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example at about 50 cm, would represent a major expansion
of the scientific capabilities of the VLA. Even longer-
wavelength capabilities are possible by constructing an
independent Yagi array tied together with the present
waveguide system. m e overall resolving power of the VLA
could be increased by placing outrigger antennas well
beyond the arms of the present Y and con- nested to it by
microwave links. m e nature of the VLA system is well
adapted to these improvements, which will ensure that it
remains the state-of-the-art instrument for a continually
growing area of observational radio astronomy.
The Panel also recommends that serious consideration
be given to upgrading the 300-m antenna at Arecibo, the
nation's (and the world's) largest radio telescope in
terms of collecting area. The highest priority is for a
300-m extension to the radius of the large fixed primary
antenna, to a diameter of 360 m. The estimated cost is
$11 million, or about 25 percent of the replacement cost
of the entire telescope. The telescope would remain in
operation throughout the construction.
Extension of the reflector would increase the average
signal-to-noise ratio of the instrument by 90 percent and
would increase the number of objects of any given type
that can be observed by a factor of about 2.5. The
greatest use of Arecibo at present is in the observation
of gas in distant galaxies; not only would the observable
number be increased 2.5 times, but the red shifts that
could be reached would be increased by 40 percent. The
ability to detect radio spectral lines in very distant
quasars would be greatly improved. Perhaps of most impor-
tance, the speed at which observations could be made would
be increased by a factor of 3.6--in effect tripling the
number of 1000-ft telescopes available to U.S. scientists.
In view of the heavy demand for this uniquely powerful
instrument, this great increase in sensitivity would be a
major benefit to American astronomers.
IV. SCIENTIFIC PRIORITIES
A. Cosmology
The dream of understanding the origin and large-scale
structure of the Universe is at least as old as civili-
zation. The last three decades have seen an explosive
growth in our scientific knowledge of the Universe on a
grand scale, owing mostly to new astronomical instruments
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and observations. Large optical telescopes and the young
science of radio astronomy have led the way by permitting
astronomers to probe deeper into space and hence further
back in time. The current picture of the Universe is that
it is expanding and evolving from a tremendous explosion
that occurred about 15 billion years ago. The origin of
this incredible event, what (if anything) came before,
and even whether the concept of previous existence has
any meaning are questions about which nothing is known:
science today stops short at the big bang.
One of the first cosmological studies in radio astron-
omy was to measure the number of extragalactic sources as
a function of intensity. The goal was to look for
departures from classical geometry and thereby to learn
the rate by which gravitational forces are slowing the
-
expansion of the Universe as a whole. Is there enough
attraction ultimately to stop the expansion and cause the
Universe to collapse, or is the momentum so large that
the Universe expands forever?
By looking at fainter and
fainter sources one is looking further and further back
in time. Source counts versus intensity then measure the
change in source density due to the changing rate of
expansion. Unfortunately, this program failed for the
same reason that similar work with optical sources failed:
evolution in the intrinsic luminosity of the radio
sources.
The dramatic identification of the radio source 3C295
with a galaxy at the large red shift z = 0.5 was a major
step out into the Universe. Galaxies at this distance
and beyond are so faint that optical spectra require long
observations even with the largest telescopes. So, to
increase the probability of finding a high-red-shift
galaxy, optical observers search the region of otherwise
unidentified radio sources. Such searches have identi-
fied galaxies at red shifts with z about 1, and show
promise at even larger red shifts. Galaxies with large
red shifts are the best understood bright objects in the
Universe and the most promising probes of large-scale
space-time structure.
Radio astronomy also pointed the way to quasars.
These enigmatic sources have high red shifts and, if at
the distances implied by their red shifts, their output
of light and radio waves is enormous. The high red
shifts of quasars (up to z about 3.5) make them extremely
interesting as cosmological probes. Again one needs to
know more about the intrinsic source mechanisms and
evolution, before quasar red shifts can be used as probes
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of the size and history of the Universe, but already they
are giving important clues to the energetics of galaxies,
the composition of the intergalactic medium, and the
coarse distribution of matter beyond z = 1.
By far our deepest probe into the Universe is the 3 K
background radiation, accidentally discovered in 1965,
though it had been predicted many years earlier. me 3 K
background is the thermal radiation left over from the
compact and hot early Universe. The subsequent expansion
has cooled the radiation (by red shifting wavelengths)
from more than 101° K to the current temperature of 3 K.
Because thermal radiation at this temperature has wave-
lengths in the centimeter and millimeter ranges, microwave
and radio techniques are essential to its study. m e
radiation has had practically no interaction with matter
for billions of years, coming to us from a very early
epoch of the Universe.
The detection of the 3 K radiation has caused most
cosmologists to reject the steady-state and other non-
evolving cosmological models, since these theories cannot
plausibly account for such a large quantity of thermal
radiation. Assuming the big-bang model to calculate
helium production in the early Universe, one obtains the
temperature, density, and expansion rate; these values
are just right to give the currently observed helium
abundance--20 percent to 25 percent by mass, thus sup-
oortino the big-bang model. Over the last 15 years, the
spectrum and spatial distribution of the 3 K radiation
have been observed extensively and are in substantial
agreement with theory at the current level of experi-
mental accuracy. We can learn a great deal about
processes in the very early Universe by measuring the 3 K
radiation with the highest possible precision.
A question that still confounds cosmologists is how
galaxies manage to form in an expanding Universe. When
were they produced? How long did it take? What was the
sequence of events? Almost nothing is known about this
process, which ranks among the most interesting and
important in natural science. One possibility is that
the 3 K photons suffered some scattering when passing
through the hot, dense plasma that should have existed
around collapsing galaxies and clusters of galaxies. If
so, the spatial distribution of the 3 K radiation may
possess very weak ripples on small angular scales (10
arcsec to 30 arcmin), but so far none have been observed.
