. they move apan, and they almost always obey Hubble's law relating velocity and distance. Some exceptional peculiar galaxies seem not to obey the Hubble law, having quite different red shifts from galaxies quite close to them in the sky. This puzzling phenomenon defies explanation at present. Astronomers using the largest optical telescopes have therefore been straining to determine which of the Friedmann models describes the universe. Do we live in a closed universe. with an inevitable collapse scheduled for the future. or are we in an open universe, which will eJ<pand forever? Or do we live in a steady·slate unh·erse, quite different in nature from other relativistic models? Because of the finite speed of light. as one probes deeper into space, one is looking back into time. One therefore sees galaxies moving as they did in the past, and that motion is sUghtly different for different models. Un- fortunately, these eJfeets are discernable only for red shifts of the order of unity and, hence, for speeds approaching that of light, and there just are not enough galaxies of large red shift observed to discriminate between rival cosmologies. Construction of one or more additional large optical telescopes and instrumenting all large telescopes with fast electronic sys- tems will permit progress on this problem. Radio aS1ronomers in the 1950's discovered distant, strong radio sources like Cygnus A, which is at a distance of 500 million light-years. Because of great improvements, present radio instruments can detect sources like Cygnus A at enormous distances. The Friedmann model pre- dicts a maximum distance of roughly 10 billion light-years-the distance light can go in the age of the universe. Objects at that distance would have extremely large red shift and could be useful for determining the model of the universe. Unfortunately. even though tens of thousands of faint rad io 

16 ASTRONOMY AND ASTROPHYSICS FOR THE 1970's sources have been pinpointed. relatively few of the radio galaxies among them have proved to have large red shifts. This may be beeause most of them are intrinsically faint and rather close. On the other hand. there is some controversial evidence that the number of distant radio galaxies is smaller than one would predict from unifonn density-it is as if, in the evolution of the universe. radio galaxies simply did not exist before a cer· tain cosmic time. A subclass of radio sources, the quasars. is much more puzzling. Ex· tremely small in comparison with normal galaxies. these objects are ob· served to emit large fluxes of radio and optical energy and in some cases infrared and x radiation as well. Generally. they have large red shifts-up to nearly 3 in one case. If they are at the huge distances indicated by their red shifts (a.s supported by observations of one quasar in a group of gal· axies having the same red shift). they must be emitting unprecedented amounts of energy. The physics by which such immense energy would be released is completely obscure but may be connected with violent events in ''ery massive general·relativistic configurations of matter. Some scientists believe that the quasar red shifts are not of cosmological origin. noting that some quasars seem to be located on the sky close to ob· jects believed to be at relatively small distances. One quasar is connected by a luminous bridge to a relative.ly nearby galaxy with a different red shift. It is difficult with this view to explain the large observed red shifts. If they are Doppler shifts. i1 is strange 1ha1 1hey are always posilive. Un· known and exJraordinary processes must operate in order 10 accelerate huge masses of matter to relativistic speeds. Whether quasars are cosmological or not, they pose serious problems for current physics. It is possible that solution of these problems will re· quire almost revolutionary new ideas in cosmology. In 1965. a truly sensational breakthrough occurred in cosmology. Sci· enlists of Ihe Bell Telephone Laboratories. in attempting to eliminate all sources of noise fro m a sensitive radio telescope. concluded after a year of effort that a very faint signal from space was confounding their best effons. They estimated its intensity at their operating wa,•elength of 7 em 10 be equivalenlto that of a blackbody of J K. It seemed 10 come equally from all directions. as expected for a cosmological effect. Hearing of this discovery. scienlists recalled that Gamow. a cosmologist. had predicted such an effect in the 1940's. Reasoning that if t.he early phases of the uni· verse were very dense, they likely were hot. we would expect the radiation from 1he primordial fireball to still be visible today as we look back in time to 1he "big bang." as Gamow called the fireball. A key point is the spectrum of the radiation. Gamow predicted lhat it should be truly blackbody, like 1hat of a star or an incandescent ligh! 

Astrophysical Frontier3 17 bulb. So far this has been verified with roughly 20 percent accuracy in the region from 21 em to 2.6 mm, although some discrepancies may have been observed in the far infrared. The best estimate of the temperature is 2. 7 K , and the radiation is uniform in all directions to within 0.1 percent, as predicted. Discovery of the cosmic blackbody radiation has given a tremendous impetus to cosmology. If this radiation was produced in a big bang, it may have last interacted with matter when the universe was only 100,000 years old and only 1/ 1000 of its present size. At that time, matter had a tem- perature of JOOO K. We can think of the radiation as propagating from a "cosmic photosphere," rather like the surface of the sun but, because of the expansion of the universe, receding from us at a speed differing from the speed of light by only one part in a million. If it were not for this recession, and the consequent red shift of the photons from the visible to the radio range, life on earth would be impossible because of the intense heat from this fireball. Unfortunately, the differences in the spectrum of the cosmic blackbody background associated with different Friedmann models are extremely small, so there seems to be little hope of determining the correct cosmological model by such observations. On the other hand, nuclear physicists have shown that the temperature of the radiation is related to the nuclear reactions that would have taken place in the fireball. Indeed, subject to certain qualifications, they predict that about a quarter of the hydrogen should be converted to helium if the radiation temperature is 2.7 K. The sun and most of the nearby stars contain about that amount of helium and so does the interstellar gas. both in the Milky Way and in nearby galaxies. The trouble is, some of this could have been produced i.n stellar interiors, and what is needed therefore is an assessment of the helium content of old stars born at the same time as the galaxy. So far, the indications here are contradictory, and further artacks using optical methods are necessary. While cosmic blackbody radiation is of great significance, recent ex- periments have cast some doubt upon the most straightforward in- terpretation. Difficult measurements are now in progness at wavelengths of I mm and below, where atmospheric attenuation makes it imperative to use balloons and rockets. Until these are completed. the cosmological interpretation is uncertain. Another phenomenon of cosmological interest is the diffuse x-ray background. now observed over a wide range of energy, from 0.1 to 1000 keY. The spectrum in the 1-1000-keY range is described by two power laws joining at about 10 keY. An additional component below I keY may be due to emission from a hot gas at 3,000,000 K. As yet the location of 

18 ASTRONOMY AND ASTROPHYSICS FOR THE 1970's this gas is unknown-for example, it could originate in our own galaxy- and detailed maps made from large spacecraft will do much to clarity this. One hypothesis is that it is distributed uniformly in intergalactic space, where it is heated by the fast panicles ejected from quasars. If this is so, it is of great importance, because the required density of gas is calculated to be about equal to that which would close the universe and yields a mass that is a factor of 30 more than seen in galaxies. As a result of these discoveries. the outlines of a possible evolutionary histoty of the universe are becoming visible, although, as we have pointed out, some puzzling phenomena remain. First, there was a big bang about 10 billion years ago, which flung matter out with tremendous speed. During the first few minutes it was so hot that part of the hydrogen fused to helium, which is still seen in our galaxy today. After 100,000 years, the gas had cooled to about 3000 K, and the radiation that we now detect as the cosmic blackbody radiation was launched on its way. As the gas cooled, clumps were then drawn together to form galaxies; among them our own Milky Way was born. Some of these galaxies exploded, and by looking back in time with large radio telescopes we can now detect these events as quasars and radio galuies. The fast particles that they ejected heated the intergalactic gas that still remained from the big bang and that may provide the bulk of gravitation of the universe. The x rays from this gas may be what we see today. As each galaxy settled down. generations of stars were born, among them the sun about 5 billion years ago, which was about 5 billion years, at least, after our galaxy was formed. Countless planets came into being. Geological processes churned up the young planets, and on our small, rocky world , with water and an atmosphere, eventually life emerged about 3 billion years ago. Ufe evolved, here and perhaps elsewhere, to a state of intelligence. Through the vast reaches of space and time, part of the matter of the universe has evolved into living matter, of which a tiny part is in the form of brains capable of intelligent reasoning. As a result, the universe is now able to reflect upon itself. In this respect, at least, the whole evolutionary chain of events is endowed with meaning. We see a major task for astronomy in the next decade to test this broad picture of the evolution of the universe critically at evety point. Of course, flaws may be found, and even the main concepts may be found to be seriously in error. There are critics who find that the available data move them to suggest more radical cosmologies. Some question the in - terpretation of the red shifts themselves. Others argue for creation of new matter in explosive small objects. Intergalactic matter has not yet been found, but it may be detected through its x-ray emission. Appa.r ently 

Astrophysical Frontiers 19 recently formed galaxies, or even groups of galaxies, do exist. Perhaps some type of "new physics" may ye. be required to understand the t universe of cosmic rays, radio sources, and quasars. But ,.•ith or without such an upheaval, the cosmological questions remain of central importance. Even if t.h e basic picture survives, we are left with an extraordinary puttle. Earlier. we spoke loosely of the big bang-the fireball at the beginning of time. Einstein's equations tell us that matter was extremely dense and hot at that moment-so much so that the equations themselves break down. Basic problems of particle physics and gravitation remain to be solved before we can describe adequately what happened at that moment and understand why there is no meaningful way to discuss what preceded it. In the final analysis. the same questions of beginnings and endings will remain. The contribution of cosmology may well be simply to push back the limits of our ignorance to the basic problem of creation itself. THE SUN Among all the astronomical objects, the sun occupies a very special place for mankind. As the prime source of our energy and light. it is in· dispensable to our existence. The earth, moving in its orbit. is immersed in the outer atmosphere of the sun. Solar physics has. therefore. a practical aspect not shared by all areas of astrophysics. Within astrophysics, the sun occupies a unique position as the closest and, therefore, brightest star. It can be studied in far greater detail than any other star. Knowledge gained from studying the sun therefore often has direct application to the un- derstanding of the rest of the universe. Observations of the solar chromosphere, solar Oares. and magnetic fields stimulated the search for these phenomena in the stars. Analogs have been found, and in some stars the effects are present on an enor- mously greater scale. The solar cycle has parallels in many stars, and measurements of the intensity of chromospheric activity lead to a method for age-dating stars. The magnetic fields in neutron stars are enormously large and play a fundamental role in the plasma physics ofthese objects. Major areas of physics have found important applications in solar phys· ics and have developed in new directions because of this interaction. The interpretation of the complex absorption line spectrum of the sun led to the theory of complex spectra and determination of the selection rules and the effects of magnetic and electric fields that culminated In quantum mechanics. 

