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RADIO ASTRONOMY,
branch of astronomy in which celestial objects and astrophysical phenomena
are studied by examining their emission of electromagnetic radiation
in the radio portion of the spectrum.
Unsuccessful attempts to detect celestial radio emission were
made during the latter part of the 19th century. The American radio
engineer Karl G. Jansky, while working at Bell Telephone Laboratories,
in 1932, was the first to detect radio noise from the region near
the center of the Milky Way, during an experiment to locate distant
sources of terrestrial radio interference. The distribution of this
galactic radio emission was mapped by the American engineer Grote
Reber (1911– ),
using a 9.5-m (31-ft) paraboloid that he built in his backyard in
Wheaton, Ill. In 1943 Reber also discovered the long-sought-after radio
emission from the sun. It was later realized, however, that solar
radio emission had been detected a few years earlier, when strong
solar bursts had interfered with the operation of British, American,
and German radar systems designed to detect aircraft.
As a result of the great improvements made during World War
II in radio antennas and sensitive receivers, radio astronomy flourished
in the 1950s. Radio scientists adapted their wartime radar techniques
to the construction of a variety of radio telescopes in Australia,
Great Britain, the Netherlands, the U.S., and the USSR, and the
interest of professional astronomers was soon aroused by a series
of remarkable discoveries.
Discrete sources of radio emission were cataloged in increasing
numbers, and beginning in the 1950s many radio sources were identified
with distant visible galaxies. In 1963 the continuing investigation
of very small radio sources led to the discovery of quasi-stellar
radio sources, called quasars , which, because of red shift of unprecedented
magnitude, could be placed at enormous distances from the Earth.
Soon afterward, in 1965, the American radio astronomers Arno Penzias
and Robert W. Wilson announced the discovery of a 3 K (–454° F)
cosmic background radio emission, which has many implications for
theories of the origin and evolution of the universe. An entirely
new type of radio source, the pulsar, was discovered in 1968 and
was quickly identified as a rapidly rotating neutron star.
For many years radio astronomers concentrated on studying
relatively long wavelengths near 1 m (about 3.3 ft), for which large
antenna structures and sensitive receivers were easy to build. As
techniques were developed to build larger and more precise structures,
and as sensitive short-wavelength receiving equipment was perfected,
the wavelength bands down to 1 mm (about 0.04 in) received increased
attention. At the same time, the development of space technology
allowed observations to be made at very long wavelengths from above
the ionosphere, which is normally opaque to radiation longer than
about 20 m (about 66 ft).
Cosmic radio emission, insofar as is known, comes entirely
from natural processes, although from time to time radio telescopes
are also used to search (so far unsuccessfully) for possible sources
of radio emission from extraterrestrial intelligence. Several physical mechanisms
are recognized that produce the observed radio emission.
Because of the random motions of electrons, all bodies emit thermal,
or heat, radiation characteristic of their temperature. Careful
measurements of the intensity and spectrum of emissions are used
to calculate the temperature of distant celestial bodies, such as
the planets in the Earth’s solar system, as well as of
hot clouds of ionized gas located throughout the Galaxy.
Radio astronomy measurements, however, are often concerned
with the much more intense nonthermal emission arising from charged
particles such as electrons and positrons moving through weak galactic
and intergalactic magnetic fields. When the particle energy is so
high that its velocity is close to the speed of light, the radio
emission from these “ultra-relativistic” particles
is referred to as synchrotron radiation, a term borrowed from the
high-energy physics laboratory, where this type of radiation was
first discovered.
Both the synchrotron (nonthermal) and thermal radio sources
radiate over a wide range of wavelengths. By contrast, a third category
of matter—excited atoms, ions, and molecules—radiate
at discrete wavelengths characteristic of the atom or molecule and
the state of excitation. Wide-range radio emission is referred to
as continuum emission, and discrete radio emission as line emission.
Radio wavelengths are relatively long, extending from about
1 mm (about 0.04 in) to more than 1 km (about 0.6 mi), and radio
telescopes must be extremely large in order to focus the incoming
signals to produce a sharp radio image. The world’s largest
stationary radio telescope, Arecibo Observatory (q.v.) in Puerto
Rico, is a bowl-shaped dish 305 m (1000 ft) in diameter. The largest
fully steerable parabolic dish-type antennas are 50 to 100 m (about
165 to 330 ft) in diameter, and they have a resolution of about
1 arc minute, equivalent to that of the unaided human eye at optical
wavelengths. Incoming radio waves are focused by the parabolic surface
onto a small horn antenna that leads to an extremely sensitive radio
receiver. These receivers, although similar in principle to the
home radio, are able to detect signals as weak as 10–17 W.
