[An earlier version of the article presented here appeared in the magazine Amateur Radio Action, volume 6, issue 8 (1983). ]
GOLDEN JUBILEE OF RADIO ASTRONOMY
by John Kennewell and Malcolm Wilkinson
Nineteen eighty-three may well be regarded as the golden jubilee of radio astronomy. Fifty years ago man first realised that an astronomical source was transmitting radio waves. The man was Karl Jansky and the source was the milky way. Never again would the universe look the same. During the next 50 years ever more sensitive radio telescopes would wrest secrets from the cosmos forever hidden from their optical counterparts.
Karl Jansky was employed by the Bell Telephone Laboratories in Holmdel, New Jersey, and was given the task of investigating static and other radio interference to long distance communications. Commencing toward the end of 1931, he employed a sensitive receiver, together with a rotatable antenna in his studies on 20.5 MHz. By 1932 he had managed to discriminate between three different types of static or atmospheric. The first type originated from nearby thunderstorms. It was very intermittent, composed of "crashes" that often induced high voltages inthe receiving antenna. The second type was similar, although more continuous and weaker in amplitude. It was identified with distant thunderstorms whose signals were propagated to the receiver by reflections from the ionosphere. The third type was a steady hiss type static whose origin was initially quite unknown.
Careful observation and analysis of a year's worth of data led Jansky to confidently state the origin of this type of noise. In 1933 he announced to the world that the source of this noise was fixed in space at a position very similar to the centre of our own galaxy, in the astronomical constellation Sagittarius. This was the first time that anyone had proven that an astronomical source actually radiated radio energy. The science of radio astronomy was born.
Growth was at first very slow. Not until 1937 was an attempt made to systematically map the celestial sphere using radio waves. This attempt was made by a dedicated radio amateur Grote Reber in Wheaton, Illinois. Using his own money and time he constructed a 9.5 metre parabolic dish in his backyard (surely a tremendous engineering feat for an individual today, let alone over 4 decades ago). Together with a home-built 'ultra-short-wave' receiver for 160 MHz, he produced the first contour maps of the sky. It was not until some time after Reber published these maps that astronomers generally realised what Jansky had first discovered.
The second world deviated interest in radio astronomy, but from the mid-forties onward it grew in leaps and bounds. Even during the war, interest was not totally lacking, particularly when it was discovered that a branch of radio astronomy had military significance. In 1942, sever jamming of British Coastal radar operating around 60 MHz was experienced for several days. Detective work by Stanley Hey revealed that the interference was not produced by the enemy, but by the Sun. From that onward, the radio activities of our nearest star have continued to be of great interest to both military and civilian users of radio communication, navigation and radar systems. Man's first steps into space have served to further intensify this interest.
If radio astronomy was of some interest to the military, radio astronomers were considerably more interested in the radio equipment and techniques developed during the war for military use. Bernard Lovell, at Jodrell Bank in England, was quick to use military surplus gear to make radar observations of meteors. The first radar reflections from the moon were received at 111 MHz, in 1946. The world of microwaves was becoming available to radio-astronomical observations. Australians played a significant part in this early development, particularly in the field of solar radio astronomy. The key organisation was the Radiophysics Laboratory of the CSIRO. To this day they have continued to lead the world in many facets of this exciting science.
|In the 1940's, CSIRO scientist J G Bolton identified the radio source called Taurus A with the optical object called the Crab Nebula, the remnant debris left over from a supernova explosion in 1054 AD. This was the first identification of an extrasolar radio source with an optical counterpart. [Image: Hubble Space Telescope (STScI/NASA/ESA)]|
There have been many discoveries made in the field of radio astronomy during the fifty years since Jansky. Some have been point events of the 'Eureka' type, but many have resulted from years of painstaking data collection and sky surveys. The end results have come from the efforts of many people: from the physicists, engineers and technicians who designed and built the radio telescopes, to the observers and astronomers who made the final deductions. Although many of these discoveries are worthy of discussion, seven contributions in particular stand above the rest. They do so because they are contributions that have revolutionised our thinking about the universe, and they are contributions that is general could have been made only by radio astronomical observations.