Another clue to the behavior of the Universe at high red
shift comes from the "large-scale anisotropy" observations
~ _
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of the background radiation, which look for irregularities
on angular scales from 10° to 180°. These observations
have detected a "dipole" effect (variation with the cosine
of the angle in the sky), which is thought to result from
the motion of the Galaxy through the 3 K radiation. This
dipole variation is an intriguing result in itself, but
cosmologists are even more interested in whether the radi-
ation possesses any intrinsic structure on large angular
scales. A rotating Universe, very-large-scale shear
motion, or a slightly nonspherical expansion could each
produce large-scale anisotropy in the 3 K background.
The opportunity to look for such large-scale structure in
the Universe was beyond our means before the discovery of
the 3 K radiation.
The following are among the key questions in cosmology
that radio observations during the coming decade may help
to answer:
· Is the Universe homogenous and isotropic on large
scales? m is was Einstein's fundamental postulate in
applying General Relativity to the Universe and seems to
be roughly true from galaxy counts. However, the best
evidence is the remarkable isotropy of the 3 K radiation,
the dipolar effect of the Earth's peculiar motion being
only 0.1 percent. More measurements, with better
receivers and sky coverage, will greatly improve the
current result. The Cosmic Background Explorer (COBE)
satellite, to be launched in the 1980's, will reach an
accuracy of 0.01 percent in this fundamental observation.
· Is the Universe open or closed? One way to find
out is to measure the mass density of the Universe. If
it is greater than 2 X 10-29 g/cm3, then gravitational
attraction will overcome the momentum of expansion and
the Universe will eventually collapse. Known luminous
matter falls short by a factor of 10, but much of the
matter in the Universe could be dark. Galaxies may have
large halos made up of very-low-luminosity stars or black
holes. Plots of radial velocity as a function of posi-
tion for galaxies are flat out to very large radii,
indicating high mass density inside--more than can be
accounted for by visible starlight, and such "rotation
curves" give a means of probing for dark matter in the
halos of galaxies. Radio measurements of the 21-cm
hydrogen line are particularly useful on large nearby
galaxies, where the rotation curve can be followed out
beyond the visible edge of the galaxy. The motion of
hydrogen clouds around galaxies is another promising way
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to study the amount of dark matter they contain. Here
again, sensitive 21-cm measurements are needed.
· When and how did galaxies form? If current theo-
ries are correct, galaxies must have formed sometime after
the cosmic temperature had dropped low enough for protons
and electrons to combine to neutral hydrogen. Then grav-
itational forces overcame radiation drag, and matter con-
densed into clouds. Galaxy formation could have taken
place at any red shift between z = 1000 (when the temper-
ature dropped below 3000 K) and z = 3 (where galaxies in
the process of formation would have been detected at
visible wavelengths).
The formation of galaxies should perturb the 3 K back-
ground radiation and show up as ripples or granulation on
small angular scales. Small-scale anisotropy measurements
are already very close to the levels predicted, and better
measurements will soon be made. Quiet receivers, well-
shielded antennas, and stable atmospheric conditions are
~ ~ ~ ~ ~ ~. A positive measurement
of the intensity and angular scale of small-scale anisot-
requlred to detect this effect
ropy would be a major advance toward understanding galaxy
formation.
Two other effects of galaxy formation may be observable
at radio frequencies. Theorists have wondered whether
star formation in young galaxies, much more intense than
that which occurs today, might re-ionize the remaining
hydrogen gas. ~
bremsstrahlung (thermal) emission.
The resulting Plasma would radiate by
_ The detection of this
radiation--probably now at decimeter wavelengths--would
give us a direct view of the epoch of galaxy formation.
Another possibility is to look for 21-cm line emission
from neutral hydrogen before it collapse into stars or is
re-ionized. Red shifting would have moved this line to
long wavelengths, so the search must be done against an
intense background from the Galaxy and man-made inter-
ference. Some observational work has already been done
on this difficult, but important, problem.
· How big and how old is the Universe? If one
assumes that the current picture of an expanding, evolv-
ing Universe is correct, then by measuring the velocities
and distances of galaxies one can estimate the age of the
Universe. The hard observational problem is to measure
A new radio method is
the distances to other galaxies.
extremely promising. It uses the apparent tight correla-
tion between infrared brightness and rotational speed of
spiral galaxies, the rotational speed being determined
from the 21-cm line. The sensitivity of large antennas
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allows hundreds of galaxies at large distances to be
measured. Not only is the technique giving Hubble's
constant, but it shows promise for answering old ques-
tions about the dynamics of galaxies in our neighborhood.
For example, is the Galaxy falling toward the Virgo
cluster? The early results of these measurements are
affirmative.
An important lesson from the history of cosmology is
that even over a decade one cannot expect to forecast
progress accurately. In fact, quite the opposite is
true; one expects major discoveries, brand new insights,
and perhaps complete changes of course in such a young
and innovative science. How then does one plan? Again,
history tells us that a good strategy is to extrapolate
from current research and ideas--plan for the progress
that seems possible. The process of pushing back the
frontiers with good ideas and careful science will
trigger the important discoveries.
Except for the COBE satellite, there are no major new
radio facilities whose main justification is cosmological.
