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evolution of the interstellar medium, the formation of
galactic halos, and the formation of stars.
Understanding the formation of stars is critical to
understanding the origin of our Sun and solar system and,
ultimately, the formation and evolution of galaxies.
Advances in infrared and millimeter-wave techniques have
permitted astronomers to probe the current birthplaces of
stars--dark interstellar clouds. We have begun to under-
stand the early evolution of stars and perhaps have ob-
served circumstellar disks similar in appearance to the
solar system during its formative phases. How star forma-
tion is triggered, how protostellar gas clouds fragment
to form protostars, how, when, and with what frequency
multiple stars or solar systems form are, however, funda-
mental questions as yet unanswered. The 1980's will wit-
ness major efforts aimed at understanding these problems.
Most stars appear to be losing significant amounts of
material, either in slow leaks of gas, in more vigorous
winds, or in spectacular outbursts. Infrared, ultra-
violet, and optical studies have permitted astronomers to
piece together a rough outline of how mass is lost by
stars of various types, the range of mass loss, and the
chemical composition of the ejected material. m e recent
discovery of stellar coronas in almost all stellar types
together with detailed studies of stellar chromospheres
show that stars have far more complex atmospheres than
previously suspected. Monumental work still needs to be
done to understand these phenomena and their role in
stellar evolution.
The Sun provides the only accessible laboratory for
probing into the physics of, for example, energy gen-
eration, magnetic-field generation, internal convection
and circulation, chromospheric and coronal heating,
explosive dissipation of fields, mass loss and stellar
winds, and short-term and long-term activity cycles, not
to mention all the many possible planetary implications
of these activities. We must exploit these opportunities
for study afforded by the solar laboratory in order to
make progress in understanding the activity of other
stars.
I I . HIGHLIGHTS OF ASTRONOMY IN THE 1970'S
A. Management, Facilities, and Instrumentation
During the 1970's, the Kitt Peak National Observatory
(KPNO) and the Cerro Tololo Inter-American Observatory
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(CTIO), both National Astronomy Centers, were equipped
with 4-m telescopes and became fully competitive with the
best university and private observatories. The growth in
optical facilities at KPNO and CTIO, the conversion of
the Sacramento Peak Observatory into a National Center,
the completion of the NASA 3-m infrared (JR) telescope on
Mauna Kea, and the initiation of extensive guest-investi-
gator programs on the Copernicus, IUE, and Einstein x-ray
observatories have opened observing opportunities to many
more scientists than ever before. A few private and uni-
versity observatories, employing privileged staffs working
with the world's largest telescopes, no longer dominate
observational astronomy. National Center facilities,
located at excellent sites and available to all astrono-
mers, are today playing an increasingly important role in
furthering our knowledge of the Universe.
Despite the growth of ground-based facilities and im-
provements in instrumentation, requirements for observa-
tional programs to support the expanded space effort, in
combination with the traditional ground-based programs,
are now placing such heavy demands on the National Center
facilities that the rational assignment of telescope time
is almost impossible. Usually the choice lies among sev-
eral programs of comparably high merit. In an effort to
schedule as many programs as possible, assignment commit-
tees often give too little observing time to worthy pro-
posals. One clear result of the pressure for telescope
time is a marked shift in the style of observational
astronomy, away from the lone investigator working at the
telescope night after night, and toward the team of
astronomers attempting to make a single pivotal observa-
tion. Both styles can yield outstanding results: the
classic work that led to the concept of stellar popula-
tions required the wartime blackouts of Los Angeles and
many nights at the 2.5-m Mt. Wilson telescope, whereas
the discovery of optical pulses from the Crab nebula
pulsar needed only a few hours on a small telescope.
When, under astropolitical pressure, time-assignment
committees allocate too little time to a worthy project,
it sometimes happens that even the assigned time is
wasted if the project is not completed. We clearly need
more telescopes, together with even more efficient ways
to handle the increasing demand for telescope time.
