In addition to celestial bodies that radiate visible light, the universe contains many objects that emit radiation of various nonvisible wavelengths, such as radio waves. In fact, some astronomical objects emit very little light (or even none at all) and are, therefore, unimpressive when observed visually. They may, however, radiate relatively large amounts of energy at nonvisible wavelengths. The range of such nonvisible radiations is extremely broad, spanning almost all the electromagnetic spectrum (of which visible light constitutes only a small part). Therefore, it is important to study as wide a range of electromagnetic emissions as possible in order to gain the fullest possible understanding of the universe.
Historically, the nonvisible part of the electromagnetic spectrum was first studied in 1931-32, when the American Karl Jansky detected radio emissions (at a frequency of 20MHz—equivalent to a wavelength of 49 feet or 15 meters) emanating from the center of our galaxy. Since then, radioastronomy has developed considerably and has made substantial contributions of our knowledge of the universe.
The nature of radio waves
Radio waves are low frequency and, therefore, long wavelength electromagnetic radiations. The radio part of the electromagnetic spectrum lies beyond the infrared region and comprises all radiation with a frequency lower than about 300,000 million hertz (300,000MHz) or with a wavelength longer than about 1 mm. Because of their low frequency, radio waves are not absorbed by the earth’s atmosphere. As a result, radio observations of celestial objects can be made from the earth’s surface during the day and at night, both in cloudy and clear weather.
Types of celestial radio emissions
There are three main types of radio emissions: thermal emissions, synchrotron (or nonthermal) emissions, and radio spectral-line emissions.
Thermal radio-wave emission occurs as a result of the acceleration of electrically-charged particles in a hot gas. When the temperature of a gas is high enough, its neutral atoms break up into negatively-charged electrons and positively-charged ions. Every charged particle moves continuously, interacting with other particles as it does so. (Such interactions are called collisions, although the particles do not normally hit each other.) These collisions cause some of the particles to accelerate, as a result of which they emit radio waves. The higher the temperature of the gas, the greater the number of collisions, and the higher the intensity of radio emissions. Hence, the temperature of a thermal radio source can be calculated from the strength of its radio emission.
Like thermal emission, synchrotron radiowave emission is produced by the acceleration of charged particles. Unlike thermal emission, however, the acceleration is caused by a magnetic field. The characteristic feature of synchrotron emissions is that the radio waves are polarized (that is, they vibrate in only one plane) unlike those given out by thermal sources. Polarized radio waves have been detected from the Crab Nebula (which also emits unpolarized thermal radio waves). The precise source of these emissions is thought to be the star designated NP 0532, situated in the middle of the nebula. This star produces the magnetic field necessary to accelerate the charged particles. Most other powerful celestial radio sources emit polarized radio waves, which indicates that they are produced by the synchrotron process.
Radio spectral-line emission, the third type of celestial radio emission, is concentrated in a narrow band about one specific frequency-just as an optical spectral line corresponds to a single frequency in the visible electromagnetic spectrum. Radio line emissions usually originate in clouds of hydrogen gas, a relatively common constituent of the universe, which is found in our galaxy’s spiral arms, among other places. Therefore, these lines can be used to map the distribution of hydrogen gas, even in regions where interstellar dust prevents optical observations of the gas clouds.
Line emissions are produced only from the relatively few hydrogen atoms in which the proton and its orbiting electron are initially spinning in the same direction (in the nucleus). In this situation, the hydrogen atom as a whole is in an unstable high-energy state. The result is that the electron changes its direction, spinning in the opposite direction to the proton. When this change occurs, the hydrogen atom falls to a lower, more stable energy state, and the excess energy is radiated as radio waves at the single frequency of approximately 1,420 MHz (commonly known as the 21 -centimeter wavelength). »
In practice, however, line emissions cover a broader range of frequencies than the 1,420 MHz line. This is because collisions between atoms affect their individual energies. This, in turn, alters the frequency of the radio waves emitted when the atoms fall to a lower, more stable energy state. The extent of this frequency broadening can be used to determine the temperature of celestial hydrogen clouds (typically about —274° F. ( — 170° O). Furthermore, the frequency of emissions detected on earth also varies as a result of movements of the clouds (an effect known as the Doppler frequency shift). This phenomenon can be used to calculate the clouds’ motions.