Because nearly all of astronomy interacts with cosmology,
the field will be advanced by implementation of many of
the Panel's recommendations. However, some general char-
acteristics of the field place emphasis on certain areas
of planning for the 1980's. First, there is always a
need to observe fainter sources, with a premium on larger
telescopes, lower-noise receivers, and more angular and
spectral resolution. The high cost of building and
operating such instruments has always been a major jus-
tification for the National Astronomy Centers, and it is
hence important to cosmology to keep these Centers healthy
and productive. Second, as with any rapidly developing
field, it is important to encourage new ideas and novel
instrumentation. Adequate support of good theoretical
and experimental research by small groups in university,
governmental, and industrial laboratories is essential to
progress in a field that puts a high premium on innova-
tion. The exciting problems and opportunities in cos-
mological research have attracted many young scientists,
as well as established scientists from other fields. It
is important that the level of research support be commen-
surate with the level of quality and interest in this
area.
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235
B. Galaxies
With unaided eyes we can see only a small fraction of our
Galaxy; its local features appear as the Milky Way. Our
solar system is imbedded in this structure at a location
not particularly favored or unique. We lie beyond its
most prominent spiral features, toward an edge that we
are as yet unable to define precisely. Galactic studies
describe the properties of this system--its size, mass,
dynamics, content, and evolutionary history--a descrip-
tion of our environment in its broadest sense. This
environment is far from static, for the Galaxy is differ-
entially rotating:
than do the outer regions.
changing.
.
the inner parts circle more often
Our neighbors are constantly
~ _ _ ~ _ _ _ A _
Shocks perhaps driven by density waves give rise to
regions of star formation on a Galactic scale, and these
in turn give rise to the spiral arms. Shocks may also be
triggered by supernova explosions. Chemically evolved
material is violently ejected and mixed through differ-
ential rotation and turbulence. We observe this chemical
evolution as an abundance gradient across our Galaxy. In
addition to these explosive events we find that there is
also a steady stream of matter from the surfaces of stars
entering the interstellar medium. m e mechanism here, a
stellar wind, is similar to the solar wind. The life
cycle continues through the continual formation of new
stars from this interstellar material. At the very center
of our Galaxy are massive molecular clouds in the environ-
ment of a strong radiation field. Various aspects of
these events are seen in other galaxies, and we are often
guided in the study of our Galaxy by what we see in these
other systems.
That our Milky Way is a spiral system was first sug-
gested over a century ago, but observational evidence
supporting large-scale spiral structures has been obtained
only in the second half of this century. Application of
radio spectral-line techniques to the distribution of
interstellar matter supplied the necessary information.
The first indications of spiral structure came from 21-cm
hydrogen-line studies and more recently from radio studies
of the carbon monoxide molecule. Hydrogen-line studies
have also shown that the outer regions of a spiral galaxy
are often bent or warped. m ere are a number of competing
explanations: tidal perturbation by nearby galaxies, gal-
actic winds, precession--but none is entirely adequate.
The understanding of such warps may be relevant to the
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possible existence and form of dark massive halos sur-
rounding galaxies.
The last decade has seen the discovery of several
dozen molecules, organic and inorganic, in the inter-
stellar medium. A new discipline, astrochemistry, has
developed, in which radio astronomers, chemists, and
physicists are collaborating profitably. The growing
number of molecular radio lines in external galaxies, now
half a dozen, offers promise of a new tool for extraga-
lactic research.
m e enigmatic properties of galactic nuclei have been
extensively studied at optical and at radio wavelengths.
However, we lack detailed knowledge of the structure of
the central energy source--not only in normal galaxies
such as our own but also in the active nuclei of radio
galaxies and quasars. Similarly, we lack knowledge of
the fundamental physical processes involved in the energy
release from these nuclei.
m e study of other galaxies has guided us to a better
understanding of the kinematics within our own Galaxy.
Hydrogen-line studies have shown a flat rotation curve;
that is, the rotational velocity in other galaxies remains
essentially constant as far from the galactic center as
can be measured. This is in sharp contrast to the pre-
viously predicted Keplerian decrease, a prediction pat-
terned after the decrease of surface brightness toward
the edge of a galaxy. Our own Galaxy has recently been
shown to have such a flat rotation curve.
The shape of the galactic rotation curve is determined
by the mass distribution. The failure of rotational vel-
ocities to decrease at large distances from the center
implies a significant mass distribution exterior to the
visible galaxy. Together with the decreasing light at
large distances, this requires a significant change in
the stellar population of the galaxy with distance from
the center. We are abysmally ignorant of the amount,
extent, and nature of the mass that surrounds a normal
isolated galaxy, so our knowledge of galactic structure
in a fundamental sense remains extremely primitive.
The quest for intergalactic matter is ongoing. X-ray
studies indicate a hot component, but one that appears to
involve only a moderate amount of material--an amount far
from sufficient to explain why galaxies appear to be
gravitationally bound in clusters--or to close the Uni-
verse. There have been many searches for a cooler com-
ponent, presumably some form of hydrogen, but these
searches have been unsuccessful, placing serious con-
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straints on the amount of such cool gas either in clus-
ters or distributed along the line of sight. No isolated
hydrogen clouds have been found in the intergalactic
medium, but near some galaxies there is 21-cm evidence
for tidal disruption--material pulled out by the gravity
of neighboring galaxies. Many such examples are known,
the nearest and most conspicuous being the Magellanic
Stream. Such 21-cm plumes and streamers may ultimately
become valuable probes of the intergalactic medium.
Twenty years after their discovery, the high-velocity
hydrogen clouds widely and prominently distributed in our
own Galaxy are still poorly understood. Are they Galactic
or extragalactic? Are they primordial or secondary in
nature? What is the relationship, if any, between them
and the Magellanic Stream?