Very rapid growth in instrumental techniques and
capabilities occurred during the 1970's. Highly sensi-
tive spectrometers capable of accurate subtraction of the
night-sky background were developed and put into regular
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operation. The improvement in the performance of these
devices over that of older techniques is impressive. At
the beginning of the past decade, the most advanced spec-
trophotometer in existence was the 32-channel instrument
used on the 5-m Hale telescope. Modern spectrophotometers
provide an increase of nearly a hundredfold in the number
of available pixels and, with the deployment of COD detec-
tors and simple optical systems, they have achieved more
than a tenfold increase in sensitivity per pixel over the
state of the art in 1970. Our current ability to obtain
accurate spectrophotometry of sources only slightly
brighter than the night sky is a major achievement of
instrument design and fabrication. Without this gain, it
would not be possible to follow up observations at radio,
ultraviolet (W), and x-ray wavelengths with the required
optical work.
These achievements at optical wavelengths are fully
matched by improvements in IR detectors and instruments.
The military interest in IR systems has led to very rapid
advances in detector technology throughout the IR region,
particularly the InSb detectors for use in the 1-5-pm
region, which have achieved orders-of-magnitude improve-
ment over devices available in the early 1970's. The
requirements for a high-quality IR observing site are
different from those for optical observatories in that
the amount of water vapor is a crucial factor. The
addition of telescopes at high-altitude locations has
therefore also been important for progress in ground-
based IR astronomy. Similarly, the design requirements
for an optimized IR telescope differ from those of a
conventional optical telescope, since the thermal back-
ground must be minimized; it is only within the last
decade that large, IR-optimized telescopes have become
operational.
Throughout the whole of the IR spectrum, atmospheric
absorption is troublesome, and it is only within the
1-30-pm region that atmospheric winnows exist that
permit high-quality measurements from the ground. With
the development of IR observatories that operate either
high in the Earth's atmosphere (e.g., balloons, the Kuiper
Airborne Observatory) or above it entirely, the full
benefits of IR astronomy are beginning to be realized.
The IR region also gleans advantages from its inter-
mediate wavelength location between the radio (where wave
detectors are used) and the optical (where photon counting
prevails) wavelength regions. This has allowed hybrid
technologies to be developed, such as spatial interfero-
-
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meters, Fourier-transform spectrometers, and heterodyne
spectrometers. These instruments often allow spatial or
spectral resolution to be obtained that exceeds that which
is possible for stellar sources at other wavelengths.
Moreover, two-dimensional IR detectors seem to be just
over the horizon. Because the atmospheric "seeing" is
better at IR than at optical wavelengths, two-dimensional
pictures in the IR region with a large telescope should
show more detail than comparable photographs in the
optical region.
Major instrumental developments in solar astronomy
include the construction of high-angular-resolution vacuum
solar telescopes on good sites and the construction of
spectrographs with a velocity resolution of the order of
1 m/sec. Together, these instruments yield unprecedented
details of the spatial distribution of velocity fields in
the solar atmosphere. Observations with high-spatial-
resolution magnetometers in the past decade have com-
pletely revolutionized our concepts of the solar magnetic
field. We now know that virtually all magnetic fields on
the Sun occur in regions of high field strength (some 1500
gauss); the small general fields observed earlier may be
explained in terms of the very small filling factor of
these intense fields.
Astrometric astronomers achieved a major breakthrough
in the past decade with the use of finer-grained emulsions
and sophisticated image analysis, which produced a tenfold
increase in the precision with which astrometric parame-
ters can be determined photographically. Since the devel-
opment of the photographic plate, the magnitude of typical
external parallax error has been reduced from 0.02 arcsec
to 0.002 arcsec. This advance has made possible the
determination of parallaxes, and hence luminosities, for
many faint dwarf and degenerate stars. Reliable paral-
laxes and luminosities for stars in the middle and upper
main sequence have been obtained, and astrometric binaries
are being detected with increased frequency.
Speckle interferometry is an emerging field and has
already greatly increased the precision with which the
separation of close binaries can be measured. Together
with the improved parallaxes it is now possible to obtain
much better knowledge of stellar masses, and hence a more
precise determination of the mass-luminosity law, with
implications for the theory of stellar evolution. Inten-
sity interferometers have given us our first reliable
estimates of the diameters of nearby blue stars. Modern
techniques have also permitted the establishment of
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fundamental positions with respect to extragalactic
objects and the subsequent correction of the optical
coordinate system to the very precise radio system.