20 ASTRONOMY AND ASTROPHYS ICS FOR THE 1970's The solar corona photographed in visible light and x rays- a montage showlng the rela- tionship between ground- and space-based observations. (Photo coul'lfiY ofAmeriam Scl~nu and Engineering, Inc., and High Altilude ObserWitory,) The seeond most abundant element in the universe was actually dis· covered through spectroscopy of the sun and consequently was named helium. Today. the question of distribution of helium in the universe is one of the most important cosmological questions. The best present·day 

A:stroplrysical Fronti~r$ 21 determination of helium abundance comes from a c~mieal analysis of the composition of the solas wind and cosmic rays by means of satellites. Abundance determinations from lhe solas specuum. verified by the chemical analysis of lhe solas corpusculas radiation. gi-res t~ tool for a similar analysis of stellar specua. In particulas, isocopic ratios. seldom determinable for stars. are measured in lhe solas wind. Why is the c~mical composition of a stu important? Because it allows us, provided we know the stellar mass and radius. to determine the energy production rate in the stu's nucleas furnace and to predict the future development and life of the star. The sun's energy production rate is known from the energy rcceived at the earth. l n addition. Its mass. radius, and chemical composition are also well determined. Thus the sun serves as a useful check on theories of stellar structure and evolution. Recently. an excitina new observational checlc of the theory of solar structure has been developed. Deep in a mine in South Dakota. physicists have been able to capture lhe elusive solas neutrino radiation with what must be the most remarkable teleseope in existence, a JO.OOO.aallon tank of cleaning Ouid. Such an exocic detector is needed beeause of t~ low interaction rate of neutrinos with matter; they pass almost unimpeded through the enormous mass of sun and eanh. Occasionally. however. a neutrino reacts with a chlorine atom in t~ clunina Ouid and causes a measurable nuclear transmutation. The cleaning Ouid is hoosed deep in a mine to avoid accidental tra.n smutations from stray cosmic rays. Because the few neutrinos that are captured are generated as a consequence of the nuclear reactions that produce eneray in the solar eore. they give a direct measurement of the structure of the solar interior. This check shows that the sun produces six times fewe r neutrinos than theory had predicted. T he consequences of this experiment for solar physics are great. Either the central temperature is lov.-er than expected, or the "-eak-interactlon theory of panicle physics is called into question. Ever since Carrington in 1859 observed the lim solar eruption, solar Oares have been studied intensely. We now know that the fantastic energies released in Oares must ha,.., been bo«led up in lhe maanetic fields of the solar active regions in which Oares always occur. The source of this eneray is thouaht to lie below lhe solar surfaee, perhaps inside sunspocs. It takes aboot a day to store enoogh eneray to cause a large solar Oare. The energy release. which takes plaee in ooly minutes, is a dramatic example of a nonthermal phenomenon in astrophysics. Durina the explosive release, maanetic fields are apparently annihilated; and the resulting large electrical fields accelerate electrons. protons. and o ther charged panicles to very hiah velocities corresponding to energies up to and sometimes exceeding a billion electron volts. 

22 ASTRONOMY AND ASTROPHYS ICS FO R Tit£ 1970'• The generation of cosmic rays by the sun is such an improbable t'\'cnt that it might nt\·cr have been imagined had h not actually been observed. The convcnion of the disorganized slow motions of gu in Ihe convec:~t:h•e layer of the sun into the ordered motion of a ft"W panicles with vetocities close to that or light stands as one of the most uncx-pecced natural events in the univtrse. Solar physicists are fascinated by the complex physical processes that cause the a«eleration of protons and electrons to cosmic· ray energies. The study of particle events in the sun opens up the possibility of understanding similar but more energetic phenomena throughout the universe. Interaction of fast particles with the plasma and with the imbedded magnetic fields produces a wide variety of phenomena that tire interesting ftom the s tand point of plasma physics and, mon."<::ver, offer us the opportunity to interpret comparable events in more distant and less·well·resolvcd objects. This interaction bet-ween plasmas and magnetic fields Is well observable in the very-short· and very·long·wavelength regions of the solar spectrum. With the availability of satellite observatories outside the earth's at· mosphere. radiation from solar flares has been obsr.rved with wavelengths as shon as IO·l and as long as 1012 -\. This wide sp«tral distribution of flare radiation is: possible because of t.ht. very nonthermal nature of solar OaJU. The • · and gamma·ray radiation between 10 and 0.001 J. is caused by Ihe impac1 of tbe very fast solar electrons wi1h 1he dense plasma of the lo-"Cr solar almosphere. This impact radiation is very impulsive. the radiation showing Ouctuations as rapid as 1 St't' and perhaps shorter. Apart from x and gamma rays, these impacts should also c.au,s.e a very energetic but as ye1 unobserved neutron radiation. The accelerated electrons interact ~·ith their environment also in another way. The loc:aJ magnetic fields make them travel in spirals centered on the magnetic field Hnes. The resulting accelerations are. the origin for part or the long· wavelength radio radiation. The impact of the fast panicles with 1he plasma and Iheir interaction wilh the magnetic field cause the s urround ings of the solar flare 10 heat up. This so-called thermaliz.ation takes only a few minutes. Because of the fantaSiic energies involved, the resulting temperatures a.r e as high as 10 million 10 100 million dcgncs, exceeding the temperature in the very center of the sun. In a flare we have, therefore, an exceptionally hot plasma, which, be<:ause of its nearness and brightness, invites extensive study by refined specnoseople techniques. Sp«tra of Oares made with Ulellilebome telescope$ have revealed lines from highly ionized a1oms, •uch as 2S times ioniud iron and 19 times ionized calcium. These observations have en· oouraged theoretical and laboratory studies of spectra of plasmas at very 

Astrophysical Fronti~l'$ 23 high temperatures. As was the case with the spectroscopy of the neutral or moderately ioniud atoms a few decades ago. we expect this new efl'on to provide us with powerful diaanostlc tools for the examination of very hot plasmas whercve.- they may exist-on the sun, elsewhere In the universe, or in the laboratory In controlled fusion experiments. for example. A fraction of the cosmic-ray panicles escapes from the flare. Before arriving in the vicinity of the eanh, they, and a slower traveling blast wave also gcMrated in the Dare. upsc1 the solar corona and t.h e lntcrplaMtary medium. Various types of radio emission originate when the particles and the blast wa•-e pass through these very outer regions of the sun. An ingenious new type of radio te.lescope recent.ly constructed in Australia clarifies bow these disturbances propagate. The resulting physical pic:ture is again applicable to similas. but more intense. radio burstS that ba•-e been observed on stass. The casth moves in iu orbit through the outer corona and cannot fail to sense the tremendous chanacs induced by Dare radiations. The cosmic-ray storms. the enormous enhancement of x rays and ult.raviolet radiation, and the interplaMtary blast wave play havoc with the casth's upper at· mosphere. changina its ionization balance, affecting the aeomaanetic field. and possibly influencing climate. larae·scale weather. and human well-being in subtle ways. Among the better-known influences of the sol as flares on our human environment are the Interruptions of radio com· munications and the disturbance of space weather affecting many of the satellite experiments and especially human spaccOiaht. Until no"'• most observations of flares have been limited to secondary phenomena, such as the x and gamma rays emitted by the panicles ac· celerated by the larae electric fields occurring in the Oaring region. Most evidence indicates that the Oaring region is very small-of the order of 100 km. None of the satellite or even ground-based observatories has even approached the spatial resolution necessary to resolve t.hcsc regions. ln order to make funher proeress In our understanding ofthe origin of solas flares, it will be necessary to increase the spatial resolution of the ob· servations by an order of maanitude. especially in the x.gamma. and radio region ofthe spectrum. Improvements in the Orbiting Solar Observatories seem to make it possible to approach the required resolution in the foreseeable future. In the absence of solar Dares. the solar corona ncas the eanh is steadil)' expandina outward with a velocity of about SOO kmlsec-the so-called solar wind. By carrying away angular momentum, this solar wind tends to slow down the rotation of the surface layers of the sun. The observed rotation of the solar surface is indeed much less than that observed in other stass. whic:b are believed to have no steUar winds. A recent ob- 