A quantitative description of the color, H I content,
luminosity, and other gross properties of galaxies can be
obtained with present instruments. Many of these proper-
ties, and especially the intrinsic dispersion, are poorly
understood. At least some of these properties must be
continually changing, such as the composition of the in-
terstellar medium within a galaxy. Some changes must
result from the continual evolution of the stellar popula-
tion, others appear related to environment.
The significant data that come from radio studies often
suffer from poor angular resolution, a property inherent
in the relatively long wavelength of such electromagnetic
radiation. However, interferometric techniques on a large
scale such as with the VLA, or on an even larger scale
with VLBI, yield a very high resolution, often better than
at optical wavelengths. We are thus able to use the radio
window as a probe with exceedingly fine resolution, giving
detail in our own Galaxy and others on a scale previously
unattainable. A relatively unexplored window still
remains--the submillimeter region. Both continuum and
spectral-line studies will profit greatly from work in
this unexplored part of the radio spectrum.
C. Quasars and Galactic Nuclei
One of the most challenging scientific problems of the
past 20 years has been the nature of quasars and their
relationship to galaxies in the scheme of cosmic evolu-
tion. After the discovery of quasars in the early 1960's,
it was clear that their remarkable properties raised fun-
damental questions in physics and cosmology. Their large
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red shifts implied large distances, placing them near the
edge of the observable Universe. Quasars were possible
tracers of the large-scale geometry of space and might
provide decisive clues about the past and future of the
Universe. At the same time, at the distances implied by
their red shifts, their energy output had to be enormous,
exceeding that of normal galaxies by several orders of
magnitude. The energy source would have to be quite
unlike those in any well-understood object. If the red
shifts were not of cosmological origin, but perhaps
caused by gravitational or ballistic effects, other
puzzles arose.
In recent years the red-shift puzzle has come close to
resolution. A number of low-red-shift quasars have been
found to reside in groups of galaxies having the same red
shift. Furthermore, some BL Lacertae objects (compact
sources thought to be closely related to quasars) have
been found embedded in galaxies, again with the same red
shift. Because the galactic red shifts are presumably
cosmological, both discoveries strongly support the hy-
pothesis that the red shifts of quasars are also cosmo-
logical and that most quasars are indeed very far away.
One of the main goals today is to use quasars as cosmo-
logical probes and to understand the physical processes
that generate such enormous quantities of energy.
At the center of most galaxies (including our own) is
a concentrated nucleus that seems to cause a variety of
phenomena. Because gas motions in galactic nuclei are so
irregular, one suspects that large quantities of energy
are explosively released from time to time. VLBI at radio
wavelengths has shown that the nucleus is typically only
a few tens of astronomical units in size. So far, these
size measurements seem to be limited to scattering of the
radio emission from surrounding clouds of ionized gas;
VLBI measurements at wavelengths of 1 cm and shorter are
under way to avoid the scattering effects and reveal the
actual size and shape of the active region.
Seyfert galaxies represent the next step up the energy
scale in activity, their nuclei showing evidence of abun-
dant quantities of ionized gas in violent agitation. m e
radio emission generally exceeds that of our own Galaxy
by several factors of 10, and the nucleus is clearly the
seat of the activity. No clear division may exist between
the nuclei of normal galaxies and those of Seyferts; from
high-resolution studies of the structure of such nuclei
we hope to understand the similarities and differences.
Activity in the nuclei of galaxies is probably an episodic
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affair, and Seyfert galaxies may represent the active
phase of the nuclear outburst.
Giant radio galaxies, whose radio luminosities may
exceed that of our own Galaxy by a factor of 106, also
have active nuclei. m ese are generally unresolved with
the longest existing interferometers and again seem to be
the seat of the energetic events. A combination of syn-
thesis mapping and long-baseline interferometry has
revealed in some radio Galaxies a striking hierarchy of
structures: -
first an unresolved radio nucleus of less
than 10 light-years across, then jets from the nucleus
several thousand light-years long, finally enormous radio
lobes stretching millions of light-years into space. In
several cases, the structures are remarkably well aligned
over size scales that differ by a million or more. The
alignment constitutes fairly direct evidence that the
energy source lies in the compact nucleus.
Quasars often exhibit similar hierarchies of scale and
alignment, suggesting that quasars and active galactic
nuclei are different aspects of the same fundamental
mechanism. Statistical studies at both radio and optical
wavelengths show that quasars were much more common in
the early Universe. Perhaps the activity seen in nearby
galaxies (and our own)
is the relic of earlier, more
vigorous activity. The jury is still out, and further
observation of angular structure, time variations in the
radio spectrum, and the visual properties of quasars are
required to reach a verdict.
At present, VLBI explores angular sizes from 3 X 10-4
to 1 X 10-2 arcsec.
At such resolution the radio emis-
sion from both quasars and the nuclei of radio galaxies
often comes from several independent "hot spots" that move
about with time. In several cases, the apparent relative
velocities of the spots exceed the speed of light. Such
velocities are forbidden by the present laws of physics,
and one must, of course, examine all alternatives very
carefully. It may be that we are not observing a true
velocity--the spot from a searchlight beam moving across
clouds, for example, can appear to move more rapidly than
the speed of light, but it is only an apparent or "phase"
velocity. In a similar way one can imagine that the
superluminal velocities in quasars arise from moving
"searchlight beams" of relativistic particles that only
radiate when they encounter surrounding clouds of gas and
magnetic fields. Many models have been proposed, but to
test them requires very accurate, high-contrast maps from
a dedicated VLBI array.