Astronomy from space came of age in the 1970's. The
operation of stable, sophisticated satellites sensitive
to radiation across the electromagnetic spectrum provided
the entire astronomical community, through extensive guest
investigator programs, with the ability to obtain observa-
tions over a large range of wavelengths. Copernicus, IUE,
and the Orbiting Solar Observatory (OSO) series have
opened wavelength windows that once were seen only in
brief glimpses from sounding rockets. Together with x-ray
satellites, these instruments have profoundly changed our
views of the interstellar medium, the physics of collapsed
objects, the interaction of plasmas with magnetic fields,
and the interrelationship of galaxies with the intergalac-
tic medium. mese new facilities have placed heavy de-
mands on ground-based telescopes used to follow up and
extend the space observations; the space facilities pro-
jected for the 1980's will only increase the demand for
ground-based optical and IR spectroscopy and imaging,
extended monitoring, and synoptic observation.
B. Scientific Programs
1. Galactic Astronomy
The Milky Way galaxy is a highly complex system whose
structure has resulted from the cumulative effects of
physical processes occurring in interrelated subcompo-
nents, e.g., stars, interstellar clouds, globular clus-
ters, and supernovae. Because of this diversity and
complexity, our Galaxy continues to be the primary source
of information concerning fundamental galactic properties
in general, such as the stellar luminosity function or
the conditions that cause interstellar clouds to form
stars.
The 1970's saw an impressive series of scientific
successes, ranging from the interpretation of the thermo-
nuclear evolution of stars to the discovery of theoreti-
cally predicted neutron stars and even objects that are
good candidates for black holes. Galactic astronomy has
benefited from the normal progress of a vigorous research
area and, like the rest of astronomy, has received a major
push from the opening of previously inaccessible electro-
magnetic spectral regions to routine astronomical observa-
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Lions. This has been a primary factor in the establish-
ment of W and much of IR astronomy as major branches of
the Galactic research effort. Simultaneously, the devel-
opment of x-ray, gamma-ray, and more sophisticated radio-
observational techniques, plus improvements in instrumen-
tation in the traditional optical region, have revitalized
areas of classical Galactic studies. Finally, major
impacts on the observational effort have come as a result
of new theoretical insights. Because of the great breadth
of what we consider Galactic astronomy, it is not feasible
to provide a comprehensive review of the field. Instead
we include a representative sample of research programs.
a. Interstellar Medium
Sounding-rocket detections of interstellar H2 and
CO were followed up by far more detailed measurements
with the Copernicus satellite, which also made the first
observations of many other interstellar absorption lines,
such as those of D, HO, and O VI. Further analysis of
Orbiting Astronomical Observatory-2 (OAO-2) observations,
and of new observations by OAO-3, showed that the distri-
bution of atomic hydrogen in the local region of the
Galaxy is highly inhomogeneous, with regions of very low
density in the solar neighborhood extending to large
distances in certain directions. This was also substan-
tiated by an experiment carried on the 1975 Apollo-Soyuz
mission, which made the first detection of hot stars in
the extreme ultraviolet (below 912 A) wavelength range.
OAO-2 and Copernicus observations of interstellar dust
extinction investigated the absorbing properties of the
dust in the W region and its variation in different
regions of space. Far- W photometry and imagery of dust
reflection nebulae and the diffuse Galactic background
radiation revealed that interstellar dust is highly
efficient at scattering W starlight.
The detection of O VI as a ubiquitous component of
interstellar space started a theoretical revolution: old
models gave way to new. We now believe that most of
interstellar space is filled with million-degree gas
instead of cool gas at a temperature less than 104 K.
Stellar winds and supernovae are now thought to provide a
major source of energy affecting virtually all gas, not
just that in the vicinity of these objects.
The study of dominant ionization stages of the first
30 elements confirmed the notion that some heavy elements
are, on average, depleted in space, probably by being
bound in dust grains. The range of depletions is, how-
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ever, very large (up to 103) from region to region and
from cloud to cloud. The theory of grain destruction in
shock waves plausibly explains this result, but detailed
abundance studies have still not isolated a unique grain-
formation mechanism. m e major component of the grains,
which is probably C, N. or O. has not been empirically
identified.
