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OCR for page 8
2
Science Objectives
INTRODUCTION
Astronomy encompasses an enormous range of interesting and
fundamental questions. The universe exhibits regions far more va-
cant than the best vacuum that we could hope to generate in
the laboratory, and compressed matter so dense that a thimbleful
would weigh a billion tons. Temperatures, pressures, magnetic
field strengths, and radiation densities range similarly across ex-
tremes. A practical consequence of this great variety of conditions
is a huge range of cosmic sizes, of velocities, and of time scales
on which dynamic processes take place. Three decades ago, it
was still possible to think of a universe in which changes occurred
ponderously, if at all, over the aeons. Now we detect compact neu-
tron stars whirling about their axes 1000 times a second, emitting
streams of pulses at millisecond intervals and there are processes
that take place on every other conceivable time scale between mil-
liseconds and billions of years. These same neutron stars are no
more than a few kilometers across, and there is accumulating evi-
dence for the existence of stellar black holes that would be another
10 times smaller. At the other extreme we see radio jets that span
an entire cluster of galaxies. The relativistic energies of such jets
are the handiwork of electrons and protons traveling along thin
8
OCR for page 9
9
filaments at close to the speed of light. These particles, which
stem from an unknown source, eventually are spent in gigantic
billowing clouds known through their powerful radio emission.
Closer to home, and of special interest, are planetary pro-
cesses. Here the history of a system can be read in its chemical
composition, and we may hope to trace the origins of life and
determine the prevalence (or absence) of biological systems else-
where in the solar system, in our galaxy, and potentially in the
universe.
To study all these different conditions and construct an un-
derstanciable picture, we require an ability to detect the very small
as well as the very large. To probe the great variety of dynamical
processes, we need to be able to measure both extremely high
and very low velocities; to recognize different chemical species, we
need to be able to unravel their spectral signatures. These require-
ments dictate that the instruments that we wit! need must cover
the widest range of photon energies from the extreme gamma-
ray portion of the spectrum to the longest radio waves. In each of
these ranges we will require large, sensitive instruments that can
follow rapid time variations. Higher angular resolution, as well,
is needed to focus on extremely compact sources. Such compact
sources will also need to be studied both at high spectral resolution
and across the widest possible dynamic range to reveal internal
motions and chemical composition.
BASIC ASTROPHYSICAL QUESTIONS
In identifying scientific objectives for astronomical research
in the period 1995 to 2015, the Task Group on Astronomy and
Astrophysics accepts that there will be major developments in as-
tronomy and related sciences between now and then. Priorities will
necessarily have to change, and entirely new projects may evolve.
However, the study Astronomy and Astrophysics for the 1980s,
conducted by the Astronomy Survey Committee of the National
Research Council, enunciated a number of basic questions that
are as relevant today as when they were published in 1982. The
task group believes that these will remain compelling questions
for the foreseeable future as well. The fundamental astrophysical
issues fall into three major groups (a fourth topic-solar and stel-
lar activity is discussed by the Task Group on Solar and Space
OCR for page 10
10
Physics in the companion volume Space Science in the Twenty-
First Century: Solar and; Space Physics):
.
galaxies.
The early universe, unidentified matter, and the origin of
The physics of collapse and the physics of strong fields.
The formation of stars, planets, and life.
In what follows, the task group has not attempted to make
predictions about the discoveries possible in each area before 1995.
The task group points out, however, the relevance of major NASA
missions to be initiated before that time.
The Early Universe, Unidentified Matter, and
the Origin of Galaxies
"Big bang" cosmology the theory espousing the explosive
origin of the universe met a crucial experimental test with the
1965 discovery of the cosmic blackbody background radiation.
That measurement yielded a ratio of photons to protons in the
universe and permitted calculation of the abundances of light
elements produced in the first few minutes after the big bang.
Studies of these abundances have since led to the conclusion that
the density of normal nuclear matter in the universe is about 10
percent of that expected of a closed universe in fair agreement
with dynamical constraints on the total amount of its mass.