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247
providing detailed clues about the way in which massive
stars are born. However, we have only vague observational
clues regarding the general distribution of gas within
the clouds or the way in which stars of lower mass, like
our Sun, condense. Maser emission from OH and especially
H2O imply small regions of high density, but detailed
interpretation of the maser process is difficult. Infra-
red studies show pointlike objects, which are presumably
protostars. Until recently, the angular resolution of
radio lines has been too poor to reveal sufficient detail,
and it will be the task of the new millimeter interfer-
ometers and the 25-m dish to provide essential high-
resolution data during the 1980's.
Many questions need to be answered. For example, can
the effects of magnetic field be seen in the collapse of
gas into stars? How does the fraction of mass that ends
up in stars depend on their mass? What triggers star
formation--expanding H II regions, supernova remnants,
galactic shocks? Do many stars form at once, or is it
largely a sequential process? Do cloudless condense into
rings followed by further collapse into binary or multiple
star systems as suggested by theory? mese are examples
of the many questions for which the tools are available
for the decade ahead.
Radio studies continue to expand our understanding of
the interstellar medium. Early observations of the
polarized low-frequency radio emission from the Milky Way
demonstrated the pervasive character of the cosmic-rays
and magnetic fields in the Galaxy.
During the 1970's the
detection of the Zeeman effect (the splitting of emission
or absorption lines into several components by a magnetic
field) has yielded definitive values for the field
strength in a few places. The field is surprisingly weak
in regions of low density and, on the other hand, strik-
ingly strong where it has been observed in molecular
clouds. Recent improvements in receiver sensitivity at
centimeter wavelengths may make lame-scale mapping of
the magnetic field possible in the 1980's.
Depending on
its strength, the field may have important dynamical
effects on the large-scale structure of the Galaxy and on
the way in which the molecular clouds give birth to stars.
The emission from many molecular species in galactic
clouds exhibits departures from local thermodynamic equi-
librium. The most extreme examples are the interstellar
masers, which show emission from narrow, highly amplified
spectral lines. In these sources, VLBI reveals that the
radiation comes from a cluster of bright spots having
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248
angular diameters of a 10-3 arcsec or less. The masers
often occur in regions of star formation, near newly
formed O and B stars. It is plausible that every newly
formed massive star may pass through a maser emission
stage lasting 104 years or so. Because newly formed
stars are often optically obscured by dense dust clouds,
and because their surrounding ionization shells are small,
the maser radiation may be the best way of studying their
earliest stages of development. The velocities and loca-
tion of the bright spots serve to outline the dynamical
structure of the circumstellar envelopes of the stars,
and Zeeman splitting of the lines can reveal the spatial
structure of the magnetic field. In many cases, evolution
of the emitting region is indicated by variations in the
source structure over times as short as a week. As yet
we do not know how the masers are formed or how they are
"pumped" by a nearby energy source. Further observations
with VLBI techniques are necessary for progress on these
important questions.
During the 1970's our notion of the overall equi-
librium of the interstellar medium has changed consider-
ably. Observations a decade ago seemed consistent with
the view that two phases were in equilibrium, one dense
and cold (10-100 K) and the other thin and warm (10,000
K). The discovery of highly ionized oxygen in the W ,
the discovery of soft x-ray background radiation, and the
identification of large gas bubbles in radio pictures of
neutral hydrogen have led to the view that there is also
a large-scale component that is very hot--near a million
degrees. To elucidate this view, the crucial radio
studies in the 1980's will be very sensitive hydrogen
absorption observations and accurate pulsar distance
determinations to find the electron density.
From a technological standpoint, the discovery of
interstellar molecules has been part of one of the major
themes of radio astronomy and indeed radio science--an
advance to ever higher frequencies. Experience has shown
that the millimeter-wave band--the short-wavelength or
high-frequency end of the radio "window" in the terres-
trial atmosphere--is the most profitable region in which
to find new molecules and study molecular clouds. In
response to this finding, a worldwide effort began in the
1970's to construct new millimeter-wave telescopes and to
provide the few existing telescopes with better receivers
and spectrometers. Over one half of the molecules dis-
covered to date were found with one instrument, the 11-m
telescope of the National Radio Astronomy Observatory
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249
(NRAO) at Kitt Peak. A successor to this productive
facility, a 25-m telescope to be located at high altitude
so that it can work as far as possible in the infrared
region, has been proposed by NRAO and if constructed will
constitute the most powerful U.S. facility for millimeter-
wave astronomy.
E. Stars and Pulsars
Many of the important discoveries of radio astronomy over
its 40-year history have been of objects that even by
astronomical standards are extremely large: the remains
of exploded stars filling regions of space tens of light-
years in diameter, the gaseous components of whole gal-
axies at least a hundred thousand light-years across, and
the giant radio galaxies stretching tens of millions of
light-years. On the other hand, in the past decade many
of the important astrophysical advances have involved
radio-frequency observations of very small objects, rang-
ing downward from the compact active nuclei of galaxies--
perhaps only light-weeks in size--to circumstellar envel-
opes, closely spaced pairs of orbiting stars, stars
undergoing mass loss and other types of surface activity,
and the minute, ultradense pulsars with diameters of only
a few miles. Enormous advances were made in understanding
these phenomena during the 1970's, in part because radio-
frequency observations of objects traditionally studied
in other wavelength regions have shed new light on their
structure, their physical conditions and environment, and
their places in the scheme of galactic evolution.
For some time now, our understanding of the life cycles
of stars has been generally satisfactory, except for the
earliest and latest phases: star birth and star death.
As described elsewhere in this chapter, star formation
takes place in the cold, neutral gas clouds of inter-
stellar space, and radio observations of the more abundant
molecular species are very effective probes of this intri-
cate process. Once formed, a stable, isolated star radi-
ates most of its energy at (or near) visible wavelengths.