24 ASTRONOMY AND ASTROPHYS ICS FOR THE 1970's servation of the shape of the sun has led to the suggestion that the core of the sun still rotates at its primeval rapid rate. invisible beneath the slowly rotating surface. The ramifications of this concept are great. It would restore the majority of the angular momentum in the solar system to the sun and make its angular momentum con.sistent with that of stars without convection zones or stellar winds. The quadrupole moment of the sun's gravitational field produced by the rapidly rotating core would cause a small change in the motion of the perihelion of the planet Mercury. This would destroy the agreement betw~n lhe observed advance and the prediction of Einslein 's general1heory of relativity. However, the observed advance could insread agr~ wi1h 1he prediction of the Brans-Oicke scalar-ten.s or theory. The careful investigation of such an apparently simple question as " Is the sun really round?" may therefore provide a major clue to the understanding of our physical universe at its most fundamental level. STELLAR EVOLUTION As recently as the last century. astronomers we.re almost totally ignorant of the basic nature of stars. but by 50 years ago the first tentative guesses were being made as to how stars might change with time. More daring were those astrophysicists who began to ask whether the relative abun- dance of the chemical elemenls themselves- the building blocks of the universe-instead of being given once and for all at a moment of creation, might have evolved with time, and indeed might still be changing. The last few years have made il abundantly clear that even the stately, stupendous, seemingly eternal galaxies have their own evolutionary processes, and that at least in the beginning, such events proceeded at what must be con- sidered, astronomically speaking, a breakneck pace. The basic physical laws and processes governing the structure and evolution of stars are now quhe well known, Stars condense from the gas and dust of the interstellar medium, spend typically a few million years deriving their initial energy and form through gravitational collapse, attain in their deep interiors the multimillion-degree te.m perature needed to ignite processes of nuclear fusion (primarily the conversion of hydrogen lo helium), and thereby become almost stationary configurations that show little outward change for intervals typically of from I million to potentially 100 billion years. However. in the core where the temperature is highest and the reactions proceed most rapidly, the central supplies of hydrogen are eventually exhausted. A new phase of rapid evolution then begins, during which hydrogen fusion is restricted to a shell around the 

Astroplrysical Fro11tien 25 core; the co.-now heliu~ntracts, the outer surface of the star expands and cools, and the star becomes a red giant or even a supergiant. Subsequent evolution depends principally on the mass of the star; the greater the mass. the greater the tendency for funher processes of fusion to occur in the deep interior. Thus helium ''burns" to carbon, then to oxy~n. neon. magnesium. and so on up the periodic table toward iron- the process of nucleosynthesis. These steps are in general well correlated with results obtained in laboratories. Eventually, when central supplies of accessible fuel are exhausted. its final evolutionary act is to contract to an extremely dense configuration. Depending on mass. it may become a white dwarf. a neutron star. or. in extreme cases. possibly a relativistic "black hole." The sun and its planetary system condensed from the interstellar me· dium nearly 5 billion years ago. We are learning more about the "solar nebula," from which we came. During the last brief accelerating stages of gravitational collapse, the planets and remainder of the solar system were formed from a small percentage of debris not used in building the sun. Five billion years from now the sun will become a red giant. remaining in that state for a few hundred million years. Any life then remaining on eanh would experience a huge red sun looming across more than 30 percent of the sky and an environment of evaporating air and oceans, at a temperature that would melt lead. Still later, a frozen and presumably lifeless eanh will swing bleakly around a faint white-dwarf sun appearing no larger in the sky than the tiny planet Mars. Astronomers are increasingly confident of the basic va.lidity of this picture of stellar evolution. We cannot follow the life history of any given star, but we see stars in various stages of evolution. We see vast irregularly concentrated clouds of swirling interstellar gas and dust. Here and there we see individual stars or, even more conspicuously. clusters of stars condensing out of this medium-the galactic clusters that lie in the layer of gas and dust in the plane of the galaxy. In many of these still·fbrming, or recently formed. galactic clusters we can directly study details of recent stellar contraction and evolution. By noting differences between stars in different clusters. we can deduce some of the effects of age and of initial chemical composition. Fonunately. the galaxy also contains a few very large and ancient clusters, forming a vaguely spheroidal distribution about the rest of the galaxy. These globular clusters appear to be more than 10 billion years old and are possibly the oldest recognizable survivors from the time when our galaxy was condensing out of whatever primordial medium may then have permeated the universe. Insight into the state of truly primitive matter can thus come directly from the study of these globular-cluster fossils. In particular, we can hope 

26 ASTRONOMY AND ASTROPHYSICS FOR THE 1970's 

Astroplrysical Frontiers 27 to learn the original proportion of helium produced by the big bang. Hydrogen, stiU overwhelmingly the most abundant element in the universe. was formed preferentially in the bang; probably, no element heavier than helium could be brewed under those conditions. But the precise perc:entage of helium that emerged from this aboriginal cosmic cooker ofl'en us the most specific information we are likely ever to obtain about the detailed physical properties of the univene a few moments after its .. cre.ation ... It appean that about one fourth of the original material was helium, but this fraction could still be seriously in error. Even as early as the time when the oldest of globular clusters were formed, it seems that already some process in the galaxy had created and distributed a small but spectroscopically detectable amount of the heavier elements (about I percent of present abundance) plus an unknown amount of helium. Was this process a quasarlike outburst at the galactic center or a sudden effiorescence of supermassive supernovae among the very first stars to form, or was it a process as yet quite unknown to us? In any event, it appean that the initial collapse of the gas and dust of the galaxy. from the original spheroidal form into a spinning Oattened disk, took place in the rather short time of less than a billion years-less than a tenth of the age of the oldest known stan. During each successive stage in the collapse, stars and dusters of stan formed with material progressively more highly enriched in the heavier elements. By the time of formation of the youngest globular clusters, still some 10 billion years ago, the proportion of heavier elements seems to have approached the present value. This is a painful point for astronomen to try to explain. We are certain that nucleosynthesis occurs in all normal stan, and we are nearly certain that such stars, during late stages of their red giant evolution , rather placidly shed much of their matter back into space. At least a fraction of this matter should be enriched in helium and in slightly heavier elements. And we know that supernovae must have been ex- ploding in quite appreciable numbers for all the billions of yean since the globular clusten were formed. Why then do we fail to find incontrovertible evidence of a steady enrichment of elements heavier than hydrogen in the younger objects of the galaxy. for example, in the galactic clusten? The more closely one looks at details of nucleosynthesis and stellar and galactic evolution, the clearer it becomes that our present undentanding of these fundamental topics is incomplete at best. Thus. for example, observations of luminosity and surface temperature agree fairly well with predictions from the theory of stellar evolution for the commoner kinds of stars in globular clusters. RR Lyrae stars, which are dynamically unstable and pulsate rhythmically with periods of approximately hnlf a day, are 

28 ASTRONOMY AND ASTROPHYSICS FOR T H E 1970'• frequently found in globular clusters. By quite independent arguments o f classical physics. it is possible to calculate the pulsation period of an RR l..yrae star of given mass and luminosity. This prediction gives results radically different from observations in globular clusters. Is the classical theory wrong. has it been misapplied. or are the stars different from what st.ellar evolutionary theory would have us believe? Or. consider red giant stars located relatively near the center of our galaxy. There is observational evidence that most of them are rather similar in age and composition to red giants in our solar neighborhood. Yet there are many RR lyrae stars in the solar vicinity ofa type that seems to be complet ely lacking ncar the galactic center. Clearly. some major factors of stellar evolution are quite unknown . New spectroscopic and photometric observations are needed to un· derstnnd the most basic evolutionary processe.' in globular clusters and to test our notions of how nucleogenesis enriches the heavy element content of the interstellar medium. For example. theory predicts the loss of 20 percent of the mass of the giant stars of globular clusters over a time of 100 million years. Very-high·resolution spectroscopic observations are needed to detect this source of heavy metals pouring out of the oldest stars. Supernovae in external galaxies are so far away that the late stages of the outburst become exceedingly faint and very difficult to observe. Yet these stages are just the ones in which spectros.:opic evidence for the buildup of very heavy elements would be expected to be found. Despite these circumstances and needs. the over-all picture of stellar evolution is well understood, and we should be able to apply it with confidence to nearby galaxies. Some parts indeed do 6c, but ouCstanding anomalies remain in the few detailed observations thac it has so far been possible to make of these nearest galaxy neighbors. the closest of which still range from hundreds of thousands to millions of light-years. In pnrCicular, some appear to have globular clusters that differ considerably from those of our own galaxy, suggesting dilfere nt original cond itions and possibly subsequent evolution. Reaching a secure understanding o f the formation of the elements and stellar evolution, with all of their implication• for the rest of astronomy and cosmology, will require decades of the most careful and detailed work on objects mostly so distant as to be beyond the reach of any but the largest telescopes equipped with the most modem eltc1rooptieal devices. It also will require energetic pursuit of the most classical of astronomical studies. astrometry, which provides the positions and apparent motions of the scars. Only in this way can we derive unambiguous data on the distances and nlOtions of stars. on which aU the rest of Che astronomica1 

