Astronomy and Astrophysics for the 1980's, Volume 1 Report of the Astronomy Survey Committee (1982) / Chapter Skim
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3 FRONTIERS OF ASTROPHYSICS
3 FRONTIERS OF ASTROPHYSICS
Pages 37-100
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From page 37... ...
According to this model, the geometry of space-time is curved by matter, and the curvature forces the matter to move: at any epoch the Universe must be either expanding or contracting. Hubble's discovery in 1929 that the Universe is actually expanding forces us to confront a bizarre implication of the theory: that an expanding Universe must have originated in a powerful explosion-referred to as the big bang before which neither time nor space had any meaning.
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From page 38... ...
With microwave antennas, they have discovered a faint radio noise that they interpret as the remnant of the big bang itself. From the theory of the nuclear reactions that must have taken place during the first 3 minutes, they have calculated the abundances of key elements and isotopes such as hydrogen, deuterium, and helium, which were produced in the big bang; with ground-based telescopes and ultraviolet spectrographs in Earth orbit they have verified that the actual relative numbers of these atoms in space agree surprisingly well with theoretical predictions.
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From page 39... ...
If, on the other hand, intergalactic gas is responsible for at least part of the x-ray background, one can infer that it is distributed uniformly; moreover, the amount of gas required is an important datum for the theory of evolution of galaxies. The cosmic microwave background radiation also gives information about the large-scale structure of the Universe.
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From page 40... ...
will for the first time resolve Cepheid variable stars in the Virgo cluster, thereby eliminating an uncertain intermediate step of the distance ladder. The continued deployment of advanced optical detectors at ground-based telescopes will make possible the rapid measurement of red shifts of galaxies at moderately large distances, where the velocity field should be one of nearly pure expansion; ST can determine the distances of the same galaxies by comparing the brightness of their globular clusters with the bright
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From page 41... ...
Observations of helium and deuterium in such gas, however, must be made at much shorter wavelengths than are accessible to groundbased observatories; they require ST. With ST we can study helium lines in clouds of red shift greater than unity and deuterium lines in clouds of all but very low red shifts.
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From page 42... ...
Telescopes of the 5-m class will make important contributions, but only a new telescope of the 15-m class, such as the New Technology Telescope (NTT) , can measure the red shifts of galaxies at large distances rapidly enough to accumulate the required number of galaxies.
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From page 43... ...
Various possibilities have been suggested to account for hidden mass: diffuse gas, massive neutrinos, collapsed stars (white dwarfs, neutron stars, black holes) , and faint red dwarfs.
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From page 44... ...
However, if that were so, one would expect a higher concentration of heavy elements in the outer parts of the galaxies, since massive stars synthesize heavy elements and eject them into the interstellar medium; this is contrary to observation. Faint red dwarfs could also account for the hidden mass, as large numbers of them in the outer parts of galaxies would be consistent with both the lower concentrations of heavy elements and the lower light levels observed there.
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From page 45... ...
Now the Very Large Array (VLA) radio telescope can image galaxies both in the 21-cm line produced by interstellar atomic hydrogen and in the synchrotron radiation produced by relativistic electrons gyrating in interstellar magnetic fields; it can thus trace the distribution and state of the interstellar medium with angular resolution comparable with that of optical telescopes.
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From page 46... ...
The spectra of isolated elliptical galaxies should manifest subtle changes that reflect the evolution of the stars that they contain, while isolated spiral galaxies should in addition manifest the progressive depletion of interstellar matter, as well as its enrichment in heavy elements produced by supernova explosions. A major indirect effect will be the reduction in the number of short-lived massive stars as the gas required to form them is depleted.
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From page 47... ...
The fact that globular clusters containing 105 to 106 solar masses are so common would be a natural result of isothermal fluctuations. If galaxies formed out of objects having 105 to 106 solar masses, then groups and clusters of galaxies must have formed subsequently through gravitational clustering of the galaxies themselves.
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From page 48... ...
Most of the energy injected into the interstellar medium, on the other hand, comes from the explosions of massive stars, in which an entire star is disrupted to form a supernova. Supernova explosions are also the principal sources of heavy elements.
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From page 49... ...