Even so, much detailed information concerning stellar
surface phenomena has come from radio observations of the
Sun and the solar wind, both from the ground and from
Earth-orbiting spacecraft. Observations of red dwarf
flare stars, both at radio and optical wavelengths, have
demonstrated the existence of similar (though much more
violent) surface phenomena on other stars as well.
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When a star exhausts its original supply of fuel for
nuclear reactions and enters the last phases of its life,
other important changes occur. Many of these changes
involve the ejection of large quantities of material from
the outer layers of the star, and these processes and
their consequences are often best observed at radio wave-
lengths. Several major new classes of stellar phenomena
were first recognized and widely studied with radio tech-
niques during the 1970's, including the expanding shells
around stars that recently have undergone nova outbursts,
shells of relatively cool gas and dust in circumstellar
envelopes around red giant stars, mass exchange processes
between members of close binary pairs, steady mass outflow
from highly luminous, isolated stars of different types,
and pulsars or neutron stars, thought to be remnants of
supernova explosions.
Much of this work involves the ways in which stars
shed substantial fractions of their mass before collaps-
ing to form white dwarfs or neutron stars. For example,
stellar astronomers have recently discovered that young,
rapidly evolving stars lose much of their mass in power-
ful stellar winds driven by radiation pressure. m is
discovery has influenced our understanding of star for-
mation and evolution and has had an impact on theories of
H II regions, supernova remnants, and binary x-ray sys-
tems. Observations of stellar winds and their effects
are now possible in the radio, infrared, optical, ultra-
violet, and x-ray spectral ranges, and such data provide
the basic information needed to construct a model of the
underlying physical processes. Accurate mass-loss rates
for individual stars are probably best determined from
measurements of radio flux, together with estimates of
wind velocity obtained from ultraviolet, infrared, or
Balmer line observations. Only a small number of young,
early-type stars were detectable with the radio tele-
scopes of the 1970's, but this situation is now rapidly
changing. The VLA, for example, is probably able to
detect every Galactic Wolf-Rayet star in its field of
view. High-resolution radio interferometry will enable
direct measurement of the angular sizes of stellar winds,
from which the gas temperature can be directly calculated.
The mass lost by stars in stellar winds has implica-
tions for many other fields of astronomy. For example,
the more luminous early-type stars lose mass at a rate
sufficient to modify drastically their evolution, and
this effect must be considered when such stars are used
as extragalactic distance standards. In a dense inter-
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stellar cloud, a stellar wind will modify the radio and
infrared emission from the associated compact H II region,
causing shocks and x-ray emission and perhaps affecting
the formation and excitation of molecules. Winds may
even influence star formation, initiating the process by
gas compression or terminating it by disrupting the gas
cloud. The huge cavities created by stellar winds in
diffuse interstellar gas must be taken into account when
interpreting the size and structure of giant H II regions,
both Galactic and extragalactic, and also when interpret-
ing the structure and evolution of supernova shells.
These shells have characteristics determined in part by
the dynamical action of a stellar wind on the surrounding
interstellar gas before the supernova occurred.
The same physical processes involved in the gas dynam-
ics and radiation transfer within stellar winds are
thought to be important in the emission line regions of
quasars and active galaxies. Since stellar winds are
easier to observe than are quasars, the development of a
physical theory to explain stellar winds may contribute
to understanding quasars as well.
Another exciting discovery of the past few years, a
result of x-ray observations, is the detection of stellar
coronas around a wide variety of stars. It had been
thought that coronas surround only solar-type stars with
shallow convection zones and sufficient mechanical energy
to heat the outer atmosphere. For the Sun, heating by
acoustic waves is now believed to be negligible, and dis-
sipation of magnetic energy is the favored energy source.
Magnetic fields in the hot coronas of other stars can be
studied by observing their radio emission. As the x rays
are sensitive to the temperature and emitting volume and
the radio emission to magnetic fields, the combination can
probe the stellar coronas in great detail. Such informa-
tion is necessary to understand how much the coronas
affect the stellar winds and ultimately to understand the
origin of magnetic activity in stars.
In addition to mass outflow from early-type stars,
observations reveal the existence of extended circum-
stellar gas and dust shells around a variety of other
stars, most of which seem to be losing mass at very high
rates. Knowledge of the chemical composition of the
shells, as well as an understanding of the subsequent
injection of this material into the interstellar medium,
is crucial to unraveling the mystery of how the galactic
material is enriched in heavy elements and complex
molecules.
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A notable class of stars with dense circumstellar
shells consists of the luminous, highly evolved red
giants, which may be the principal source of "processed"
interstellar material--gas that has been enriched in
heavy elements. Density and temperature are such that
the shells are best observed at millimeter, submillimeter,
and infrared wavelengths. At millimeter wavelengths,
observations have shown that many of these stars produce
maser emission from SiO, OH, and H2O. Further studies
of these maser sources with VLBI techniques will provide
information about physical conditions in the shells. As
is the case with the shells surrounding early-type stars,
spatial mapping of the continuum emission is also of high
priority, because one needs shell sizes to determine
opacities and masses. Pushing interferometric techniques
toward shorter radio wavelengths and into the infrared
region is needed to achieve this goal.