Grand unified theories of elementary particles postulate that
the universe underwent a transition from a symmetric to an asym-
metric phase 10-35 S after the big bang, when the temperature
was 1028K. A rapid inflation of the universe then resulted in a
present total density of matter almost exactly that of a closec3
universe according to thorn The Nits era Rile ~1~,~
for matter, however, tolls 10 times short of that predicted den-
sity. Such theories thus imply that 90 percent of the mass of the
universe is in unidentified form; if this mass is nonbaryonic, it
could take the form of massive neutrinos or even more exotic new
particles like photinos or anions. This would have important im-
plications for the origin of galaxies, which must have formed in the
gravitational collapse of density perturbations in the cosmically
expanding matter. As the universe evolved, most of the "normal"
matter has, theoretically, been compressed by gravitation to form
the stars and galaxies that we observe; these processes would be
~ · ~flus ~- ~UL4~11] ~1 VO
OCR for page 11
11
profoundly influenced by the presence of the greater bulk of matter
in unidentified form.
Astronomers began to realize some years ago that there is
some unidentified form of mass present in the universe. Galaxies
have since been shown to be embedded in giant halos composed
of material whose presence can be inferred from its gravitational
effects, but which has not yet been detected directly at radio,
infrared, optical, or x-ray wavelengths. The matter in such ha-
los appears to make up 5 or 10 percent of the critical cosmic
density, but the data are not accurate enough to decide whether
this unidentified mass could be in the form of very faint stars or
whether additional matter of novel form is required.
Density estimates based on the dynamics of clusters of galaxies
do not exceed 20 percent of the closure density that is calculated for
a closed universe. Thus, these estimates are apparently not consis-
tent with an inflationary universe based on grand unified theories.
On the other hand, almost all visible galaxies are observed to lie
within clusters or filamentary structures that occupy a small frac-
tion of the volume of the universe. The rest of space seems to be
largely void; but these voids in the distribution of galaxies may
yet contain large amounts of hidden matter not counted in the
analyses of clusters and superclusters enough, in fact, to make
up the 100 percent of closure density predicted by grand unified
theories. Current observations have not resolved this point.
The 1982 report of the Astronomy Survey Committee recom-
mended a number of facilities that promise to contribute to our
understanding of the unaccounted mass. HST will determine the
distribution of hidden mass in galactic halos by determining the
motions of globular clusters. AXAF would permit a search for
the x rays emitted by the hot envelopes of faint stars in galactic
halos. SIRTF will search for cold matter, such as brown dwarfs-
substelIar bodies emitting energy by virtue of a slow contraction
that liberates gravitational potential energy. This search for cold
matter by SIRTF will set limits on the baryonic composition of the
dark matter component in our galaxy. AXAF can also exploit the
recent discovery of hot gas in the halos of some elliptical galaxies
to determine the halo mass through its gravitational ejects.
The NASA instruments that will be operational or under con-
struction by 1995 have extraordinary sensitivity at wavelengths
ranging from the x-ray to the infrared region. They will thus per-
mit us to Took across large distances, far back in time to the epoch
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12
of galaxy formation, when the universe was probably only 10 or
20 percent of its current size. Theorists are currently developing
computer simulations of galaxy formation in a universe dominated
by various types of unidentified matter. Such models have al-
ready demonstrated that if the unidentified matter is composed of
massive neutrinos, then the large-scale structures such as clusters
and sup erclusters should be more highly developed than galaxies
themselves are observed to be. Hence, current attention is focused
upon other particles that remain consistent with grand unified
theories but are more exotic than neutrinos.
· . .
The evidence for
exotic particles may come from direct measurements of their anni-
hilation products such as proton/antiproton pairs and gamma-ray
photons. By 1995, it is expected that theoretical models of galaxy
formation will be well-developed, and it will be possible to com-
pare observations of distant matter in the remote past with the
predictions of particle physics. S~RTF is essential for t.hi~ work
~ _~ 1:_L~ 31 _ ~ 1 · ~e ~ ~
;5111~: 1l'~ll' embed nv n~lmnr~l~ I an I=Y1~O ;C! h^~r;l~r m1~;~
the infrared.
