So What Is Radio Astronomy

Radio astronomy involves the study of radio waves from the depths of space. Many objects in the universe, including stars, galaxies, and nebulae, as well as a wide variety of peculiar, fascinating, and often mysterious objects, emit radio waves through naturally occurring processes.

1.3.1. How Radio Waves from Space are Generated

Cosmic radio waves are created in several ways, depending on the physical conditions in the radio-emitting object. All the processes involve the movement of electrons, in particular, changes in their velocity during which the electrons lose energy, which can be radiated away as a radio wave. Radio energy is produced either by slow-moving electrons (traveling between tens and hundreds of kilometers per second) within hot clouds of gas that surround very hot stars, for example, or by electrons that have been accelerated to near the speed of light through stellar or larger scale explosions, which energize the particles. The two radio emission processes are known, respectively, as thermal and nonthermal.

Nonthermal emission, sometimes called synchrotron radiation, involves cosmic ray electrons that spiral around magnetic fields and radiate energy in the form of radio waves. Depending on the energy of the particle and the strength of the magnetic field involved, this process can produce emission at any of the wavelengths across the electromagnetic spectrum (see Appendix A.2).

1.3.2. Radio Telescopes

For hundreds of years, ever since Galileo, in 1609 AD, first used an optical telescope to study the moon, stars, and planets, astronomers have used glass lenses or a mirror to gather and concentrate light from distant stars and galaxies. The light is then passed through more lenses to bring it to focus on a photographic plate or on an electronic detector in the best of modern telescopes.

A radio telescope is similar to an optical telescope, but it reflects radio waves oft a metal surface instead of a glass mirror. The larger the reflecting surface the greater the amount of energy gathered and the fainter the radio signals that can be sensed. For decades the 250-ft diameter Lovell telescope at the Nuffield Radio Astronomy Laboratories of the University of Manchester in England was the largest fully steerable radio telescope in the world, see Figure 1.2.

figure 1.2. The 250-ft diameter radio telescope at the Jodrell Bank Observatory in England. At the top of the 60 ft high mast in the center of the dish a box houses electronic amplifiers attached to a small antenna mounted just beneath the box at the focus of the telescope. The amplified signal is then fed down to a laboratory at ground level or, in the days that I was using the dish, to our lab in the tower at the right. This image is of the upgraded version known as the Lovell telescope. The version we used in the early 1960s had only one stabilizing girder carrying much of the weight and the surface was not as carefully constructed. (Photo courtesy of Nuffield Radio Astronomy Laboratories, University of Manchester.)

figure 1.2. The 250-ft diameter radio telescope at the Jodrell Bank Observatory in England. At the top of the 60 ft high mast in the center of the dish a box houses electronic amplifiers attached to a small antenna mounted just beneath the box at the focus of the telescope. The amplified signal is then fed down to a laboratory at ground level or, in the days that I was using the dish, to our lab in the tower at the right. This image is of the upgraded version known as the Lovell telescope. The version we used in the early 1960s had only one stabilizing girder carrying much of the weight and the surface was not as carefully constructed. (Photo courtesy of Nuffield Radio Astronomy Laboratories, University of Manchester.)

In January 19611 arrived at Jodrell Bank to begin my postgraduate work, having just arrived from South Africa. It was a foggy day and I could not see the giant telescope until I was nearly under it. Then 1 was overwhelmed by its awesome size as it loomed through the fog That image remains deeply etched in my memory. Our lab was situated 120-ft above ground in the enclosed space seen in Figure 1.2 in what was called the green tower, on the right. The walls of this lab were made of steel plates and had no insulation. The important elements of the receiving system were in a small, heated enclosure that barely allowed us to enter to make adjustments when needed. During the infamous winter of 1962-1963 in England the temperature inside this lab dipped below freezing and remained there for weeks despite having heaters going to keep us warm. After tedious trips up to the focus located 60 feet above the dish, which required a4-minute one-way ride in a small funicular platform, any attempts to warm my feet by an electric fire upon my return to the metal box tended to set my socks smoldering before I even knew that the heat was on. Our midnight observing runs, which required remaining awake from midnight until 10 am tending the paper charts recorders, were a test of endurance. They were also dramatic and fun.

Radio waves from space are reflected off the parabolic surface to a focus (top of the mast in the center of the disli in Figure 1.2) where a small antenna is placed that may look similar to a conventional TV or FM antenna but far more. There the concentrated radio signals that are converted into minute electrical currents in amplifiers connected to the antenna. This is known as the "front end" of the receiver. These currents are then sent to the control room where they are amplified a million or more times in the "back end" of the receiver before being processed in a computer or displayed in such a way that the radio astronomer can "see" what the data indicate.

A single-dish radio telescope will collect all the radio energy coming from some small area in the heavens at any instant. That area is called the beam and defines the resolution of the telescope, which depends on the observing frequency and the diameter of the dish. The larger the diameter or the higher the frequency, the better the resolution (smaller beam width). The 250-ft radio telescope shown in Figure 1.2 has a beamwidth of about 12 arcminutes at a frequency of 1400 MHz.