Astrophysical Frontiers 29 pyramid is built. It will require the closest attention to nearly every other branch of astronomy. including especially radio astronomy and ultraviolet space observations of interstellar gas. which can help to determine the proper1ies of the swirling masses of gas destined to become stars. THE DEATH OF STARS AND THE BIRTH OF NUCLEI The past few years have seen the growth of an enormous astrophysical interest in stellar deaths: how stars die and the nature of the corpses. This interest stems in part from growing confidence. in theories and calculations of stellar evolution but even more from the recognition of the enormous variety and significance of the phenomena associated with stellar de- mise-the probability that almost all the chemical elements heavier tha n carbon were formed in the few seconds of a violent supernova explosion; the discovery of pulsars and their identification with rotating. collapsed stellar corpses; clues that dying and collapsed stars may be the source of cosmic rays, perhaps even of gravitational radiation and of other high- energy activity in our galuy and in other, more explosive. large. and dis- turbed stellar systems. There are many suggested ways in which a star. after its nuclear fuel has been burned. either explosive.ly or gradually, contracts to its final state. But there are only four possible types of stellar corpse; only two have been observed so far. I. In some cases. nearly all the matter in a star may be blown off into the interstellar medium. This is an extremely important process, because it is a major way in which heavy elements formed within the star are distributed for incorporation into other younger stars and solar systems. Partial disruption by steady Oow or explosive ejection has been observed. 2. The star may implode toward a "black hole," a collapsed state in which the pull of gravity is so strong that even light signals cannot escape. A nonrotating, uncharged black hole has an external gravitational field. which may lead to observable x rays produced by falling charged particles, but no other contact with the universe around it. The state of maner in its interior cannot be known and has no reference to the rest of the universe. It is conjectured that a rotating black hole is axially symmetric with no external properties that can change with time, and so far none of the observable effects that might be associated with a rotating black hole have been identified. A black hole is the only possible final state for a stellar mass greater than approximately two to four sola.r masses. Stars with still greater masses may eject their outer layers and collapse in the center to 

30 ASTRONOMY AND ASTROPHYSICS FOR TilE 1970'• A supernova remnant - the Crab pubat photographed by the 12Q.in. telescope. The upper picture nc1ar mui.mum liJht; in the lower, nearly lnvi!llble; the puhar O hes u 30 times a ~~eoond, (Photo courtety of Lick Obsenwtory . ) 

Astrophysical Fro111ien 31 form a black hole; again, we have no firm evidence for this process, except for a suggestion of a black hole as a component of an x·ray source. 3. The contraction of a star under gravity, when other internal pres· sures fail. can be halted by the motion of high·velocity electrons- a neces· sary consequence when electrons are forced close to each other. Such objeds were discovered long ago; they are the white dwarfs whose mass can extend up to slightly greater than that of the sun. The central density of a white dwarf can reach 10' g cm·J . where the properties of matter still can be calculated with reasonable confidence. Beyond this limit, the pull of gravity is sufficient to crush such stars toward another even denser final state. There remain various unresolved problems associated with white dwarfs. such as the origin and stabilization of the huge magnetic fields that seem necessary to produce the circularly polarized light and strange spectra of some white dwarfs or the elfects of differential rotation, con· vection. and crystallization in such superdense matter. 4. FtDally. when a star has contracted so far that the atomic nuclei within it actually touch. further contraction is resisted by the same combination of nucleon motion and repulsive nuclear forces that make the nuclei themselves almost incompressible. This can happen only for stellar densities exceeding 10,. g cm·J . At such densities. all except a few percent of the electrons have been absorbed by the constituent protons, converting them to neutrons. Since these are the main components in such ultradense matter. the star is called a neutron sta.r or a pulsar. The upper limit to the mass of a neutron star depends on nuclear forces at short range and the presence of the normally unstable, strange particles. It probably lies between one and two solar masses. These theoretical pictures are confirmed by observations of stars that lose mass, or explode violently. and of white dwarfs, the classical stellar remnants. The search for direct observational evidence for black holes may never succeed even though they may contain much of the mass of the universe. But the neutron star has been found. and its properties can best be undel'$tood by study of the best·known remnant of a stellar explosion, the Crab nebula. The Crab illustrates in a most vivid manner the value of historical, accumulative data. of observations made at all possible wavelengths, from radio to x·ray. and the interaction of theory and observation in modem astronomy. In A.D. 1054. on the fourth of July. the court astronomel'$ of ancient China noted the appearance of a new star. a supernova, bright enough to be seen in daytime and visible for most of a year. Its place was taken by what Messier described in 1758 tO be a "nebulosity. It contains no star; it is a whitish light. e.longated like the Dame of a t.aper. It was observed by Dr. Bevis in about 173 I." The otar had been replaced by a 

32 ASTRONOMY AND ASTROPHYSICS FO R THE 19 70's filamentary. gaseous nebula. expanding at a thousand miles a second. The nebula has an inner amorphous mass. whose optical radiation was not understood until it was shown to be almost completely polarized and until the radio astronomers found it to be the third strongest radio sou= in the sky. The amorphous mass consists of tangled magnetic fields in which high-energy electrons spiral. at speeds approaching that of light. with energies of billions of electron volts. Their total rest mass is about one millionth the mass of the sun. The filamentary expanding gas cloud has about a solar mass and will eventually be slowed down by plowing into ordinary interstellar gas. It is now a few light-years in diameter. about ten thousand light-years distant. Both the amorphous mass and the high- energy electrons bring messages !Tom the initial explosion of the super· nova. The deceleration of an electron as it swings about the magnetic fields produces synchrotron radiation, over a wide band of optical, in- frared. and radio wavelengths. when the electron energy is high. The Crab nebula emits x rays. which were observed during one brief rocket Oight; during another. while the source was being eclipsed by the moon, it was found to be an extended rather than a point source. Most imponantly. the high-energy elttt:rons lose energy rapidly by emitting radio to x-ray photons and must, in fact. be renewed every few decades from the central source. Photographs taken with large telescopes had revealed the expansion of the filamentary nebula, and spectra had shown it to be of slightly peculiar composition. lacking hydrogen (presumably consumed as nuclear fuel and convened into helium). The center of expnnsion was located by accurate positional measurements of the shorter filaments. on photographs taken over many years. Nearly at this center were two faint stars, one of which had no detectable spectral lines, absorption. or emission; its visible light output made it slightly brighter than the sun. while the nebula was hundreds of times brighter- not unusual for a very hot star immersed in a normal gaseous nebula. But direct photography of the amorphous mass, near the center star, added a new mystery. Small. di!fuse clouds moved as if struck by waves of ex- citation (or high-energy elttt:rons) and changed in brightness and structure. The disturbances propagated at nearly the speed of light. The discovery of pulsars in England in J 967 revealed the existence of variable radio sources with periods from 0.033 to 2 sec. The 0.033-sec pulsar was near the Crab. The timing of the pulses was extraordinarily accurate and soon revealed that the periods were slowly lengthening. Three young and ingenious astrophysicists, using only a 36-in. telescope, searched in J 969 for the 0.033-sec period in the light of one of the central stars of the Crab nebu la and immediately found it. Other telescopes determined very accurate light curves, which resembled the radio light 

Astrophysical Frontiers 33 curves. and found that the star disappeared at pulse minimum; the spectrum was re-examined and found again to be featureless, with a continuous energy distribution that could be an extension of the radio- frequency pulse spectrum. Re-examination of rocket-flight data with the now known period showed that the x rays also pulsed, i.e., t.h at the star as weU as the amorphous continuum produced x rays. The supernova bad left behind an observable stellar remnant, radiating enormous energies, from radio to x rays. There is extremely strong support for t.he nodon that such a pulsar is a rotating neutron star containing a huge (perhaps 1012 G or greater) magnetic field. The precise observations possible for pulsar signals, and the period between pulses, suggest many ways of exploring their internal structure. Neutron stars are objects of an intense interest, which is unlikely to diminish. They supply the only known mechanism for efficient conversion of enormous amounts of gravitational potential energy into ,-ery-high-energy cosmic rays. The remnant pulsar at the center of the Crab nebula is still emitting over ten thousand times the solar luminosity into that nebula 900 years after the supernova explosion in which it was formed. During the coUapse of a dying star, angular momentum conservation demands that it spin faster. Much of its gravitational potential energy is converted into the kinetic energy of its increasingly rapid rotation, some is used to compress and thereby greatly amplifY whatever magnetic fields it contains. The "dead" star together with its huge field spins many times per second. This comprises an enormous electric power generator that efficiently accelerates surrounding electrically charged particles to cosmic- ray energies. It slowly converts the rotational energy that it gained from gravitational contraction to that of cosmic-ray protons, nuclei, and electrons. Understanding a neutron star will require extending present frontiers in nuclear physics, elementary-particle theory, solid-state theory, and low- temperature physics. A typical neutron star of one solar mass has a radius of only 10 krn. It would fit inside Los Angeles! The outer layer, of the order of a few kilometers, until the density reaches about 3 x 10" g cm·3 , contains nuclei in a crystalline lattice well below t.he melting temperature. Thus the neutron star has a thick crust. The inner part of the crust is 10" times stiffer than steel and lOS times better an electrical conductor than copper; it is almost entirely free of impurities and contains a neutron fluid that together with free electrons fills the region between nuclei. Below the crust, neutron star matter consists mostly of neutrons and a small per· centage of electrons and protons. The neutrons form an anisotropic super1luid, similar to that of terrestrial liquid helium at a temperature 