Supernova explosions apparently occur frequently enough today to keep ellipticals swept clean of interstellar matter. In spirals, on the other hand, the initial supernova rate may not have been great enough to cause a catastrophic purging of the interstellar medium, and so a sufficient amount of interstellar matter still remains today to support active star formation.
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From page 50... ...
In effect, cosmic rays constitute a relativistic gas with a pressure comparable with that exerted both by random motions of the interstellar gas and by the interstellar magnetic field; the cosmic-ray gas therefore plays a critical role in the equilibrium inflation of the gaseous disk, in the fragmentation of the interstellar medium into molecular-cloud complexes, and, presumably, in the support and activation of halos of galaxies. Determining the origin and propagation of cosmic rays is therefore an important aspect of the overall effort to understand the course of galactic evolution.
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From page 51... ...
NTT~ whose large collecting area will make it possible to obtain spectra of various subsystems of nearby galaxies, will reveal variations in abundances expected to develop as the result of different rates of evolution at different points within galaxies; the 25-Meter Millimeter-Wave Radio Telescope will permit determination of isotopic abundances of CO and other molecules with sufficient angular resolution to detect variations in abundances across the faces of galaxies; its beamwidth corresponds to 60 parsecs at M31. By yielding sharp images of large-red-shift galaxies, ST will permit them to be classified morphologically for the first time; this is essential in order correctly to interpret observed correlations between the red shifts and other properties of galaxies.
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From page 52... ...
Here AXAF, with its ability to observe the diffuse, hot gas with high angular resolution, increased sensitivity, and high spectral resolution, will be able to examine many radio galaxies at larger distances, for evidence of the M87 phenomenon. The whole problem of activity in the nuclei of galaxies poses a major puzzle in galactic evolution.
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From page 53... ...
Until the 1950's, studies of cosmic rays were limited to those particles reaching the Earth; the bending of the trajectories of cosmic rays by interstellar magnetic fields precludes the identification of their sources by observing the directions from which they arrive at the Earth. A breakthrough in our understanding of the role of high-energy particles in cosmic processes occurred in the 1950's, when optical and radio astronomers discovered polarized emission from supernova remnants and showed that it is synchrotron radiation from relativistic electrons accelerated and trapped in magnetic fields.
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From page 54... ...
Optical, radio, and x-ray observations of supernova remnants strongly suggest that most Galactic cosmic rays are initially accelerated in supernova explosions. Supernovae are also believed to be responsible for the synthesis of heavy elements, and thus they play a critical role in the chemical evolution of galaxies and, ultimately, in the origin of stars, of planets, and of life.
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From page 55... ...
The pulsar in the Crab nebula, which is the remnant of supernova 1054, has been observed over the entire electromagnetic spectrum from radio waves to gamma rays. The known origin of the nebula in a supernova explosion together with the indication from the observed synchrotron radiation that electrons are accelerated by the central pulsar strongly suggest that supernovae are a prime source of Galactic cosmic rays.
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From page 56... ...
Measurements of the relative abundances of radioactive nuclei in cosmic rays, which have a variety of mean lifetimes, have shown that cosmic rays are contained for a few million years in the Galaxy; the constraints this puts on the required energy sources are consistent with acceleration in supernova shock waves. Binary Star Systems A large fraction of all stars is found in binary systems.
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From page 57... ...
Although an isolated black hole cannot be detected with current techniques, a black hole that is accreting gas from a close binary companion star may be revealed by the x-ray emission of its accretion disk. The analysis of Cygnus X-1 shows that it is a binary system containing a compact object of about 10 solar masses; this object may be a black hole, as no satisfactory alternative interpretation of x-ray emission from the system has been found.
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From page 58... ...
Found in the central regions of our Galaxy and at the centers of condensed globular clusters, these systems emit up to 1039 ergs of x rays in bursts lasting only 10 to 100 sec. These bursts appear to originate in weakly magnetic or nonmagnetic neutron stars that are members of close binary systems of low mass.
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From page 59... ...
If the Doppler broadening of this line is due to the revolution of the emitting gas about a central gravitating object, the region within about 1 parsec of the Galactic center must contain some 106 solar masses of material. While this may be explained by a dense concentration of stars, its association with Sgr A makes it more likely that it is a single massive object, whose accretion disk produces the relativistic electrons responsible for the radio emission of Sgr A
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From page 60... ...