Theories of molecule formation were sorted out with
the detection of H2 and HD. At least for diffuse
clouds, charge-exchange reactions between ions and mole-
cules can explain most observations, as opposed to for-
mation on dust grains. Formation of H2 itself was con-
firmed to occur on grains, the general theory agreeing
with observations of the ratio of H2 to total hydrogen
over a factor of 107
Studies of light elements (Li, B. and Be) and isotopes
(for example, D) helped to confirm older ideas on the
origin of these elements, which are destroyed as gas is
processed through stars. The unexpectedly large amount
of D suggests that it is primeval in origin and that the
density of the explosion in which D was created was quite
low, suggesting an open Universe if the standard ages and
simple theories of the Universe are correct. Data on
interstellar B. Be, and Li are consistent with their for-
mation in situ by reactions between C, N. and O atoms and
. .
cosmic rays.
Near the end of the decade verY-hinh-resolution obser-
vations using Michelson techniques allowed detection of
hyperfine structure in Na I. m is long-searched-for
result implies that internal motions in a few clouds are
much more nearly thermal than had been previously thought.
In conjunction with new global models of the interstellar
medium, this result suggests that interstellar clouds are
a complex of cold, quiescent regions and expanding, evapo-
rating surfaces, impinged upon from all directions by
shock waves, sometimes with quite high velocities.
Direct observations have been made of the stellar,
ionized-gas, dust, and probably the nonthermal components
of the Galactic center region.
Within a complex structure
with scale sizes down to less than 1 parsec, there exist
intricate rotational and random motions as large as 300
km/sec. These data have been interpreted as providing
indications of continuing star formation and perhaps even
for the presence of a massive black hole.
The dense interstellar clouds within which star for-
mation occurs shroud the stellar-birth process in the
optical spectral region but not in the IR. Large molecu-
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lar clouds are detected as luminous, low-temperature ther-
mal sources, but the energy-production process remains
uncertain. At later stages, protostars are found to emit
copious amounts of IR radiation, but detailed knowledge
of the structure and evolution of the protostar is still
missing.
The discovery of vibrational emission from hydrogen
molecules at a temperature greater than 1000 K, near the
core of the Orion Molecular Cloud, provides evidence that
energetic dynamical phenomena are associated with young
stars. Subsequent high spectral resolution IR observa-
tions of CO, H2, and ionized gas leave little doubt
that a shock front is moving out from a central source
with a velocity of 30 to 50 km/see and is at a radius of
about 1017 cm. Millimeter observations of broad CO
emission also indicate an outflow; the total energy
involved has been estimated at more than 1047 ergs.
Thus an evolutionary process on a time scale of about
1000-3000 years is taking place inside the molecular
cloud. The observation of dynamical events associated
with young stars is an exciting research area in which
rapid advances are now possible. Future observations
taking full advantage of new techniques may yield obser-
vations of protostellar collapse, which in spite of much
effort has not yet been observed. -
This is also an imPor-
tant area of research, particularly necessary for compari-
son with theories of star formation, which require more
observational guidance.
More and better data are now being acquired for the
distribution of elements in H II regions that show that
the Milky Way, like other spiral galaxies, is likely to
have a radial gradient in the abundances of common ele-
ments such as C, N. O. and S. Observations of planetary
nebulae are suggestive of a similar trend, indicating that
the degree to which matter has been processed within the
Galaxy has varied systematically with position throughout
much of the Galaxy's history. The origin of the abundance
gradient is not fully understood, but it could result from
the more frequent processing of matter by spiral-arm-
induced star formation at smaller radii.