34 ASTRONOMY AND ASTROPHYSICS FOR THE 1970'• close to absolute zero: it has essentially no viscosity. negligible heat capacity. and remarkable flow properties. The protons probably constitute a superconductor in which electric currents producing a magnetic field Dow without loss. When the density exceeds 10 1 ~ g cm ·J. even the con- stituents of matter are not well understood; in the ultndense environment of a neutron star core. normally unstable heavy particles become quite stable and mesons and strange heavier nucleons will be present. The nature of matter. and its pressure, is not yet known at such high densities. What happens to lhe matter ejected from a star that has suffered a violent central collapse? The observed supernova spectra, for many classes of supernovae. show essentially normal composition. except that the hydrogen has been depleted and the heavy elements somewhat increased. (For one lype, the brightest of all. no spectral features have yet been identified.) Velocities of expansion up to 15,000 km see ·1 are found. But the light fades before material from the deeper interior can be spectro- scopically observed. Advances in theoretical study of the hydrodynamics of the collapse and the nuclear reactions that occur currently suggest Ihat stars in the range of four to eight solar masses are the source for most of the nucleosynthesis of heavy elements. The details of the collapse are somewhat speculative. When the core has burned aU its hydrogen and helium. it is left with too much mass to be supported by the degenerate electron gas pressure. Rapid contraction of the hot core causes a thermal runaway. igniting thermonuclear reactions of carbon and oxygen. The detonation wave races through the core, burning additional nuclear fuel and creating a shock wave that ejects the outer layers of the star. A neutron slar remnant may be left at the center of the exploding super· nova. Study of the details of nuclear reactions that occur in the shocked, rapidly exploding envelope has been carried through. using a complicated network of hundreds of possible nuclear reactions. For many of these, laboratory data exist; for others, nuclear cross sections must be computed or estimated. The outer part of the star (two to six solar masses, if a neutron star is left behind) is exploding in a little bang like primal matter of the big bang. The density and temperature vary as a function of time and initial location within the star. All the envelope material will be ejected and the reactions terminated in a few seconds. Material that started near the core, at high density. will build huvy elements; slightly lower initial temperatures yield the iron group, and still lower, the elements from silicon to scandium. There is striking similarity to details of the abundance curve of the elements and isotopes found on earth, the moon, and in the meteorites. Certain old stars. with low total metal content, show elemental abundances remarkably similar to those pre- dicled by explosive nucleosynthesis. The theoretical predictions can be 

Astrophysical Frontiers 35 improved by better knowledge of reaction cross sections and the dynamics of the collapse. But the astronomers must also provide better data on supernova light curves at all wavelengths. including observations in the infrared and in the far ultraviolet from space. They must also obtain the spectrum at least two or three years after the explosion, when it has faded into near invisibility at optical wavelengths, since only then will the cloud be transparent and will material from the hot, inner regions, containing heavier elements, become observable. As the cloud density drops. the spectrum is expected to change until it will show only the "forbidden" lines of partially stripped ions. as in the solar corona ; spectroscopic ob- servations of supernovae years after the explosion will be possible only with the largest telescopes, equipped with the modern electronic auxiliaries now becoming available-television finders, image intensifiers. and digital-output Tv tubes. Clearly, more than one large telescope is needed. Are there optical pulsars in other supernova remnants? Are there superpulsars in the very luminous core of a galaxy? If so, how are the abundances of the elements changed? Again, only the largest telescopes can supply the answers. Or. in space, accurate mapping of x-ray sources and detailed study of very-high-energy brightness may reveal a neutron star core. Thus, from the supernova outburst, to pulsar, to the origin of the elements out of which our bodies are made. our minds must traverse a continuous line of observation, search, thought, speculation, calculation, and measurement. Could we bypass all this speculation by traveling through space ten thousand years and studying the pulsar as we Oy past it? A young astrophysicist has written the following: We will never see wen a neutron star. lmotgine chat one has seen a dis tam light. Is it emined by a llashlight or a bonfire? One doc-.s not know, for one has only seen the. light. Similarly, we see only flasht$, not the neutron sta.r emitting them. h is not because it is invisible in some magical way. but because it is small. If an astronaut. more foolhardy than brave, were to venture sufficiently dose that il loomed as large as the moon in our sky he woukl be irradiated (high-energy panicles}. burnt to a crisp (thermal .x rays). torn in shreds {tidal force), and blown away. If he were to live. he would be traveling so fast that he would have only a thousandth or a second for a glimpse of it whh the naked eye as he sped by. EXPLODING CORES OF GALAXIES Large optical and radio telescopes have discovered extraordinary phenomena in the centers of galaxies, the nearly pointlike nuclei. Ob- served first visually as a point, photographed next as a tiny region, and measured by very-long-baseline interferometry as still essentially un- 

:s: > ... "' "' 0 7 . 0 J: -< ::-.....-.-. .... -~ > z 0 > ... "' ,. t .. 0 ::t -< - "' n "' Cl The tptctrum or 1 quu:ar wilh one or the lar,esl known red shifts. oblaincd in a 6-hour uposuro on the 200-in. lbJe telt:tcopt whh an Integral• " ... me TV camera. More than. week or Upo$Ute on an ordinary photographic plale would be rtqu in~ to obtain this spectrum. The dark line is ::t li&hl polludon- mercury arc: lighu from Sa.n Die,o. (fholo coutUty of Prin~lon and""'~ Obta'V#Iories. ) .. "' ~ 0 •• 

Astroplrysical Fro,tiers 37 resolved. the nuclei of certain galaxies seem to be the site of unbelievably energetic explosions. To the astronomy of the first half of the twentieth century belongs the revelation that we live in an expanding universe. Galuies rush away with ever-increasing speed the farther out we look. as the universe expands from the cataclysmic event of whose wrenching paroxysm we are but dimly aware in the most remote and oldest reaches of space. To the astronomers of the 1930's and 1940's, these galaxies, grand and stately. seemed im· mutable, great beacons that rushed through space changing slowly in response to the evolution of their stars but otherwise as solid as the rocks of the ancient earth from which they were observed. To most astronomers of that era, explosive events were confined to a few curious and peculiar stars and to the colossal singular moment at the beginning of time. But with the advent of radio astronomy, the discovery of quasars, and the closer optical examination of many strange galaxies, astronomers of the 1960's and 1970's have found that the universe teems with explosive events, as bewildering, outrageous, appalling. amusing. and sobering as they are ubiquitous. The tiny, remote, but fantastically energetic quasars show evidence for the ejection of matter at high velocity (100,000 km per sec). Many po"-erful radio galaxies are doubled sources, with evidence that two great magnetized clouds of gas. with masses of millions of suns, are exploding away from each other with velocities close to the velocity of light. Embedded in the centers of certain galaxies. called Seyfert galaxies, are tiny sources. whose dimensions are only ten times that of a supergiant star but that act like a permanent supernova, ejecting matter into space at a velocity of 1000 km per sec. In one such object, ejection has been going on for at least 40 years, but the significance of the early observations was not appreciated until recently. Something like 2 to 5 percent of galaxies show evidence for explosions and violent events in their nuclei, and it is estimated that. if all galaxies evolve through a stage in which such violent events take place, the ex· plosive stage lasts some 100 million years. During that time, some galaxies may eject quantities of matter equal to that of a small or possibly even a normal galaxy. What mechanism is responsible for the ejection of such a seemingly inexhaustible cornucopia of matter? What is the maner and energy balance in the nucleus? Could the energy of the nucleus be sup- plied by collision.s berween millions of stars confined to a volume of space less than a light -year in diameter? Is the nucleus itself, having a mass of about 100 million suns, some strange massive object whose physics is only dimly understood? Though truly violent events seem to be confined to the nuclei of Seyferts and quasars and to certain kinds of disturbed radio galaxy, even "nor· 