The first quasars were discovered through analysis of the spectra of optical counterparts of pointlike radio sources, but it is now clear that only a small fraction of quasars are strong radio sources. Many new quasars have been found recently through spectroscopy of the optical counterparts of faint xray sources discovered by the Einstein x-ray observatory.
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From page 61... ...
Except for the big bang itself, quasars are the most powerful explosions in nature. Quasars resemble radio galaxies in that compact, active regions are connected by jets to distant radio lobes.
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From page 62... ...
Because of the enormous amounts of energy produced much of it in the form of relativistic particles-in such a small volume of space, the nature of the central energy source of quasars is of exceptional interest. Among the possibilities are a compact cluster of neutron stars or stellar black holes or both undergoing frequent collisions, a massive plasma cloud stabilized by rotation, and an accretion disk formed from matter spiraling in toward a single massive black hole containing 100 million solar masses or more.
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From page 63... ...
The distribution and spectrum of cosmic rays in the Galaxy can be inferred from the detection by GRO of gamma rays resulting from the interaction of cosmic rays with the
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From page 64... ...
Theories for the formation of heavy elements in supernova explosions will also be tested by GRO; the characteristics specified by the Space Science Board's Committee on Space Astronomy and Astrophysics (CSAA) in its 1979 report would permit it to detect gammaray emission lines from freshly synthesized nuclei in supernova explosions out to the Virgo cluster, where a new supernova occurs about once every year.
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From page 65... ...
It is not known why there are no known quasars with red shifts greater than 3.5. Did quasars simply not form until 2 billion or 3 billion years after the big bang, or are more distant ones being obscured by intervening dust?
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From page 66... ...
FORMATION OF STARS AND PLANETS The Interstellar Medium Interstellar space is not empty. All along the Milky Way are dark clouds containing microscopic particles of interstellar dust.
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From page 67... ...
As predicted, the presence of H2 is correlated with that of dust. Roughly half of the interstellar hydrogen in our Galaxy (about 5 x 109 solar masses)
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From page 68... ...
U.S. radio astronomers have led the world in the study of molecular clouds as a result of the availability of suitable radio antennas and good millimeter-wave receivers; the 11-m millimeter-wave antenna on Kitt Peak operated by the National Radio Astronomy Observatory has played a particularly important role in such studies.
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From page 69... ...
The higher sensitivity and angular resolution to be provided by GRO will permit many more of these clouds to be studied at gamma-ray wavelengths. Our current understanding of the interstellar medium is based on the fact that the energy radiated by interstellar matter must be supplied by the absorption of energy from supernova shock waves, from ultraviolet radiation from hot stars, and from interactions with cosmic rays.
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From page 70... ...
Molecular Clouds and Star Formation The fundamental rotational transition of the carbon monoxide molecule CO at a wavelength of 2.6 mm is a widely used tracer of the distribution of molecular clouds in the Galaxy. An important frontier for future research is to understand the distribution of such clouds in other galaxies; the 25-Meter Millimeter-Wave Radio Telescope (at 2.6-mm and 1.3-mm wavelengths)
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From page 71... ...
Massive stars form in the dense cores of molecular clouds, as indicated by the observation of infrared sources, H2O and OH masers, and microwave-continuum sources in these regions. According to one theory, dense cores of clouds form when the ultraviolet ra A 12.4-,um map of the central region of the Orion molecular cloud, including the BecklinNeugebauer "protostar" and the Kleinmann-Low nebula, made with the Goddard Space Flight Center infrared CID array camera.
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From page 72... ...
For example, the study of planetary atmospheres has led to the development of concepts and techniques that apply more broadly to the dynamics of stellar atmospheres and the interstellar medium; in situ measurements of planetary magnetospheres have provided new insights into the processes of magnetic-field reconnection and particle acceleration; the theory of the internal heating of Jupiter-like planets has consequences for more general studies of the collapse and structure of low-mass protostellar and protoplanetary objects; and studies of comets, meteorites, and other primitive bodies of the solar system have provided insights into the chemical composition of the original solar nebula that may well apply to interstellar clouds generally. The mechanisms that led to the formation of the Sun and its planets must be at work in the Galaxy today.