Since the discovery of 21-centimeter hydrogen line emission, scientists have found that other molecules also produce line emissions when falling to lower energy states. Each type of molecule can be identified from the characteristic frequency and wavelength of its emissions. This is the same way that hydrogen can be identified from its 21-centimeter radiation. More than 50 different sorts of molecules have now been detected in space using this method.
The basic design of most radio telescopes is similar to that of optical reflecting telescopes: both use a parabolic reflector (called a dish in radio telescopes) to collect incoming electromagnetic waves and bring them to a point of focus. But unlike optical reflectors, which collect and focus light, radio telescopes collect and measure the minute amounts of energy in radio waves. (As an indication of how little energy there is in celestial radio waves, it has been calculated that if the energy emitted by a quasar—one of the strongest celestial radio sources—were tp be collected by a radio telescope for 10,000 years, it would only be enough to light a small bulb for a fraction of a second.) Because of the infinitesimal amounts of energy involved, the largest possible radio dishes are needed to study radio sources at the edge of the detectable universe. Moreover, the larger the radio dish, the greater the amount of detail it can reveal about the structure of a radio source. That is, the resolving power of radio telescopes increases with increasing diameter of the radio dish. (The resolving power of both radio and optical telescopes also increases with increasing frequency of the incoming electromagnetic waves. Thus, optical telescopes can, in theory, resolve greater detail than can radio telescopes because visible light has a higher frequency than radio waves. In practice, however, other factors—such as turbulence of the earth’s atmosphere—limit the resolving power of optical telescopes, and radio telescopes are generally more effective.)
Nearly all radio telescopes are steerable so that they can track a radio source as it moves across the sky. (This movement is caused by rotation of the earth.) The first fully steerable radio telescope was built in 1957 at Jodrell Bank, England, by the University of Manchester and has a dish that is about 250 feet (76 meters) across. The largest radio telescope of this type is the 330-foot (100-meter) Effelsberg telescope at the Max Planck Institute for Radio Astronomy, Bonn, Germany. There is, however, a practical upper limit to the size of individual radio dishes, due to the difficulty in making very large parabolic dishes that are also accurately shaped—principally, because large dishes tend to deform under their own weight. This limitation creates a problem when astronomers want to study the finest details of distant radio sources, because this requires dishes that are tens of miles across. The problem can be overcome, however, by using radio interferometry.
Radio interferometry is a technique whereby two or more moderate-sized radio telescopes are separated by several miles and (coordinated by means of a cable link) simultaneously study the same radio-emitting object. By combining the signals received by each of the telescopes (which can usually be moved on tracks), it is possible to obtain the same degree of resolving power that would be obtained from a single dish several miles across. In addition, radio interferometry enables the positions of radio sources to be precisely determined. This is very useful for optical astronomers because a radio source may be optically very dim and, therefore, extremely difficult to find without knowing where to look. The angular sizes of radio sources can also be obtained by radio interferometry, as can the distance between two neighboring sources.
Moreover, despite the low frequency of radio waves, radio interferometers can resolve the structure of celestial objects in far greater detail than can optical telescopes. This is partly because of the large effective size of radio interferometers (the effective size being equivalent to the distance between the dishes) and partly because the practical resolving power of optical telescopes is restricted by the turbulence of the earth’s atmosphere. But even the best conventional radio interferometers are inadequate for studying extremely small and distant objects, so a technique called Very Long Baseline Interferometry has been developed.
Very Long Baseline Interferometry
The principle of Very Long Baseline Interferometry (VLBI) is essentially the same as that of normal radio interferometry, except that VLBI employs several radio dishes hundreds of miles apart, sometimes on different continents. In Europe, one such VLBI array comprises the 250-foot (76-meter) Jodrell Bank telescope in Britain, the 320-foot(100-meter) Effelsberg telescope in Germany, and 14 dishes at Westerbork, Holland. The large separation in VLBI arrays causes difficulties in coordinating the individual dishes and in analyzing their results, so most arrays are controlled by a central computer, which sends instructions to each dish by radio. But even with this arrangement, the signals transmitting the data collected by the dishes back to the computer are often of poor quality. To overcome this problem the individual radio observatories record their data on tapes, which are later collated and analyzed by the computer.
Using Very Long Baseline Interferometry, a small region (less than about 200 astronomical units—0.003 light-years—in diameter) of unusually strong radio emission has been detected in the center of our galaxy. The nature of this object is unknown, although some astronomers believe it to be a black hole that is releasing large amounts of radio energy as it captures dust and stars from the galactic center.