38 ASTRONOMY AND ASTROPHYSICS FOR THE 1970's mal" galaxies show evidence for the ejection of matter from their nuclei on a smaller scale and at a slower rate. Our own galaxy ejects matter from its nucleus at the rate of about 1 solar mass per year, as revealed by radio· astronomy observations. Infrared, radio, and • ·ray observations show that this tiny nucleus has some of the properties of a modest, but disturbed, Seyfert nucleus. Because of the enormous obscuration of the center of our galaxy by interstellar dust, v•e cannot "see" it in optical wavelengths. It may be that all spiral galaxies show evidence for at least some modest activity in their nudei. What role. then, does the nucleus of a galaxy play in its evolution? Could it be that all the matter of a galaxy is ejected (even, possibly, created) from its nucleus, and that as galaxies age, the explosive natures of the nuclei gradually die away with time? Further study of the astrophysics of these nuclei not only will tax the ingenuity of the theoretician dealing with new and unexpected states of matter but will require significantly greater observational effort with the largest telescopes and the most sophisticated electrooptical technology. What plausible theory can explain this implausible behavior? Specula· tions are many; since the concentration of so much energy into a small volume is common to the observed compact pans of galaxies, or quasi·stellarobjects. some theoreticians believe that gravitational collapse is involved. A dense many-star system could evolve thermodynamically by ejecting stars, supplying work done in the ejection from gravitational contraction by the balance of the stars. The system becomes denser, the near collisions and ejections more rapid , until the remaining stars undergo relativistic collapse into a black hole, releasing enormous amounts of gravitational energy. This dramatic picture is less radical in its physics than the speculation that a very large concentration of matter, such as is found at the center of a galaxy, may be the source of rapid, continuing creation of new matter. Here a new physical principle is required, not testable on the small scale of the earth, working only on the large scale- requiring deep changes in general relativity. In still another model, the dense core of a galaxy may contain some matter of extremely high density, not yet expanded from the early moments of creation; matter and energy equivalent to a hundred million suns ltave been formed in some unknown way, the equivalent of a single giant atom. which eventually explodes. In many other ways explosions in galaxies have strange and not yet un· derstood effects. Evidence for the recent appearance of whole galaxies has been derived from study of radio-quiet compact gala.xies. dominated by the emission of hot gas and very young stars. Where did their matter come from ? A fascinating link with the theory of the origin of chemical elements comes from the observations of the spectra of exploding stars or super· 

Astrophysical Frontiers 39 novae. Chains of nuclear reaction occurring in the stars have been suggested as leading to the synthesis of nearly all the chemical elements. Have we found any classes of stars, or of galaxies, so young that only the hydrogen and helium of the big bang exist in their atmospheres? The answer seems to be no; even the most metal-poor stars have at least half a percent of the solar metal abundance. What happened in our own galaxy to load the primal hydroge.n and helium with all these heavy elements? Was our galaxy filled with quasarlike explosions, or was it.s nucleus so enormously active that the heavy elements synthesized there provided this initial metal content? The emission-line spectra of quasars show most of the expected chemical elements from carbon to iron. The apparently fast· moving shells producing the absorption lines found in qua.sars were also found to show the common and expected elements. The quasars should have been the likely candidates, if any objects might be, to be young or to have had radically different types of nucleosynthesis. Perhaps the strangest feature of astronomical explosions, besides their prevalence, is their association with high-energy electrons and possible cosmic rays. Ordinary novae emit radio noise and excessive infrared. Supernovae, as evidenced by the Crab nebula and its pulsar, emit x rays, light. and radio waves. Exploding galaxies are radio sources and emit excess infrared. Yet the velocities of explosion are relatively low, 103 to to• km sec·•, corresponding to 6 to 600 keV per nucleon. The cosmic-ray electron energy required, however. to produce the radio noise is in the billion-volt range. and perhaps higher for x rays. Thus observation from space. as well as from the ground, of the sun. stars. and galaxies leads to one fundamental question-how organized motion of many particles at relatively low energies can result in such concentration of energy into the relativistic motions of a few particles. Are large electric field gradients created? Do magnetic fields accelerate a few fast particles to nearly the speed of light? Or is some mechanism like that of the pulsars (a rotating magnetic field) present on a huge scale in the nuclei of galaxies as well as on a small scale in the heart of a supernova? One lesson. for possible terrestrial application is that rapidly moving clouds or ionized gas are almost always connected with the very-high-energy phenomena. MOLECULES, DUST, AND LIFE There was a time when the formation orthe earth and the solar system was believed to be an extraordinary event, occasioned by a chance encounter with a passing star that ripped off the planets from the sun, set them spinning, and then wandered away into space never to be seen again. We 

40 ASTRONOMY AND ASTROPHYSICS FOR TilE 1970's lntcnl<llat p s and dutt- tho birthpboe of tt2J1 and planetary systemJ. Pbotocnpbed by tho l()().ln. tele1copo. l/'h0to cowtny ofH•l• Ob""""tom~) no longer think such special creation to be true, but an even more in· triguing picture has taken its place. We now think solar systems to be commonplace, arising in a natural way as a consequence of the formation of the stars themselves. The evidence for this is both observational, in that 

Astrophysical Frontien 41 rotating young stars are always seen associated with complexes of gas and dust. and theoretical, in that calculations show that if a cloud of matter becomes sufficiently dense or massive, it must inevitably collapse and fragment into a cluster of stars. How planets form is somewhat of a mystery. one problem being that so far we have only one solar system to study. How life forms once the planets are there is even more obscure, but one of the fascinating discoveries of recent Limes is that some of the molecules necessary for the development of life may already have been present in the original dust cloud out of which the solar system formed. Indeed the process of creation may be universal; there may be many solar systems, and perhaps many life forms, scattered throughout the galaxy, with whom we might someday communicate or of whose existence we might at least learn. We must uncover the initial conditions in the interstellar medium out of which the solar system formed. We lrnow both gas and dust were present. Photographic studies of the sky long ago revealed many luminous clouds, which spectroscopic analysis later showed to be mainly hydrogen gas, ionized by the hot stars embedded ,.;thin them. h was suspected that space far from stars also contained hydrogen, but in a neutral form too cold to be luminous. This was confirmed in 1951 when the 21-cm radio emission of hydrogen was first detected. Since then. radio observations have mapped hydrogen clouds in spiral arms stretching around the galaxy and detected such gas in other galaxies. The solid particles in space, the cosmic dust, have also been long known, for even a casual glance at the Milky Way, let alone examination of deep-sky photographs, reveals numerous complexes of dark patches, which arc nearby dust clouds obscuring and reddening the more d istant stars. In general, the dust is where the gas is, except that when a radio telescope is pointed at the densest dust clouds, no emission from atomic hydrogen is found at all. The dust is presumed to have catalyzed the formation of molecular hydrogen. Hz, and shielded it from the destructive power of the ultraviolet radiation filling most of space. Because it is un· detect.able at optical and radio wavelengths, the hydrogen molecule was not detected in space until recently, when rockets carried stellar spec- trographs above the atmosphere to obtain observations in the ultraviolet. Such observations are limited to clouds in the direction of bright stars, however. so we must await the detection of infrared emission lines of Hz before we shall know the true distribution of this molecule. Are there other molecules in space? Some, such as CH and CN, have strong absorption lines at optical wavelengths and were discovered early with the large spectrographs on conventional telescopes. New electronic detection techniques will doubtless lead to searches for weaker lines, 

42 ASTRONOMY ANI> ASTROPHYSICS FOR THE 1970's allowing several more to be discovered in this way. However, it is in the radio spectrum. particularly the centimeter and millimeter regions, that the most dramatic molecular discoveries have been made. The hydroxyl radical. OH. was the first sought and found . h too seems to be associated with dust clouds. Its existence is not surprising, for oxygen and hydrogen arc among the most abundant clements. However, its radio emission. the 18-cm line, was found to be exceedingly strong in some small regions, and it soon became clear that an interstellar maser was involved. Understanding the details of the maser process is a challenge to the theoreticians. It involves finding some mechanism (which may involve collisions. infrared or ultraviolet radiation) that will over- populate a molecular energy level, causing it to amplify any incident IS- em radiation. The full explication of this stimulated emission process will reveal new physical phenomena going on within the dust clouds. Radio telescopes precise enough to work at millimeter wavelengths have led to the recent discovery of sC\•eral other diatomic molecules, e.g.. CO, CS, and SiO. Unes of water and ammonia have been found, and the water emission also has maser characteristics. The presence of these molecules is not surprising. for though they contain more than two atoms. they are the stable compounds of the abundant elements oxygen and nitrogen with hydrogen. Do more complex molecules form in space? Only a fC\V years ago. we would have said no, for the rate at which atoms collide and stick together in the near vacuum of space is so low that large complexes should not build up. In addition, most complex molecules are easily broken apart by ultraviolet light and cosmic rays- it is only because our atmosphere shields us from this radiation that life can exist on earth. Yet with the decade of the 1970's barely begun, an undreamed-of array of complex forms has been found in interstellar space-molecules like formaldehyde, methyl alcohol. cyanoacetylene. formic acid, and formamide. Every radio astronomer has an extensive search list, and the known species will prob- ably be doubled in the next fC\V years. We might speculate that even very complex species. such a.s amino acids and other prebiologieal molecules. may be found. Now the astronomer must call for help from the chemist to see if, together. they can explain how such a bewildering assortment of chemicals could be born. An important factor in constructing any theories of molecular formation will be the determination of the numbers of atoms, the building blocks of molecules, within the dust clouds. Here we must look to ultraviolet spectroscopy from space vehicles to provide the data. for few of the important elements have strong absorption lines in the visible part of the spectrum. 