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From page 73... ...
Future infrared telescopes on Earth and in Earth orbit will contribute to further understanding of all the planets by imaging and spectrally analyzing their infrared emission. NTT will speed up spectroscopy in the near infrared, while the LDR, because of its large collecting area and high angular resolution at 20-llm wavelength and longer (better than 1 arcsec)
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From page 74... ...
Are supernova shock waves required to make them unstable toward collapse? Why do they fragment into objects of stellar mass as they collapse, and what determines the distribution of masses?
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From page 75... ...
In molecular clouds, extension of the accessible frequency range using the 25-Meter Millimeter-Wave Radio Telescope and the 10-m submillimeter-wave radio antenna will bring many new organic molecules, as well as excited states of known molecules such as CO and HCN, under study. In the vicinity of young stars and of shock waves formed in cloud cores, the temperature is high enough to excite submillimeter and infrared lines of molecules.
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From page 76... ...
Gradually, as instruments improved, additional features of solar activity were discovered, including eruptive prominences, small flares that appear in newly emerging active regions, and occasional large flares. It was found that flares are the source of highly energetic particles, called solar cosmic rays.
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From page 77... ...
The solar wind extends far into space, forming a "heliosphere" within the surrounding interstellar medium, which is a vast sea of plasma activity perhaps several hundred astronomical units across. It is filled with fast and slow streams of solar wind, interacting violently to produce shocks and fast particles, and is swept constantly by blast waves from solar flares.
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From page 78... ...
Renewed efforts with infrared, optical, ultraviolet, x-ray, and gammaray telescopes, together with determined theoretical studies, will be necessary for progress toward understanding some of the more complex aspects of solar activity during the 1980's. Instruments aboard the Solar Maximum Mission carried out the first precise, coordinated studies of flares in ultraviolet light and x rays, observing particularly the extremely compact and intense centers of activity with temperatures above one hundred million degrees that lie at the heart of solar flares.
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From page 79... ...
The flux of solar neutrinos detected by this experiment is less than one third as intense as predicted by current theories of stellar energy 79 A large solar flare photographed in hydrogen light. The combed appearance of the solar gases is due to the strong magnetic fields in the flare region.
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From page 80... ...
Observations of chromospheric lines in less massive stars indicate that they, like the Sun, are surrounded by superheated gases, suggesting that they also have active regions and therefore localized magnetic fields. Many of these stars display magnetic cycles, during which stellar activity waxes and wanes with a regular period like the sunspot cycle.
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From page 81... ...
satellite extended these studies to a much greater variety of stellar types. Observations with the Einstein x-ray observatory revealed that stars of nearly all types are surrounded by coronas at temperatures reaching millions of degrees, implying the existence of magnetic fields and hence active regions, stellar winds, and perhaps flares.
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From page 82... ...
It has been suggested that magnetic fields were trapped in the interiors of stars at the time they formed from interstellar gas. According to this suggestion, the field slowly rises to the surface of the star over its lifetime.
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From page 83... ...
Mathematical solutions of these equations incorporating reasonable estimates for the fluid velocities within the convective zone predict magnetic fields remarkably like the actual magnetic fields of the Sun. On the assumption that the angular velocity of solar gases increases with depth, the equations predict that an east-west magnetic-field component develops first at middle and high solar latitudes and then migrates toward the equator, where the oppositely directed fields from opposite hemispheres meet and cancel each other.
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From page 84... ...
It is now clear that both the hot, blue giant stars and the cool, red giant stars have stellar winds and that these winds are sometimes prominent features of regions of star formation. The most luminous stars appear to lose mass at rates up to a billion times the mass lost in the solar wind.
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From page 85... ...
These winds are rich in dust grains and molecules; since nearly all stars more massive than the Sun eventually evolve into red giants, such winds provide a major source of new interstellar gas and dust, furnishing a vital link in the cycle of star formation and galactic evolution. As for hot stars, the mechanism for driving these winds is not well understood.
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From page 86... ...