~ _ _ _ ~
b. Stellar Astronomy
The Copernicus and IUE spacecraft have for the first
time opened the W spectral region to the powerful tech-
niques of high-resolution spectroscopy. As a result of
extensive surveys by these spacecraft, we now realize
that stellar winds are a ubiquitous phenomenon among
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stars and that mass-loss rates range up to 109 times
that of the Sun. It now appears that high mass-loss
rates are common among highly evolved stars as well as
among very luminous young ones. The recent discovery of
large mass-loss rates, as inferred from W resonance lines
with P Cygni-type emission profiles, has major implica-
tions for the evolution of these stars, for the dynamics
of their interstellar environment, and for the dispersal
throughout the Galaxy of chemical elements produced in
stars. The quest to understand the acceleration mechan-
isms and consequences of these strong stellar winds has
become one of the most active and exciting areas of astro-
physics. Circumstellar shells, strong IR emitters in the
2-20-pm region, have been discovered in both young and
old stellar types. The detailed spatial and spectroscopic
studies of these envelopes yield much additional informa-
tion on the evolution of stars and on their interaction
with the interstellar medium.
Ultraviolet spectra from Copernicus and IUE, x-ray
observations from the High-Energy Astronomical Observa-
tory (HEAD) satellites, and ground-based optical studies
have shown that phenomena previously studied mainly on
the Sun, such as chromospheres, coronas, and flares,
occur also in a very wide range of stars. Chromospheres,
for example, are found in essentially all stars cooler
than type early F. but the chromospheric heating rates
vary by several orders of magnitude for stars of the same
type. Observations from the Einstein and IUE satellite
observatories have shown that essentially all stars, with
the probable exception of cool giants and supergiants,
have hot coronas. The roughly 3-orders-of-magnitude
spread in the x-ray surface fluxes at each spectral type
and the existence of coronas in OB stars clearly elimi-
nates the long-held idea that coronas are heated by
connectively generated acoustic waves. Instead, heating
by dynamo-generated or remnant turbulent magnetic fields,
either through magnetohydrodynamic wave processes or field
annihilation, is now felt to be likely. A major theoreti-
cal effort to understand these heating processes is now
under way, guided by in-depth studies of the spatially
resolved solar corona. Also, flares with energies up to
5 orders of magnitude higher than those of large solar
flares are now being studied in both dMe and RS CVn-type
close binary systems. These phenomena reveal the all-
pervasive role that magnetic fields play in the outer
atmospheres of stars, as was previously known for the
Sun. They also point out the critical need to measure
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magnetic fields directly in many stars, which is now
feasible, and to measure accurately stellar rotation
rates on which dynamo processes depend.
me major role of dust grains in the outer atmospheres
of stars is now recognized, and conditions favoring dust
formation are being deduced from the thermal-emission
properties of dust shells. Dust is an integral part of a
wide range of circumstellar environments, which include,
for example, cool giants and supergiants, novae, and
Wolf-Rayet stars. In most instances the dust seems to
have formed in a mass outflow, and the infrared charac-
teristics can therefore provide information on stellar
mass loss. The chemical composition of dust in circus
stellar shells has in part been revealed through the
discovery of the IR silicate emission feature. Further-
more, there are suggestions that the dust in some stars,
such as novae, may be present mainly in the form of
graphite. Curiously, the silicate-emission properties of
dust embedded in comets have proved to be similar to
those of dust formed in the winds of dying stars.
High-dispersion spectroscopy of fainter stars has
resulted from the implementation of better detectors on
existing coude spectrographs and the construction of
echelles for use at the Cassegrain focus. A variety of
metal-poor stars has been subject to detailed abundance
studies. m is has uncovered patterns in abundance ratios
as a function of overall metallicity, which can be related
to the origins of heavy elements in the early Galaxy
through the theory of stellar nucleosynthesis.
Globular clusters are of essential value to the study
of Galactic astronomy because of the relative simplicity
of their structure and dynamics, the extreme character of
their stellar population, and the information they provide
about the dynamical and chemical evolution of the Galaxy.
It now appears that there are significant variations of
age and chemical composition among the globular clusters.
These results will play an important role in future
efforts to reconstruct the evolution of the Galaxy. They
already suffice to show that the postulate of an initial
rapid collapse of the halo represents an oversimplifica-
tion of the processes that must have occurred during the
formation of the Galaxy. The discovery during the 1970's
of x-ray sources in globular clusters is especially excit-
ing because of the possibility that their presence signals
the existence of massive objects (such as black holes) in
the cores of clusters. At the least, these x-ray sources
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113
must be representative of late stages of stellar evolu-
tion.