Astrophysical Frontien 43 One current theory is that molecules are made on the surfaces of dust grains within the dark clouds. Much theoretical and laboratory work is needed to explore this idea. and astronomical work too. for we as yet do not even know what the dust is made of. The degree of reddening of starlight in the visible spectrum tells us that the particles must be small. a few tenths of a micron in diameter. but to determine the composition will require sensitive measurements in the infrared. where vibration-rotation bands of molecular solids occur. or in !he ultraviolet. where we find the electronic absorption bands. Some clues are already at hand, however. Ultraviolet spectra of reddened stars obtained with the Orbiting Astro· nomical Observatory satellite are indicative of the presence of graphite. Infrared observations show emissions like those of silicate minerals. Perhaps a mixture of many kinds of substances is the true situation. The dust clouds around stars are particularly interesting. In some cases !he dust seems to be formed by the star itself and is being blown away into interstellar space. Perhaps such stars are the source of all the interstellar dust. Are they the source of molecules as well? CO and CS have been detected in one infrared source. and OH and H 2 0 in another. High· sensitivity infrared and millimeter studies of such objects should pro•·e fruitful and may reveal the prime molecule factory. Other circumstellar dust clouds appear to be the remnants of protostars that have just formed. Perhaps we are seeing preplanetary systems under conditions like those prevailing in our own solar systemS billion years ago. The infrared s pectra of comets show the same silicate emission feature found elsewhere in the galaxy. Here, too. in objects left over from the early days of our solar system , we look for molecular clues to our origins. What does all this have to do with chemical and prebiological evolution on earth? The rapid discovery of ever more complex molecules within dust clouds sugges ts that they will, at least, lead us to a clearer delineation of !he starting point from which evolutionary theories must proceed . It might, of course, be a false lead , if all the molecules evolved in interstellar clouds and protostars may be destroyed in the condensation or subsequent hearing of the planet. Examination of !he sutface rocks of the moon and olher planets will yield useful data on this point. Perhaps the destructive hearing of planetary formation is circumvented by the temporary storage of !he cosmic organic chemicals in natural "deep freel'.CS." These could be the swarms of meteorites (some of which have complex hydrocarbons) that circulate in planetary systems or, even more likely, the comets (whose spectra are dominated by compounds of carbon, nitrogen, oxygen, and hydrogen). Here the material of life could be protected at low temperature while a planet develops the conditions suitable for life, eventually to rain 

44 ASTRONOMY AN I> AS TRO PHYSICS FOR TilE 1970., onto the planet with much ofi t suni.ving um•hercd. Thi.. implies that life e\ery~Ahere is ~imilar chemically. making biological similarity more likely. E\·tn if life is not oon~tructcd from molecules fonncd in interstellar space. study of the organi< processes and ch<mimy raking place there will gh·c insight inco organic chemical C\·olution. ~ht:rc\'t'r it look place. Wh:ate,-er the case. it is clear that in the study of the interstellar medium. stars. and preplanetary systems. we are at a frontier o( k n~ ledge: ar .,.,hich the techniqut:S of optical. radio, infrared. and space a\tronomy. combined with the laboratory and theoret ical skills of chemists and ph)·sicists as ~Atll as astronomers. arc leading toward a bener understanding of not only our ultimate beginnings but t hose of life everywhe re in the universe. TH E SO LAR SYST EM We live on one small pa rt of a giant ruin cont aining vitnl clu~ co the processes of stnr a nd planet forma tion and the origin of life. Are then: other ploneco.ry systems 1ike our own? Do they contain organiz.c:d. self- replicating entities that we might call alive? What art the processes required for life 10 evolve. and how oommonly do rhey <>«ur! One method of anacking these questions is to imrestigace our own planet and our immediate neigh~rhe other objecrs rraveling rhrough space in tbe oompany of the oun. In the nev.• eta or w lar·system in,-esrigatiom. direct probes are provid- ing information in a quantity and q ualiry impossible lo achieve by other means. Nevenheless. imponant problems can sri II be Sludied wilh ground - based observations. This is especially true of Ihe outer solar system. where journeys by sophiSiicaled probes are nor expecred before lhe end of rhis de<ade. b ur il is also rrue for many kinds o f srudies of our neighbors Mars and Venus. Space missions are necessarily few in numbe r : they often provide info rm acion only about a small region o f rhe p articu lar object to which they a re sent. Furthermore. while the instrumentation may make irreplaceable observa tions. it must be miniaturized and limited. result- ing in a loss of sensitivity compared with devices nc g round -based ob sen-atorics. For exampl<. Ihe earlier spacecraft have failed 10 deleel waler on Mars or HCJ and H F on Venus. C\'en though ground ·based mtasurements sho-..· rh<se oompounds 10 be present. The CJ<tremely low limiiS on possible ronSiilueniS oflhe Martian atmosphere set by ultraviolet obse,.ations of rhat plane1 by OAO-Al exceed the sensitivity of mass speetrometen <urrently planned for landing on Mars. In the armospheri< windows. 

Astrophysical Frontiers 45 i much can be accomplished from ground -based and near-earth ob· ~ servations, especially as sensitivity and sophistication improve over the • next few years. I Two areas of active research in physics indicate the diversity of solar· system studies. Tests of general relativity may be carried out by radar ranging of planets passing close to and behind the solar limb. The signal returned has predictable delays caused by gravitational dencction, which differ in Einstein and Brans-Dicke cosmologies, and accurate observations will permit a definitive choice between the two theories. A second subject is planetary magnetic fields, their interaction with the interplanetary medium. and the formation of belts of trapped high-energy particles. Jupiter is the only planet. other than the earth, known to have a magnetic field; it is surrounded by radiation belts similar to, but much more energetic than, those discovered around the earth by van Allen. The Jovian system is studied by the radio-frequency energy emitted by these charged particles. We do not yet understand their interaction with the planet's ionosphere and with its satellite lo. Increased understanding of the large- seale electrical and magnetohydrodynamical behavior of matter on an astronomical scale may come from such studies. Within the last decade. it became apparent that the compositions of the atmospheres of earth, Mars. and Venus. while closely related. were significantly different. By analogy with the composition of our own at - mosphere, the dominant atmospheric constituent for Mars and Venus was assu med to be nitrogen. Ground-based spectroscopic investigations (later confirmed by spacecraft) discovered that the major constituent of both atmospheres is carbon dioxide. Carbon dioxide is the second most abundant volatile gas released from the earth's crust over long geologic times. It does not dominate our atmosphere because in the presence of water it displaces Si02 in rocks to form carbonates. In the absence of water, we would expect the earth to be lifeless. with an atmosphere containing about as much carbon dioxide as the atmosphere of Venus, with nitrogen only a minor (-5 percent) constituent. But water is the most abundant volatile outgasscd by the earth. By analogy, we should expect large amounts of water on Mars and Venus also. But the surface of Venus is too hot to permit water to exist, either as a liquid or as ice or absorbed in rocks. This explains the presence of the enormous amount of carbon dioxide that has been observed: the equilibrium between silicate and carbonate rocks was established at a level that kept the ou tgassed C02 in the atmosphere. But what about the water? The amount of water presently in the atmosphere of Venus is uncertain. but it clearly is several orders of magnitude below that expected 

40 ASTRO~O.II Y AND ASTR OPH YSICS FOR TH E 1910"t 

A$1rophysicul Frontien 47 by analogy with the earth. Either it was deficient in the planet originally. or it has subsequently esc.a ped. It is vital chat v.:e di.scc:n-er the reason for this anomaly if we are to assess properly the likelihood of the d.-.·eloprnent of life elsewhere in the univene. Uqukl water is generally conceded to be essenllal for 1ife. How is it that • 'e have so high an abundance on the earth and so linle on Venus? Is distance from the sun lhe crucial factor? The ans•-crs should have equal applicability to other solar systems. even those with suns of differing luminosities and surface temperatures. On Mars the problem is slightly different. Again we observe smaller amounts of water in its atmosphere than we can reconcile with the amount of C02. The total atmospheric-water pressure is too low for liquid water to exist. Our cxpcctution on the basis of analogy wich the earth is satisfied. To account for the absence of water. we could invoke again the possibility that it escaped during the life of the solar system. But we might a lso postulate it s till present on the planet. as permafrost and adsorbed water buried beneath the surface.. It is obviously imponant to resolve such ambiguities if we are 10 develop our ideas about evolution of planetary atmospheres. We will have to resort to indirttt methods. fOT example. studies of the abundance of noble gases and their isotopes. The amount or nitrogen prc-xnt is also an important <luc: spaceeran ,..ill probably be rcquiml to mal:e these measurements. Mars still holds the special fascination associated ..-ith the pcosibility that some form or life might exist on its surface. Each increase in our knowledge of the Martian en,·ironment has appeared to make this possibility less likely. but it is by no means excluded. and experiments to detect life are planned to be landed before the end oft he decade. The Jovian planets are d raStically different from our nearer neighbors. The m ajor plonecs are much close- in composition to lhc cosmic average r found in stars and 1 1cbulae-they may represent the bosic matter from which t he sun and the planets formed . The composition nnd isotope ratios in these planets could pro,•ide information a bout conditions in the primitive solar nebula. Scientists comntonly assume thnl. In order for life to begin on eanh . it was necessary for the atmosphere of our planet to contain larae amounts of methane and ammonia. the major constituents or the present :umospheres of the outer planets. The giant planets. especially Jupiter and Saturn. may thus provide enormous natural laboratories in which processes required for the origin of life are still being rqxated. The level of complexity achie•'ed under such conditions •ill afr«t theories or the origin of life on earth and etse-•here in the unhuse. Tilere issomedoubl whether theeanh's atmosphc« was evn- h)'drogen· rich. It is more liktly that after the crust solidified. free hydrogen was less abundant than the eosmic average. There are objects in the solar system 