In order to produce the equivalent of a photograph of a section of sky the radio telescope has to be systematically "scanned" in the same way that a TV image is produced by scanning an electron beam across the TV screen. The intensity of received radio signals is recorded and the data combined to produce a radiograph, the visual image of what a particular direction in the sky looks like to the radio telescope.

1.3.3. What is a Radio Source?

One of the earliest post-World War II discoveries in radio astronomy was that specific regions of the sky seemed to emit more radio energy than their surroundings. These were given the generic name of "radio source." Whenever a larger radio telescope or more sensitive radio receiver was used, more radio sources were discovered. Today tens to thousands of radio sources are known.

The accuracy with which the first radio sources were located in the sky was insufficient to allow optical astronomers to decide which of the hundreds or thousands of images of stars, galaxies, and nebulae in their photographs of the region in question was responsible for the radio emission. In order to make an optical identification the astronomers required an accuracy of 1 arcminute or less (Appendix A.6), although by the late 1940s and early 1950s half a dozen of the strongest radio sources had been identified with obviously unusual, and hence interesting, optical objects. Those included a couple of nebulae associated with the remains of exploded stars, and several distant galaxies.

The list of various types of radio sources now known includes stars, nebulae, galaxies, quasars, pulsars, the sun, the planets, as well as amazing clouds of molecules between the stars, all of which generate radio waves. The study of the cosmic radio waves—where they come from, how they are produced, what sorts of astronomical objects are involved—is what radio astronomy is all about.

1 A. Radio Interferometers

In order to "see" more clearly the radio astronomer needs, above all, high resolution. As stated above, the larger the diameter of a single dish antenna the better its resolution, but there is a limit to how large a structure can be built before it collapses under its own weight. Instead, radio astronomers began to combine the signals from two dishes separated by miles in what is called an interferometer. Its resolution is set by the maximum distance between the component dishes.

A very beautiful variation on this was developed at Cambridge by the radio astronomers led by Sir Martin Ryle. In this technique the aperture (or area) of a very large dish is synthesized by many small dishes set far apart, and feeding their individual signals to a powerful central computer.

Imagine two 10-meter diameter dishes located on a football field and pointed at a given radio source. If you store the radio signals from each of these dishes as they are moved to every point on the held and then combine all the data, it is possible to synthesize what you would have observed had you used a single dish of the size of the entire football held. What Ryle and his team realized was that, as seen from the radio source, any two radio telescopes appeal to move around each other during the day due to the rotation of the earth. That means you don't have to physically move the dishes. You just let the earth do the walking. Enormous apertures can be synthesized in this way.

In practice, aperture synthesis involves using an array of dishes spread over dozens of miles of countryside

1.4.L Very Large Array

The world's largest aperture synthesis telescope is the Very Large Array (VLA), 50 miles west of Socorro, in New Mexico, one of the National Radio Astronomy Observatory NRAO's repertoire of beautiful radio telescopes (Figure 1.3). Observations with the VLA were used to make many of the radiographs shown in this book. Twenty-seven individual radio antennas of 25 meter diameter are located along railroad tracks, which are laid out in a Y-shape, each arm of which is 21 km long. To completely synthesize the largest possible aperture obtainable by the VLA the individual antennas have to be moved to different locations along the rail tracks ever)' few months.

One of the most stunning images made by the VLA is of the radio source known as Cygnus A, shown in Figure 1.4.

i.4.2. Very Long Baseline Array

The Very Long Baseline Array (VLBA) is a continent-sized radio telescope (Figure 1.5) is capable of enormously high resolution. Ten antennas are located from St. Croix in the Virgin Islands, to Hawaii, with eight distributed over the continental United States. As with all new antenna arrays, the resulting radio telescope

FIGURE 1.3. The VLA radio telescope of the National Radio Astronomy Observatory located west of Socorro, NM. out in the middle of nowhere, which is the way radio astronomers like it. In this view many of the dishes are spaced in the so-called compact array. The railroad tracks on which the dishes can be moved for up to 11 miles along each of three arms of the array can be seen in the foreground. (Image courtesy of NRAO/AUL)

FIGURE 1.4. A radio image, or radiograph, of Cygnus A, one of the most powerful sources of radio waves in the heavens, as observed with the VLA. Tenuous filaments of radio emitting gas constrained by magnetic fields illuminate two enormous lobes fed by jets blasting out of on either side of a central galaxy located 6(H) million light years distant (see Chapter 10). Investigators: R. Perley, C. Carilli, and J. Dreher. (Image courtesy of NRAO/AUI.)

Figure 1.5. The location of the 10 dishes that make up the VLBA radio telescope of the National Radio Astronomy Observatory. Data from all the out stations are brought together at the central processing computer in Socorro, NM. (Image courtesy of NRAO/AUI.)

operates on aperture synthesis principles. The VLBA can attain an angular resolution of two tenths of one thousandth of an arcsecond (0.2 milliarcseconds), which may be compared with 1 arcsecond for the typical radiographs shown in this book.

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