One of the most important discoveries of modern astron-
omy was the detection of the long-predicted neutron stars,
first observed as sources of pulsed radio emission. Pul-
sars have provided the first (and only) example of macro-
scopic matter at nuclear density; with masses comparable
with that of the Sun, but radii 105 times smaller, the
densities are as large as 1015 times that of normal solid
matter. The equation of state of matter in neutron star
interiors requires relativistic many-body theory and the
most advanced concepts of particle physics. Theoretical
advances over the past decade have emphasized the impor-
tance of comparing the calculated masses of neutron stars
with observations and the possibility of determining the
structure of neutron stars from observations of pulsar
periods over extended intervals of time. Radio-frequency
timing measurements, and recently hard x-ray spectral
observations, have shows that pulsars possess huge mag-
netic fields (up to 101 times that of the Earth). It
is apparent from all these remarkable properties that
neutron stars offer a unique testing ground for our under-
standing of the fundamental laws of nature.
In addition, pulsars have provided new insights into
the late stages of stellar evolution and the complex in-
terrelations among the constituents of the Galaxy. Since
pulsars are thought to be "born" in the most spectacular
stellar deaths--supernova explosions--they occupy a well-
defined place in the Galactic hierarchy of things. Sys-
tematic pulsar surveys, followed by a reckoning of the
implied galactic distribution and number density of the
pulsar population, have led to an estimate of the pulsar
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birth rate, apparently in conflict with the rate of super-
novae. Because of its implications for stellar and ga-
lactic evolution, this conflict is likely to be a major
focus of theoretical effort during the next few years.
The problem may be resolved by better determining the
mass range of stars that end their lives as supernovae.
It is clear that observations of pulsars will continue
to yield scientific rewards in the 1980's. A number of
obvious problems need to be attacked. The pulsar emission
mechanism is still not understood, nor is the process that
seems to cause pulsars to cease emitting radio waves when
they are about a million years old. More data on old pul-
sars are needed, and there is a need for further measure-
ments of distances and proper motions to improve the
accuracy of inferences based on positions and kinematics.
Pulsar observations in general require large telescopes
working at wavelengths longer than about 10 cm and (for
astrometric purposes) Very-Long-Baseline Interferometry.
A unique and fundamental testing ground for theories
of gravitation became available in 1974 with the discovery
of a binary pulsar--a pulsar orbiting another star. The
General Theory of Relativity predicts that two masses
orbiting each other will emit gravitational waves, but
until recently this fundamental prediction of the theory
had never been tested. Gravitational waves radiated by
the binary-pulsar system should carry energy away at the
expense of orbital kinetic energy, and therefore the pul-
sar and its companion should slowly spiral together. Five
years of careful timing measurements of the pulsar have
revealed just such an effect: the orbital period is
gradually decreasing. These data thus furnish the first
evidence in support of the existence of gravitational
waves, as well as verification of a previously untested
aspect of General Relativity. It is essential that obser-
vations of this binary pulsar be continued for at least 5
more years.
F. The Sun
Over the past decade, solar radio astronomy made many
advances: the detection of coronal holes, the obser-
vation of very small structures associated with the
primary energy release in solar flares, and both obser-
vational and theoretical advances in the understanding of
the complex plasma processes caused by electron streams
moving from the Sun through interplanetary space.
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The problems of solar radio astronomy can be divided
into two broad categories: what we can learn about the
Sun itself, i.e., the problems of solar and stellar
physics, and what we can learn about fundamental physical
processes, especially in plasma physics. m e corona and
solar wind provide a unique plasma laboratory with dimen-
~, _ ~
signs of 10t km and more.
Many of the phenomena that
occur in the corona are not observed under laboratory
conditions because they become important only on a large
scale. Of all astronomical sources studied, the solar
corona and wind have probably yielded the greatest wealth
of observational data, and to understand them requires
the most advanced ideas of plasma physics.
As an example, the most extensively studied problem in
astrophysical plasma physics is probably the Type III
solar radio burst. This common type of radio emission is
caused by a stream of electrons moving from the inner cor-
ona to the orbit of the Earth and beyond. This stream
excites density waves in the coronal plasma, at first
high radio frequencies in the lower corona, then lower
frequencies in the outer corona and the interplanetary
medium. The radiation (emitted at the local plasma
frequency or its second harmonic) arises as nonlinear
emission from a region of turbulence. A long-standing
problem has been that theory and laboratory experiments
predict that instabilities should destroy the streaming
motion within a few kilometers, but this is not observed.
Two theories have been developed in the past five years
to explain the continued propagation of the electron
stream and the Type III bursts. In one, the "quasi-
linear" theory, there is a backreaction of the plasma
waves on the electron velocity distribution that subverts
the instability, followed by the reabsorption of most of
the wave energy by trailing parts of the stream. In the
other, the "nonlinear theory, the plasma waves undergo
self-focusing, which concentrates the wave energy into
very small volumes, at the same time expelling the plasma
particles and subverting the instability by altering the
wave spectrum. During the collapse, plasma standing waves
would radiate at the local plasma frequency and its second
harmonic, with an intensity comparable with that observed.
The recent progress on this difficult problem comes
mainly from combining sophisticated plasma theory with
extensive measurements not only of the radio bursts but
also of the fast-electron distribution functions and the
plasma wave intensity. Such a combination of diverse
information is essential for progress because of the large
number of possible solutions to the relevant equations.
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The solar flare is another complex interaction of many
processes that, despite a century of study, remains poorly
understood. The energy for a flare is thought to be
stored as electric currents and magnetic fields in the
corona and to be released when sudden changes, perhaps
caused by shearing motions, occur within the magnetic-
field structure. The regions of energy release are probed
most directly by centimeter-wavelength radio emissions
and hard x rays. Together, the two types of data provide
detailed information on the densities and particle ener-
gies in the emitting region and the magnetic-field
strength and topology.