The critical parts of the receiver are often cooled to temperatures
close to absolute zero in order to obtain the best possible performance.
For spectral line observations, specialized receivers are used that
can be tuned to as many as 1000 frequencies simultaneously.
In order to obtain higher resolution, arrays of antennas are
used as interferometers giving resolutions of approximately 1 arc
second, equivalent to that of large optical telescopes under ideal
viewing conditions. The largest radio telescope of this type is
the Very Large Array, or VLA, located on an isolated plain near
Socorro, N.Mex. The VLA contains a total of 27 parabolic dishes,
each 25 m (82 ft) in diameter, located along three 21-km (13-mi)
arms in a Y configuration. Each antenna element contains its own
receiver, and the signals from each receiver are sent to a central
building where they are combined to form the high-resolution image
by a technique that is known as aperture synthesis. Other interferometers
may use antennas like huge television antennas. One at Cambridge,
England, uses 60 antennas to detect radiation at wavelengths of
2 m (6.6 ft).
Even higher resolutions may be achieved if individual antenna
elements are spaced thousands of kilometers apart. With these spacings
it becomes impractical to send the signals from each antenna directly
to a common point. Instead, separate broadband tape recordings are
made at each antenna, and the individual tapes are then shipped
to a central processing facility. This technique of very long baseline
interferometry (VLBI) involves using atomic clocks at each telescope
to synchronize the individual recordings to an accuracy of better
than one-millionth of a second. In this way, angular resolutions
of one-thousandth of an arc second are achieved, equivalent to the
apparent angular dimensions of a basketball at the distance of the
Moon. In 1984, the U.S. government appropriated funds for the construction
of an installation called the Very Long Baseline Array (VLBA), a
network of 10 radio antennas spread across the U.S. from Hawaii
to the Virgin Islands. The VLBA, which became operational in 1993,
is expected to provide angular resolutions in the range of 200-millionths
of an arc second. Canada and Australia are both planning similar
programs.
Many discrete radio sources have been discovered and studied
in our solar system, in our galaxy, and in the wide extent of the
universe beyond our galaxy.
The Sun is the brightest radio source in the sky. Its radio
emission is much more intense than would be expected from the thermal
emission of its visible surface, which has a temperature near 6000
K (about 10,300° F). This is because most of the radio
emission observed at longer radio wavelengths comes from the much
hotter, but optically invisible, outer atmosphere, which has temperatures
near 1,000,000 K (near 1,800,000° F). In addition to the
thermal emission, numerous nonthermal storms and bursts occur, particularly
during periods of high sunspot activity when the intensity of radio
emission may dramatically increase by a factor of 1 million or more
for brief periods of about an hour.
The only other source of nonthermal radio emission in the
solar system is the planet Jupiter . At wavelengths near 15 m (about
49 ft), Jupiter emits strong bursts of radiation that come from
relatively small regions, near the cloud surface, that rotate with
the planet. The intensity of these bursts appears to be greatly influenced
by the location of the satellite Io. In addition, Jupiter is surrounded
by extensive radiation belts that radiate in the microwave band
at wavelengths that are shorter than about 1 m (about 3.3 ft).
Thermal radiation has been observed to emanate from the surface
or atmosphere of all of the planets except Pluto. These emissions
have been used by instruments aboard spacecraft to derive information
on planetary meteorological conditions and other phenomena.
The galaxy, or the Milky Way, emits radio waves as a result of
synchrotron radiation from cosmic ray electrons moving through the
weak galactic magnetic field. The 21-cm line emission from neutral
hydrogen is also observed throughout the Galaxy. Small changes in
the observed wavelength of the 21-cm line are caused by the motion
of the hydrogen clouds toward or away from an observer. These changes
are an example of the phenomenon known as the doppler effect, or
red shift. Clouds that are most distant from the center of the Galaxy
revolve around the center with the greatest velocity, and observations
of the Doppler effect are used to measure the velocity and locate
the position of hydrogen clouds. In this way it has been possible
to trace the shapes of the Milky Way’s spiral arms, which
are not readily observed at optical wavelengths.
In addition to the diffuse background radiation, numerous
discrete sources of radio emission exist in the Galaxy. These discrete
sources include the following: supernova remnants, radio stars,
emission nebulas, molecular clouds, and pulsars.
Supernova remnants are the clouds of debris remaining from
stars that have exploded. Relativistic electrons produced in a supernova
explosion are captured by the magnetic field surrounding the location
of the explosion. As these electrons spiral around the magnetic
field lines, they continue to radiate for thousands of years. In
some cases the star itself continues to be a source of radio emission
and is referred to as a radio star. Another important class of radio
star comprises the binary (double) star systems that emit radio
waves when mass is transferred from one component to the other.