New classes of stars are now recognized on the basis
of their IR properties; some of these are new aspects of
well-known types of objects. For many transient stars,
such as Eta Carinae, the optical flux does not necessar-
ily indicate the true luminosity, since a large fraction
is often converted into IR radiation by circumstellar
dust. Other examples of unusual objects include the disk
IR emitters, such as CRL 2688, and the classes of stars
that are completely obscured in the optical range, prob-
ably the result of an optically thick dust shell.
Great advances have been made in the past decade in
the observation and theoretical understanding of late
stages of stellar evolution and in the detailed under-
standing of collapsed objects. The study of counterparts
of x-ray sources with ground-based optical spectroscopy
and the IUE satellite has revealed many close binary sys-
tems, which often include compact objects, high magnetic
fields, accretion disks, gas streams, and beamed emission.
Significant improvements in our theoretical understanding
have been achieved in the related problems of the equa-
tion of state at very high densities, the theory of black
holes, the causes of nova outbursts, the elucidation of
helium-burning phases of stellar evolution, and the theory
of stellar pulsation.
m e proof of the existence of neutron stars is one of
the major astrophysical breakthroughs of the 1970's.
These objects have masses comparable with the Sun's but
with mean densities more than 1014 times that of normal
matter. Hard x-ray spectral observations imply that mag-
netic fields associated with neutron stars may be as high
as 1012 gauss. The first neutron stars were discovered
as radio pulsars, one of which has been shown to be in a
binary system. Four, including the Crab and Vela pulsars,
were subsequently detected at gamma-ray energies. A
second group of neutron stars was discovered as pulsating
x-ray sources. All of these appear to be members of close
binary systems, in which the x-ray emission seems to be a
to a magnetic neutron star.
It is apparent from all these remarkable properties that
neutron stars offer a unique testing ground for our under-
standing of the fundamental physical laws of nature.
There has also been substantial Progress in understand-
consequence of mass transfer
~ _
ing cataclysmic variables and similar systems during the
past few years, and many questions involving these systems
now appear to be on the threshold of resolution. There
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is mounting evidence that nova outbursts are thermonuclear
explosions, while dwarf-nova outbursts appear to result
from nuclear reactions caused by accretion events. Fur-
ther theoretical studies combined with W photometry and
elemental abundance measurements could firmly establish
these models within the next few years.
2. Extragalactic Astronomy
a. Galaxies and Clusters of Galaxies
The completion of several 4-m telescopes, coupled with
major advances in detector technology during the past dec-
ade, have led to a virtual revolution in our analysis of
the large-scale mass distribution in the Universe, which
has strengthened evidence in favor of a dominant, nonlumi-
nous component of cosmic matter. Observations suggest
that the total mass distributions of ordinary spiral gal-
axies extend far beyond the optically visible disks. At
least some fraction of the nonluminous matter in the Uni-
verse resides in the outer regions of individual galaxies
themselves. Optical searches for this matter have been
inconclusive, confirming only that its luminosity per
unit mass is much lower than that of conventional stellar
matter.
A remarkable development is the recent discovery that
the rotation curve of virtually all spiral galaxies
remains flat to the limit of detectable emission. No
galaxies exhibit diminishing stellar velocities at large
nuclear distances, as would be expected for centrally
condensed objects. This result implies that significant
mass is located at large nuclear distances, so that the
total size and mass of spiral galaxies are much larger
than previously thought. Because most of the material in
the outer region is unseen, the physical properties of
the material are largely unknown. The possibility that
the unobserved mass is gas can be ruled out, but there is
still no evidence for large numbers of faint M dwarfs in
the halo.
Challenging dynamical problems must be overcome before
the disturbed nature of the outer parts of the Galactic
disk are understood. Many external galaxies show the
same nonplanar outer structure. Although the dynamics of
galactic encounters can explain some warps, they are also
observed in isolated galaxies with no obvious companion.
Problems of maintaining the warp against dispersive
effects are particularly puzzling to theorists.