86 ASTRONOMY AND ASTROPHYSICS FOR THE 1980's servations of the motions of the radio maser sources in such regions, because its enhanced sensitivity will allow the study of fainter, more numerous sources. PLANETS, LIFE, AND INTELLIGENCE Molecular biology has given penetrating insight into the nature of life.
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From page 87... ...
The terrain slopes upward to the left. (Photo courtesy of the National Aeronautics and Space Administration)
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From page 88... ...
More than 50 different molecules have now been found in molecular clouds, including long-chain molecules such as HCgN. The abundances of the simpler molecules observed have been explained by ion-molecule exchange reactions, but it is not yet certain whether the more complex molecules require other processes, such as surface catalysis on dust grains, for their explanation.
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From page 89... ...
By enabling studies of new molecules and higher energy states, with increased sensitivity and angular resolution, the millimeter, submillimeter, and infrared instruments recommended in this report offer an opportunity to pursue the chemistry of carbon in interstellar clouds, and particularly in the collapsing cores of dark clouds, where stars and planets are believed to form. Furthermore, they will offer additional opportunities to study the molecular composition of comets, which are believed to be samples of the most primitive material in the solar system.
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From page 90... ...
The technology is now available to make significant searches of this kind. The 300-m radio telescope of the National Astronomy and Ionosphere Center at Arecibo, Puerto Rico, is capable of receiving a message beamed at us from any of the hundreds of billions of stars in our Galaxy, provided the civilization sending the message were transmitting with a facility similar to that at Arecibo.
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From page 91... ...
A theory that unifies electromagnetic and weak nuclear forces has been successfully developed along with a comprehensive theory of the strong nuclear force; new theories aimed at unifying both of these theories are now being proposed. Astronomical data have played a role in these developments and may play an even greater role in the future.
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From page 92... ...
Laboratory studies of nuclear reactions show that there are actually two types of nuclear force, strong and weak; the latter is associated with an unusual particle called the neutrino.
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From page 93... ...
The gauge theory of weak and electromagnetic interactions in its original form says nothing about the problems of solar neutrinos or hidden mass. However, pursuing the principle of gauge invariance behind it, physicists have constructed a theory of the strong nuclear force, called quantum chromodynamics, or QCD.
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From page 94... ...
Some theories of this type predict that there should be the three types of electrons that are actually observed, as well as three corresponding types of neutrinos, called e, mu, and taut In some versions of the theory, e, mu, and tan neutrinos are regarded as three aspects of the same basic neutrino, which has a finite rest mass and which oscillates back and forth among its three aspects. Although the nuclear reactions in the Sun emit only e neutrinos, according to some Grand Unified Theories neutrino oscillations would be expected to occur long before the neutrinos reached the Earth, so that at the Earth one would observe a random mixture of e, mu, and tan neutrinos.
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From page 95... ...
Before the First Three Minutes Although astronomical data now available appear to be in agreement with the predictions of big-bang cosmology, the big-bang model cannot yet be considered conclusively proven, so that it is of the greatest importance to test its predictions however possible. In particular, the model predicts that the cosmic microwave background originated as high-temperature radiation in the first few minutes of time.
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From page 96... ...
Einstein's theory, unlike Newton's, is believed to be valid for very strong gravitational fields and for bodies moving close to the speed of light; it is therefore crucial for an understanding of systems such as neutron stars, black holes, and the expanding Universe. The General Theory of Relativity predicts that when any nonspherical body collapses to form a compact object or a black hole, it emits a new form of energy called gravitational radiation.
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From page 97... ...
Black holes of all sizes could have been created in the big bang; in particular, those having masses about of 10~5 g (the mass of a small mountain on Earth) would just be evaporating now, giving rise at the ends of their lives to bursts of gamma radiation.
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From page 98... ...
98 ASTRONOMY AND ASTROPHYSICS FOR THE 1980's to take account of the process of creation itself. Is it possible, as astrophysics pushes the frontiers of time back to the moment of cosmic creation, that the existence of the Universe will be recognized as a consequence of the nature of the fundamental force?
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From page 100... ...
The primary mirror for the Space Telescope being inspected after figuring. Photo courtesy of the National Aeronautics and Space Administration)
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