Radio stars are often X-ray sources as well.
Thermal radio emission is observed from clouds of ionized
hydrogen (termed H II regions) located along the spiral arms of
the Galaxy. When free electrons recombine with ions of hydrogen
or other light elements, radio energy is released that can be observed
as recombination lines in the radio portion of the spectrum.
Spectral lines also result from vibrational and rotational
transitions of such interstellar molecules as water vapor (H2O),
ammonia (NH3), formaldehyde (H2CO), and carbon
monoxide (CO). More than 50 interstellar molecules are now known,
including many complex organic molecules. In some interstellar clouds,
the radio molecular lines are unusually intense due to the maser
(microwave amplification by the stimulated emission of radiation)
effect.
The intensity of most cosmic radio sources is steady, or only
varies slowly with time. The pulsars, however, emit short periodic
bursts or pulses of radiation about once per second. Although first
discovered because of their intense pulsed radio emission, some
were later found to emit optical and X-ray pulses as well. Pulsars
are thought to form when stars like the Sun collapse under their
own gravity to dimensions of about 10 km (about 6 mi). The density
then becomes extremely great, and electrons are stripped from their atoms,
leaving a so-called neutron star.
Most galaxies probably emit radio waves and do so at energies comparable
to that of our own galaxy—about 1032 W.
In the cases of the so-called radio galaxies, however, the radio
emission is up to 100 million times stronger. Most of this energy
originates not in the galaxies themselves but in clouds of superheated,
ionized gases, or plasma, located hundreds of thousands or even millions
of light-years away from the parent galaxy. These giant radio clouds
may be 100 times the size of the galaxy itself and are among the
largest known objects in the universe.
A great deal of energy is required to generate the powerful
radio emissions from radio galaxies, and it may amount to a significant
fraction of the total energy that would result from the nuclear
burning of a whole galaxy. The origin of this energy and the manner
in which it is converted to radio emissions have been major problems
of astrophysics since the discovery of radio galaxies more than
two decades ago.
Recent detailed pictures of radio galaxies, obtained with
high resolution radio telescopes such as the VLA, often show a prominent
jet of material connecting a bright, compact radio source at the
galactic nucleus to the more extended radio lobes (clouds). It is
widely speculated that these jets or beams transport energy away
from the galactic nucleus to the radio-emitting plasma and that
the source of energy lies in a massive object, possibly a black
hole located at the galactic center. Frequently, a compact radio
source is found at the center of radio galaxies. In one unusual
radio galaxy observed in the mid-1980s, two bright clusters of stars near
its center are emitting jets apparently braided together.
Quasars appear to radiate with the luminosity of hundreds
of galaxies, but each quasar is smaller than a typical galaxy by
a factor of nearly a million. Quasars have very large red shifts, and
they are therefore believed to lie at great distances from the Milky
Way. Because quasars appear to be so powerful, and because their
radiation often varies rapidly, it was once thought they might be
relatively nearby weak objects rather than distant powerful ones.
However, evidence has accumulated supporting the cosmological interpretation
of the red shifts. Radio galaxies, quasars, and bright objects called
BL Lacertae Objects are probably closely related phenomena.
Like the radio galaxies, some quasars are also surrounded
by extended lobes of powerful radio emissions, but most of the radio
emission from quasars usually comes from a bright core only a few
light-years or less in diameter and coincident with the optically
visible quasar.
When observed with very high resolution radio interferometers,
this radio core is often found to consist of two or more smaller
regions, which may appear to be moving away from each other with
velocities considerably greater than the speed of light. Although
these remarkably high velocities may seem at first to violate Albert
Einstein’s special theory of relativity, they in fact can
be explained as a result of motion just under the speed of light,
which is directed almost toward the observer. Because the moving
radio source is nearly catching up with the emitted radiation, the
observed time interval between successive positions of relativistic
jets of material appears shortened, and the velocity appears to
be increased by a large factor over the true velocity. This phenomenon
is termed apparent superluminosity.
Because radio galaxies and quasars are such powerful radio
sources, they can be detected from a great distance. Because of
the long time it takes for signals to reach the Earth from distant
radio sources, radio astronomers are able to see the universe as
it appeared more than 10 billion years ago, or far back in time
toward the origin of the universe—the so-called big bang.
Unfortunately, determining the distance to a radio source is not
possible from radio measurements alone, so that distinguishing between a
powerful distant source and a relatively weak nearby one is impossible.
The distance may be determined only if that source is optically
identified with a galaxy or quasar that has a measurable red shift.
Nevertheless, from studies of the statistical distribution of large
numbers of radio sources, it appears that when the universe was
only a few billion years old, the number of intense radio sources
was much greater and their dimensions smaller.