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115
The molecules CO and HCN were discovered for the first
time in external galaxies during the 1970's. In the Milky
Way, CO studies have played a major role in determining
sites of active star formation. High-resolution CO maps
of galaxies of a wide variety of Hubble types could pro-
vide a cogent test of star-formation theories in other
types of galaxies.
Decades from now, the 1970's will be remembered as a
period when the full complexity of galaxy evolution was
glimpsed for the first time.
that galaxies, even after formation, are not the isolated
island universes that Hubble envisioned. On the contrary,
galaxies interact with each other and with their environ-
ments in a complex way. Some bizarre galaxy forms are
now understood as two galaxies in collision, or galaxies
tidally distorting one another. Within clusters, central
massive galaxies can grow by mergers or at the expense of
the halo stars in less massive neighbors. The sizes of
the great clusters and the extent of the holes between
clusters are larger than many would have imagined in 1970.
There is now a realization
b. Quasars
The long-standing puzzle over the nature of the red
shifts of the quasi-stellar objects is now close to solu-
tion. Groups of galaxies have been found surrounding a
number of low-red-shift quasars; these QSO's have the same
red shift as the surrounding galaxies. Furthermore, some
objects thought to be closely related to quasars appear
to be nuclei of galaxies, again with the same red shift
as the galaxy. These discoveries support the hypothesis
that the red shifts of at least some quasars are cosmo-
logical. It seems likely that quasars are really very
distant, with enormously high luminosities. Brightness
variations on short time scales indicate that the central
energy source is extremely small--comparable in size with
our own solar system.
Studies of both optically selected and radio quasars
show that quasars were much more numerous and possibly
more luminous in the past. However, a decline in detected
quasars sets in at a red shift of 3.5. Limits put on
distant quasars by the x-ray background suggest that this
apparent decline in numbers is real and that the increase
in quasar numbers does not continue beyond z = 3.
Very recent studies have suggested that the mysterious
quasar absorption lines have multiple origins. Some evi-
dently arise in the quasar, some in a surrounding galaxy,
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some in intervening galaxies, and some perhaps in inter-
galactic clouds. m ese clouds therefore provide a unique
probe of gas densities and abundances under a wide
variety of conditions at very large red shifts.
It is possible to study from the ground the spectra of
high-red-shift quasars to below the Lyman limit at 912 L.
During the 1970's it became possible to make similar
studies of the much nearer ordinary and peculiar Galaxies
and low-red-shift quasars.
First by means of OAO-B, and
later in more detail using IUE, it has been found that
between 1200 and 2000 ~ the spectrum of ordinary gal-
axies is dominated by spectra of hot stars. These stars
give clues to the evolutionary history of the galaxies.
Rocket and IUE observations of Seyfert galaxies have show
that the emission lines in the W region have far differ-
ent intensities than predicted by simple theories. The
IUE observations support the idea that, in some of these
objects, interstellar dust plays an important role. In
other Seyferts, just as in quasars, there is little or no
evidence for dust even though the line intensities are
peculiar. IUE observations of the low-red-shift quasar
3C273 show that absorption lines are absent, supporting
the view that most of the absorption lines seen in large-
red-shift quasars are produced by intervening galaxies
and gas clouds.
Among the many possible models for quasars and active
galactic nuclei, accretion of material onto black holes
with masses between 106 and 101° solar masses now appears
to be the most likely. Regardless of whether this spe-
cific model eventually proves to be correct, however, a
more important conclusion has emerged. Despite the enor-
mous energies involved in some of the outbursts observed
in quasars and active galaxies, there is no strong theo-
retical reason to doubt the cosmological nature of the
observed red shifts or to believe that "new physics" is
required to understand these objects. Still, the mystery
posed by their energetics is one of the most challenging
in contemporary astronomy.
c. Cosmology
Recent research in cosmology has been dominated by
the impact of the discovery of the cosmic microwave back-
ground radiation in 1965. Most astronomers accept this
3 K radiation as a relic of the primeval fireball created
in the big bang. During the 1970's, the blackbody nature
of the microwave radiation was generally verified, al-
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though tantalizingly small departures from a blackbody
curve may have been detected at both long and short wave-
lengths. These departures are important because they
trace the detailed early thermal history of the Universe.