48 ASTRONOMY AND ASTROPHYSICS FOR Till! 1970't with such atmospheres: Titan. the largest satellite of Saturn. and Uranus and Neptune appear to have hydrogen-deficient atmospheres. Uranus and Neptune challenge our ideas about the formation and evolution of atmospheres. How was hydrogen lost from cold. massive bodies at such great distances from the sun? Are we seeing the results of composition differences in the original cloud of gas? Some models for collapsing gas clouds suggest that such effects might occur. We must interpret the "fossil record" correctly in order to assess such models. and this requires additional studies of the atmospheric composition and radiation balance of these distant planets. The comets. planetary satellites. asteroids. nnd interplanetary dust must be encompassed by any comprehensive theory of origin and evolu- tion. Before manned landings on the moon. meteorites provided the only extraterrestrial material for laboratory analysis. They are still the oldest undisturbed samples we have and provide invaluable information about physical and chemical conditions in the early stages of solar-system formation. The rocky. metallic meteorites have received most attention and are probably associated with the asteroids. However. it has become apparent that friable meteorites. such as the type I carbonaceous chondrites. are much much more abundant than had been thought and are unique in containing relatively large amounts of complex organic compounds. A number of amino acids have been identified recently in two such meteor- ites. establishing that fundamental building blocks for living systems were formed abiogenically somewhere in space and lending support to theories that suggest that material for life may be common in the universe. At this stage of understanding we can only speculate about connections between meteorites. organic molecules recently discovered in interstellar space, and development of life. Comets seem to be likely intermediaries; they appear to contain ices that outgas some organic compounds. Comets may have been the first objects to condense from the solar nebula and may contain some of the rich mixture of complex molecules now found in dense interstellar clouds. A tie between friable meteorites and comets has been established through the study of meteor orbits; but it remains to be proven whether the type I carbonaceous chondrites are fragn1ents of extinct comet nuclei. One can see large questions about prelife organic chemistry and conditions in the solar nebula looming behind these studies. With the opportunity for investigations of the widely differing en- vironments provided by the inner and outer planets, their satellites. and the small bodies that move among them. studies of the solar system will continue to provide a foundation from which to assess the possibility that we are not the only form oflife existing in the universe. 

Astrophysical Frontiers 49 ASTRONOMY AND EXOBIOLOGY There is perhaps nothing more tantalizing than the possibility of deterting the peoples of other worlds. a possibility made real by the development of powerful radio and optical tete.copes in our era. Indeed. our current radio telescopes can detect the radiations or a civilization no more advanced than ours over distanees or many hundreds or light·years.• range within which there are millions of stars. If more advanced ci'vili:zations exist with the capability of controlling the powers of the Sllrs themselves, which is not inconceivable.tbeelfects or such cosmic engineering con be discovered by our present optical and radio telescopes throughout our own and many other galasks. Our cmtiution is within reach of one of the greatest steps in its emlu· tion: knowledge of the existence. nacure. and activities of independent civiliutions in space. At this instant. through this very document. are perhaps passing radio waves bearing the rom·ersations of distant creatures-conversations that we could ~cord if v.·e but pointed a lele· scope in the right direction and turned to the proper frequency. An assurance of rapid results cannot be made in a search for extra- terrestrial civilizations. Such a search is akin to the one for the proverbial needle. but in this case the haystack oontains three dimensions of space and two more oftime and frequency. and there may be no needle. It is only our knowledge of the value of that needle. if it exists. that compels us to pursue such a difficult objective. We can be helped greatly by any reasoning that will decrease the amount of space. time. and frequency thot must be oombed. But here. we are not only ignorant in many areas. but we nrc not even sure that we have recognized nil the imponant considerations. We do not know the longevity of civilizations that are power·radiating. hence power-dissipating. sys- tems. How quickly does n civilization. under the pressures of economy, become invisible-not as n result of inndequnte technology but rather of superior technology? Are sclf-<lestroying wars a common destiny of civiliz.ations? Despite the enormous times required for spacecran to traverse interstellar distances. might other dviliz.ntions have circumvented this problem and launched large numbers of spacecraft rather than radio or light waves into space? Do civilizations indeed convert the material of their planetary system into the greatest possible living space and thus produce an unusual. perhaps easily detectable type of object in the sky? These questions must be answered if we are to conduct efficiently a search for other civiliutions. The answers are also among the most interesting facts we could learn of life elsewhere. W e find ourselves in that worst of fixes: to solve a problem. we need to know the answer in advance. There is 

50 ASTRONOMY AND ASTROPHYSICS FOR THE 1970'• no solution but the diffic-ult one-to search a vdde ran~ of space. time. and electro magnetic freque.ncy. There are some arguments. none absolutely oompelling, that can direct us to search modes possibly more likely to succeed. Most of these argu· ments apply if other civilizations are purposely trying to make their p~ncc known. ln this case. one argument is that the methcxt that is most economieal in energy is the most likely to be u.sed. ThiJ leads to the deduction lhat electromagnetic radiation is by far the most favorable communications mode. This argument can be carried further by observing that the quantum nature oflig.ht causes lo'•,.er electromagnetic frequencies to be more economical in transmitting information. Howe\'tr. the exist- ence of cosmic radio noise renders the lowest frequencies le55 efficient. The frequency bond of maximum economy is fairly well defined and is in the centimeter·wavelength region. Thus purposeful attempts by others to call a ttention to themseh•es are likely to be at such wavelengths. and it is sensible to emphasize searches at such wavelengths. It is a favorable cir- cumstance that much con\•entional radio-astronomy research is done near 1hese optimum wavelengths. Th• 1000.(1 ,.dlo lelnco~ •• Artcibo, P\ttno llico, il e•p.~bk ot eo•u•unluliq witb • .dml• l•strument •.t • d l&u.JW:e of 1000 up.1 -yean. ("'o ro «nutuy of N •tiD,.I AJtrOIIOmy •ltd I OftO•Iflt,. Cl'ntu.) 

Astrophysical Fro11tiers 51 It is clearly restrictive. however. to construct a search that aims to detect only signals generated to attract our attention. After all, we make no such transmissions. and perhaps few other civilizations do. It would be prefer· able to be able to detect the signals a civilization uses for its o"•n pur· poses. If other civilizations exist, such signals are surely far more num· erous. but unfortunately much weaker, and at completely unpredictable frequencies. Nevertheless, methods have been found that use the in· formation from a telescope to determine if there is present an ensemble of signals typical of a civilization. Thus civilizations may be detected even though no individual signal rises above the noise of the telescope. Such methods call for observations of a large number of frequencies and ex· tensivc analyses of the recordings with high-speed computers; the technology is available. Indeed there exist the know-how and instruments to search for extra· terrestrial civilizations. The reach of our instruments is probably greatest in the radio region. which is one of the most promising wavelength regions in which to seareh. For example, the largest radio telescope, 1000 ft in diameter, could detect a civilization beaming signals with a similar tele· scope at a distance of some 1000 light-years. Some of the na1ural pulsar signals recorded by radio telescopes closely resemble signals that might be received from lransmitters of other civilizarions. Despite the power and promise of our instruments for serious searches for other civilizarions, no major seareh has taken place. The explanation lies in the intense pressure on major astronomical instruments to produce the astrophysical results that are the mainstream of astronomical re- search. Because we cannot accurately predict the effort needed to detect another civilization, quick results cannot be guaranteed. Indeed, the time estimated for a single radio telescope to yield a reasonable probability of success is a few decades, even with high-speed equipment and procedures. In today's rush such a time scale is usually considered unacceptable. Nevertheless. each passing year has seen our estimates of the probability of life in space increase, along with our capabilities for detecting it. More and more scientists feel that contact with other civilizations is no longer something beyond our dreams but a natural event in the history of man· kind that will perhaps occur in the lifetime of many of us. The promise is now too great, either to tum away from it or to wait much longer before devoting major resourees to a search for other intelligent beings. For the lime being, the discovery may come by chance, for many of the ob· servations we now make of natural objects are done using methods that are suitable for detecting intelligent life-the studies of pulsars and of infrared sourees are examples. In the relatively near future we foresee the 

52 ASTRONOMY AND ASTROPHYSICS FOR T HE 1970'' ronstructlon of major facili ties,, such as a glant radio receiving array. a nd the operation of a project that will have as its goal the detection of in· lclligcnt life elsewhere. In the long run chis may be one of science's most imponan·c and most profound contribution to mankind and to our civilizatton. I