- ~-A^ ~^ ,br~- 15= 4~at~ fly OllllUG~ lllbU
A puzzling feature of the universe is the asymmetry of the
matter/antimatter balance within the galaxy in view of the sym-
metry of the laws of nature. We have no clear-cut understanding
of mechanisms that would have led an initially hot, explosive uni-
verse to favor matter over antimatter, though several symmetry-
breaking mechanisms can be postulated. Whether the universe
on its largest scales preserves the matter/antimatter symmetry is,
therefore, a question of great interest. If matter and antimatter
exist in proximity, we mav expect annihilat.inn t.n t.~k" r~lar.= ot
the interface where gases from these regions mix. The annihila-
tion radiation has its own spectral signature, readily identified,
and we may expect to find this annihilation radiation, if it exists.
Also, antimatter of extragalactic origin might be detected with
magnetic spectrometers that could analyze the cosmic particle
radiation with high sensitivity.
- ~^ van v~ lows- mu
~. , . . _
The Physics of Collapse and Strong Fields
The discovery of radio pulsars, neutron stars, and candidate
black holes in x-ray binary stellar systems demonstrated that some
stars collapse beyond the density of atomic nuclei. Theoretical
models indicate that the collapse occurs within a fraction of a
second when the core of a star becomes unstable; the bounce of an
OCR for page 13
13
imploding stellar envelope off the just-formed neutron-star core
is sufficient to eject the stellar envelope explosively in a super-
nova eruption. Such events are important in the evolution of our
galaxy because they are the principal source of turbulence and
heat in the interstellar medium. They account for the acceleration
of cosmic rays, and create the heavy elements that later become
incorporated into stars and planets like those of our solar system.
These elements, which are believed to originate by such explo-
sive nucleosynthesis, can be detected through their characteristic
gamma-like decay.
Neutron stars represent a unique state of matter of excep-
tionally high density. In spite of their high temperatures, their
interiors are superfluid and are capable of sustaining magnetic
fields up to 10~2 Gauss. Observations of their rotational proper-
ties yield information about their interior structure. Further, the
radiation accompanying accretion of matter from companion stars
is teaching us much about the behavior of matter in strong gravi-
tational and magnetic fields. X-ray burst sources, for instance, are
believed to be neutron stars on which a sufficient layer of matter
has accumulated from a companion star to initiate a thermonu-
clear explosion. Recent observations of the x-ray spectrum during
the decay phase of such a burst in the source 4U1636-53 indicate
the presence of an absorption line. If attributed to iron (as seems
probable), this spectral line is redshifted by 40 percent from the
spectrum of iron observed in a laboratory. If this represents the
gravitational red shift at the surface of a neutron star, then such
stars are both smaller and denser than indicated by models based
on general relativity and the equation of state of nuclear matter.
If confirmed, this interpretation may teach us something quite un-
expected about particle physics or, conceivably, about gravitation.
Some collapsing cores do not stop at the neutron star stage
but plunge on toward a black hole singularity. It is of great interest
to learn about the black hole state. Astronomers have identified
several objects believed to be stellar black holes in orbit around a
normal stellar companion. With the aid of computer simulations,
theorists are now studying the accretion process near rotating
black holes. They are finding that these relativistic flows exhibit
novel features that could help us to understand temporal variabil-
ity in the emission from black hole candidates. We anticipate that
more sophisticated theoretical models will yield new insight that
may tell us whether individual sources are, in fact, black holes.
OCR for page 14
14
Before 1995, relativistic jets will be observed in greater detail
by the Very Long Baseline Array (V[BA), now under construc-
tion. Current observations have demonstrated structure on the
microarcsecond scale. In several jets there are condensations that
appear to be moving apart faster than the speed of light. The
-VCOA, worn z~u-,uarcsec resolution, will pursue this finding with
greater sensitivity and angular resolution, and the QUASAT mis-
sion will extend resolution still further, probing more deeply into
the jet formation site. Much more will remain to be done, since an-
gular resolution of a few microarcseconds will resolve details about
3 light-days across in the quasar 3C 273, and only 2 light-hours at
the nucleus of the active galaxy M87. Because the Schwarzschild
radius of a black hole whose mass is 109 times that of th`~ chin in
~ 1- L ~L _ 1 1 . ~
~ rT ~ A · I 1 rams ~ .