An anisotropy in the background due to the Earth's
motion was apparently detected, and the amount was sur-
prisingly large, near 600 km/sec. This implies that the
presence of the Virgo supercluster is sufficient to slow
the expansion in the vicinity of the Galaxy. With respect
to the background radiation, the Galaxy and the Local
Group have a velocity of about 400 km/see toward the cen-
ter of the Virgo supercluster. The small-scale anisotropy
of the background radiation is less than 10 4 on an
angular scale of 10 arcmin.
The magnitude of the Hubble constant Ho, which mea-
sures the present rate of expansion of the Universe,
remains a source of controversy, and the currently
accepted value is probably uncertain by a factor of 2.
The value assigned to Hb affects the assumed luminos-
ities, sizes, and densities of virtually all extragal-
actic objects; it also sets an upper limit to the age of
the Universe and clustering time scales for galaxies.
Classical procedures and novel techniques now being used
to evaluate Ho from cosmological observations will we
hope lead to a single value of high accuracy.
Classical cosmological tests have been pushed to
greater look-back time; the red-shift-magnitude relation
for galaxies now extends out to red shifts of unity.
However, our increasing understanding of galaxy evolution
makes it clear that such tests are more sensitive to the
evolution of galaxies than they are to the large-scale
structure of the Universe. Near the end of the decade it
was found that there is a strong correlation between the
continuum intensity of QSO spectra and the strength of
the C IV line. This promises to provide a technique for
calibrating the intrinsic luminosity of quasars. Com-
bining ground-based observations of high-red-shift quasars
with space observations of low-red-shift quasars could,
in principle, yield a firm value for the deceleration
parameter go. However, because the underlying cause of
the correlation is not understood, it is possible that
evolutionary effects could result in a major distortion
of the Hubble diagram and skew the derived value of
go. Thus, we cannot yet state whether the Universe is
open or closed.
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3. Solar Astronomy
Solar Magnetic Fields
Hale's early investigations of solar magnetic fields
distinguished between strong, sunspot-related magnetic
fields and a weak, general magnetic field of the order of
1 gauss in strength. During the last decade, high-
resolution magnetographs and sophisticated observing
techniques have revolutionized our concept of the solar
magnetic-field structure. We now believe that virtually
all solar magnetic fields occur in regions of very high
field strength (1500 gauss). The 1-gauss general magnetic
field observed by Hale was the result of the small filling
factor of the high-field magnetic elements. The true size
of these magnetic elements is unknown, since they are too
small for even the best ground-based magnetographs to
resolve. Theoretical attempts to explain the origin and
stabilities of these magnetic-flux tubes are under way.
The study of solar vector magnetic fields, spatially
resolved and with unprecedented sensitivity, will be a
prime objective of the SOT on the Space Shuttle, which is
designed to achieve 0.1 arcsec spatial resolution.
b. Coronal Holes and the Solar Wind
l
One of the outstanding discoveries in solar physics
during the past decade was the recognition that the so-
called solar M regions responsible for the geomagnetic
storms do not coincide with regions of solar activity
but, quite to the contrary, with extremely inactive
regions on the Sun. Whereas the magnetic fields in the
solar active regions of high magnetic flux are "closed,"
the fields in the low-magnetic-flux solar-polar regions
and in some other nonactive regions on the Sun are "open,
extending outward away from the Sun toward the Earth and
the other planets. For reasons not yet fully understood,
the high-speed component of the solar wind (expanding at
about 1000 km/see) originates in these open magnetic-field
regions in the solar corona, resulting in lower-density
coronal plasma and easy visibility of these so-called
"coronal holes" as dark regions in solar x-ray images.
Since coronal holes do not share the differential rota-
tion with solar latitude seen in the solar photosphere,
but rather appear to rotate solidly, they are thought to
be anchored in the solar interior. The energy balance in
coronal holes differs fundamentally from that in active
regions, as coronal-wind expansion is the dominant cooling
mechanism in holes, whereas radiative losses and thermal
conduction dominate for active regions. The all-pervasive
Representative terms from entire chapter:
neutron stars