Recent observations, for example,
suggest that the plasma in the region of a solar flare is
largely thermal at a few hundred million kelvins, rather
than being nonthermal and dominated by highly organized
beams of electrons. Nonthermal and relativistic particle
populations are probably of secondary importance in the
energetics of the flare; nevertheless, they are of great
interest because of their geophysical effects such as
auroras and geomagnetic storms.
Further progress in understanding solar flares will
require very high spatial and temporal resolution at
several radio wavelengths, accurate polarization mea-
surements, concurrent hard x-ray data with high spatial
resolution, and better theory. Before and after flares
we need accurate maps in circular polarization to deter-
mine the strength and structure of the coronal magnetic
field. m e VLA is a unique and powerful tool for pro-
viding the necessary spatial resolution, though improve-
ments are required in time resolution polarization our-
ity, and map processing.
Unfortunately, comparable
spatial resolution Is unachievable with present x-ray
telescopes. Another facility, the Owens Valley Observa-
tory, is developing powerful tools for observing details
of the spectrum of bursts with high time resolution (50
msec), though with limited information on spatial
structure.
Over the past decade it has been learned that changes
in the solar corona are not slow and steady but are domi-
nated by violent events or "coronal transients, n which
are strikingly revealed in white-light pictures. Tran-
sients involve the ejection of plasma and magnetic field
into the corona, usually in association with a flare or
the eruption of a prominence. The mechanical, kinematic,
and magnetic energy of the faster, flare-associated tran-
sients may equal or exceed that released by the flares in
all other forms. Rearrangements of magnetic fields, whose
strengths can at present be derived only from meter-
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256
wavelength radio observations, probably provide the driv-
ing forces for the transients. As yet we have no convinc-
ing theory for the transient mechanisms or their relation
to the basic cycle of solar activity.
For many transients, it appears that the moving matter
and magnetic field act as a piston that generates a shock
wave accompanied by nonthermal radio emission. The shock
sometimes reaches the Earth, causing auroras and magnetic
storms, and is observable from near-Earth spacecraft.
Detailed measurements of the shock parameters are just
becoming possible: particle densities, field variations,
electron distribution functions, plasma wave intensities,
and kilometer-wavelength radio emissions. From these mea-
surements we can expect much better theories of the struc-
ture of collisionless shocks, particle acceleration with-
in them, and the nonlinear wave-particle interactions that
give rise to the varied phenomena. The theories can then
be applied to similar phenomena (of possibly much greater
magnitude) that occur in other stars and galaxies, where
detailed observations of the plasma parameters cannot be
made.
Radio observations are also needed to advance our
understanding of the quiet solar atmosphere, especially
the transition region between the chromosphere and the
inner corona. At present there is a discrepancy between
radio and extreme ultraviolet observations, with the
observed radio brightness at decimeter wavelengths being
a factor of 10 lower than expected from models derived
from ultraviolet line intensities. It is likely that
important physical processes have been omitted when deriv-
ing these models, for example, fast electrons from the
corona may penetrate deep into the transition region giv-
ing nonequilibrium ionization of heavy elements and
enhanced line radiation--or heavy ions may diffuse toward
higher temperatures, giving anomalous line intensities--
or the continual interchange of mass and energy between
chromosphere and corona may invalidate the assumptions of
hydrostatic and thermal equilibrium. Similar effects may
occur in other stars, but until we understand the pro-
cesses occurring in the Sun it is impossible to know.
G. The Planets
Radar astronomy has produced a host of important results
over the last few decades. Detailed maps of a large
portion of the surface of Venus have been made, revealing
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a variety of geologic structures.
The nature of the par-
ticles composing Saturn's rings has been largely under
stood and the small-scale qualities of the surfaces of
the Galilean satellites determined.
In this decade, it is expected that new pictures of
the surface of Venus will be obtained with oreatlv
improved resolution.
-
_
In addition, there will be a large
number of opportunities to observe asteroids, yielding
very precise information about their sizes, rotation
rates, and surface characteristics. Existing data suggest
that some asteroids have surfaces quite different from
those of other solar-system objects. It will be of par-
ticular interest to see if radar shows that some of these
objects are metallic.
The above observations can be done well with existing
equipment, in particular the large telescope at Arecibo.
Although it would be possible to improve the radar capa-
bilities at Arecibo by adding new transmitters of higher
power or shorter wavelength, at present it would not be
cost effective to do this. On the other hand, increasing
the radius of the Arecibo reflector by 30 m would improve
the radar signal-to-noise ratio about 3.5 times. This
would provide a much greater improvement in sensitivity
for objects near the extremes of the Arecibo declination
coverage. From the standpoint of radar astronomy, it
would be extremely cost effective to carry out this
extension.
During the last decade, centimeter- and millimeter-wave
passive radio observations of the planets have produced
data important to understanding the atmospheres of the
_ , ~ _ , _ _ _ _ ~ _ _ _
nearby planets. Except at centimeter wavelengths, most
of the studies have not been made with enough angular
resolution to distinguish spatial features.
Nevertheless,
large-scale properties such as the cloud-top pressure in
the Jovian atmosphere and the CO2 partial pressure at
the surface of Venus have been deduced. Further progress
will be made in the 1980's with millimeter-wave interfer-
ometers. Angular resolution of about 1 arcsec is now pos-
sible, so individual surface features can be studied with
a resolution comparable with that of an optical photo-
graph. The most valuable work will probably be studies
of distant objects that have not received close scrutiny
by spacecraft. Examples include investigation of the
surface features of Mercury (a planet without an atmo-
sphere) and determination of the abundance of ammonia in
the atmospheres of the Jovian planets. Insight into the
distribution of nitrogen in the solar system yielded by
Representative terms from entire chapter:
radio galaxies