~ou~ ~ ~gr~-nours, sucn observations will permit study of events
less than 20 Schwarzschild radii from the black hole (the r~r~'mP~
~:~ r ~ 1 . ~ ~ ~ ~
lit OI one accretion DISKS, provided that the opacity is not
too great. Such observations will be of extraordinary interest
in testing the black hole mode} of active galactic nuclei and in
studying the novel relativistic effects expected in the vicinity of
black holes. Both the Long-Baseline Optical Space Interferometer
(LBOSI) and the large space radio array (ASTROARRAY see
Chapter 4) aim at achieving 1-,uarcsec resolution at optical wave-
lengths. We should be able to probe phenomena in active galactic
nuclei at distances even closer to the black hole (liaht-hours) than
the size of the light-emitting region ~light-days) indicated by the
time scales involved in light variations. Already, x-ray observa-
tions have demonstrated strong variability on a time scale of an
hour or less in active galactic nuclei (AGN) of a wide range of
luminosities. SIRTF will also play a vital role in the study of these
,
. . .
superenergetic phenomena.
Gamma radiation also is a tracer for highly relativistic par-
ticles cosmic rays both in the galaxy and beyond. In our galaxy
the gamma rays emanate from a number of discrete sources, such
as the Crab and Vela pulsars, but they also emanate from dense
molecular clouds. This diffuse gamma flux indicates the presence
of highly energetic particles that interact with matter in these
clouds. In ways we do not yet understand, this gamma flux could
reflect mechanisms that play a significant role in the formation
of stars. It could, for example, indicate destruction of magnetic
fields that have to be eliminated in order for protostellar matter
to collapse to form stars.
OCR for page 15
15
The mystery of the origin of cosmic rays is only partially
solved at present. Acceleration of particles to relativistic energies
occurs at a variety of scales: in the solar system in solar flares
and planetary magnetospheres, and in the galaxy probably in stel-
lar winds and supernova-driven shock fronts. Recent observations
have indicated that binary x-ray sources containing compact ob-
jects such as Cyg X-3 appear to be powerful particle accelerators
at extremely high energies. To sustain the average luminosity in
cosmic rays of our galaxy (about 104 ergs/s), a substantial energy
source is required. If supernovae are responsible, almost 10 per-
cent of their total energy output would be required to account for
the cosmic-ray luminosity. We also know through observations of
secondary photons in the radio, and sometimes in the gamma-ray
region, that powerful particle acceleration must occur in almost
all classes of extragalactic objects. In fact, the nonthermal radio
spectra of most bright spiral galaxies seem to have almost identical
slope, indicating that some common, but as yet poorly understood,
mechanism governs the acceleration and propagation of the parent
electrons in these galaxies.
Cosmic rays from our galaxy can be directly observed with
detectors in space. At their very highest energies, they can be ob-
served indirectly in ground-based air-shower installations. Most
spaceborne gamma-ray detectors have been relatively small in size
and therefore restricted to studies at low energies (below about
1 GeV) where interactions with the solar wind ("solar modula-
tion") change the composition and energy spectra of the radia-
tions reaching us from the galaxy. Radioactive dating, using the
i°Be isotope, has shown that cosmic rays are a relatively young
sample of galactic matter, with an age of about 107 years. Yet the
elemental abundance distribution of cosmic rays seems to be quite
sirn~lar to that of the much older solar system. However, subtle
but characteristic differences in the isotopic abundances reflect a
different nucleosynthesis history of the matter from which cosmic
rays are accelerated. This may constitute evidence for the contin-
uous chemical evolution of our galaxy, but much more sensitive
measurements are required before this interesting question can be
resolved.
First information on the composition of the extremely rare
ultraheavy cosmic rays, up to uranium, came from detectors on
the HEAD-3 and Ariel-6 spacecraft. Measurements on the Space
Shuttle are providing the energy spectra of individual nuclear
OCR for page 16
16
species to much higher energies (approximately 1013 eV) than
previously accessible and thus are expected to help reveal the
character of the galactic acceleration mechanism.
Cosrn~c-ray measurements could potentially also lead to major
changes in our understanding of fundamental physics and the
formation of the universe if they detected exotic particles, such as
magnetic monopoles, superheavy nuclei, or primordial antimatter.
The Formation of Stare, Planets, and Life
Our own solar system was formed 4.5 billion years ago, prob-
ably through the gravitational contraction of a fragment of an
interstellar molecular cloud. With radio and infrared telescopes
(IRAS is a notable recent example) we have the opportunity to
observe similar processes at work today. Large numbers of active
regions in molecular clouds appear to be candidates for star for-
mation. Clouds of small particles have been discovered around
several bright stars. These could conceivably represent a stage in
the process of planet formation. Astronomers are already study-
ing such events, but need higher spectral and angular resolution
to sort out the complex gas dynamics. STRTF will be able to infer
the chemical composition, temperature, and density of gas flows
involved in star formation regions. However, the angular resolu-
tion will be relatively limited (6 arcsec at 30-,um wavelength for
SIRTF), so the images of such regions will be fuzzy. The Large
Deployable Reflector (I.DR), described in Chapter 5, will attain
angular and spectral resolution that will permit study of details
as small as 50 AU, or somewhat smaller than the diameter of the
solar system, in the nearest star-forming region.
We have not yet been able to demonstrate the existence of a
single planet beyond our solar system. Our instruments have so
far been unable to detect any of the manifestations of such bodies:
reflected light, infrared reradiation of stellar light, wobble on the
plane of the sky, or periodic radial velocity variation. It should
be possible in the 1990s to undertake a meaningful search for ex-
trasolar planets by using special high-resolution spectrographs to
look for the radial velocity variations in a parent star produced
by planetary gravitational pulls. Space-based astrometric instru-
ments may also be used to Took for the corresponding periodic
wobble of the star position.
Planetary systems may well be a common phenomenon around
OCR for page 17
17
late-type dwarf stars like the Sun roughly 50 of which lie within
10 parsecs distance. Whether they are organized like our solar
system into large outer planets, rich in light elements, and smaller
inner planets that have high concentrations of the heavy elements
is more conjectural, but plausible. Both theoretical analysis and
the example of our own solar system support this view. A more
speculative, but intensely interesting subject is the prevalence of
life in the universe. We know of only one example-our own Earth.
Whether the phenomenon of life is unique, rare, or a common oc-
currence appears now to be a problem that can be approached
in a preliminary way. Instruments specifically designed for this
purpose, with spatial resolution of the order of 10-3 to 10-4 arc-
sec, could obtain images of other solar systems. Furthermore,
they could carry out spectrophotometric analyses of the chemical
composition of the planetary atmospheres.
For our own Earth, the oxygen concentration is far higher than
one would expect if it were near chemical equilibrium. Nearly two
aeons ago, the oxygen concentration was enriched dramatically by
the action of living organisms. A similarly high oxygen concentra-
tion, if found in the atmosphere of another planet, would be highly
suggestive of the presence of biological activity there as well. Our
knowledge of how planets form and how life arises is slight, and
surprises would surely result from such a program of exploration.
A further, more specialized search involves intelligent life.
Such a search has a different character than the broader-based
search for life and is not addressed in this study. Obviously, if the
NASA Search for Extra-terrestrial Intelligence (SETI) program
were to find radio signals from another planetary system, it would
be a tremendously exciting and significant event. In the case of
the more restricted study described here, even the hint of life in
another planetary system would trigger a new era in planetary
research.
RELATIONSHIP TO OTHER DISCIPLINES OF
SPACE SCIENCE
Astronomical instrumentation is becoming increasingly so-
phisticated and powerful in the high-precision measurement of
objects far beyond our immediate solar system. This remote-
sensing capability of astronomy can clearly be applied to other
· ,- · ~
space science alSClp lneS.
OCR for page 18
18
Sensitive cameras and spectrographs designed for high spatial
and spectral resolution of cosrn~c phenomena allow imaging and
spectroscopy of solar system objects that contribute to studies of
the chern~stry and physics of planetary atmospheres and magneto-
spheres. Images and spectroscopy of planets, circumsteliar disks,
and bipolar outflows from young objects can help to unravel the
sequence of events leading to formation of stars and planetary
systems. Studies of the magnetic activity of stars, and even di-
rect imaging of stellar surfaces, enable us to extrapolate physical
theories of solar activity and magnetic dynamos to objects like
our Earth. Eventually, these studies may contribute to a deeper
understanding of climatic variations on Earth, of the terrestrial
dynamo action (including periodic reversals of the Earth's mag-
netic field), and of other phenomena of long-term importance to
life on Earth.
Representative terms from entire chapter:
black hole