In historical order, the first of these seven events was the detection by Harold Ewen at Harvard, of the discrete frequency signal emitted by vast numbers of hydrogen atoms that fill the void between the stars. This was in distinct contrast to the extra-terrestrial signals observed by Jansky and Reber. These were very wideband or continuum type emissions produced by the acceleration or deceleration of charged particles, mainly electrons. The bulk of such radiation occurs either due to thermal agitation (i.e. hot objects) or to circular-type motions in a magnetic field. The single-frequency radiation from hydrogen atoms occurs at 1421 MHz (a wavelength of 21 cm) when the electron in the atom rapidly flips the direction of its 'spin axis'. The presence of such radiation had been predicted on theoretical grounds, by a young Dutch astronomer Hendrick van de Hulst in 1944. The great significance of Ewen's detection of this emission (in 1951) was its use in mapping the structure of our home galaxy, the Milky Way. When we look at the southern sky on a dark cloudless night, it is obvious that the Milky Way is a planar or flattened structure like a saucer viewe edged on. However, we cannot see very far into the plane because the light is obscured by large amounts of intervening gas and dust. Radio waves are not so hampered. Furthermore, the Doppler effect allows us to sort out the various 'arms' of the galaxy. As each arm moves at a different velocity relative to us, so its hydrogen 21 cm emission is shifted slightly in frequency. In consequence, a map showing the galactic features within the plane can be constructed.
The second significant event in the field of radio astronomy was one that evolved quite gradually, and one that is still being refined. It was the discovery and explanation of the diverse radio activity emanating from the Sun. It revealed that the Sun is a much more active body than was previously expected. In a way it typifies the whole discovery process of modern astronomy. Each new revelation of the universe shows it to be more complex, violent and turbulent than was formerly imagined. The classification and physical investigation of solar radio bursts by a team led by J Paul Wild, an Australian radio-physicist, gave much impetus to the science of astrophysics and revealed the intricacies of the Sun's atmosphere. Plasma conditions unreproducible in Earthly laboratories were opened to study. Who would have believed that when the solar surface termperature was 6000 degrees Kelvin (degrees_Celcius = degrees_Kelvin - 273), the temperature of the outer atmosphere would turn out to be around one million degrees.
The third significant event was the accidental discovery by Bernard Burke and Kenneth Franklin of the Carnegie Institute in Washington, of radio noise emanating from the planet Jupiter. This discovery, made in 1955, was completely unexpected. The planets became alive, if not in a biological sense, then certainly in a physical one. This awareness has continued through the more intimate investigation of the solar system by spacecraft. The interaction between Jupiter's magnetic field and the moon Io to produce the chorus of radio waves first heard in 1955 at around 20 MHz is still being debated.
The fourth significant event in our saga was the discovery of a very peculiar class of astronomical phenomenon known as quasi-stellar objects (QSOs) or quasars in short.
The initial discovery was made over the years 1961-63 of a radio source identified by the third radio sky survey conducted by Cambridge University. It was the 273rd object discovered in this survey, and was thus given the name 3C273. At this time radio telescopes did not have very great spatial resolving power and it was thus generally impractical to determine if the radio object could be located and studied with an optical telescope. However, Cyril Hazard, using the then newly commissioned CSIRO radio telescope at Parkes was able, with the help of a series of lunar occultations (whereby the moon obscured the source), to determine an accurate position for 3C273. Maarten Scmidt, a young Palomar Observatory astronomer was then able to examine 3C273 in great detail. Whereas it appeared on first inspection to look like a star, all other evidence said it wasn't. For a start, velocity determinations from the Doppler shift of its spectral lines seemed to indicate that it lay 2000 million light years away (our nearest well-defind galaxy, that in Andromeda, is only 1.7 million light years distant). At this distance the power output of 3C273 is 1040 watts. Together with its small size this indicates an absolutely fantastic energy consumption. It is equivalent to consuming one whole sun every month! At the time no known physical mechanism to explain the operation of quasars had been satisfactorily proposed. Later it would be suggested that a massive black hole was the driving engine, and this is now the generally accepted explanation.
Nevertheless, the quest to discover quasars more distant than 3C273 continues to this day, with radio telescopes directing optical observations. The latest identification(1983), made by a team of Australian observers using the Parkes radio telescope and the Anglo-Australian telescope (optical) at Siding Spring (NSW) , is that of the quasar PKS 2000-330 estimated to lie at a distance of nearly 20,000 million light-years. At this distance, it is the most distant known object in the universe (1983) and is running away from us with a velocity of 92 percent of the velocity of light.
The fifth significant event is one that may yet turn out to be the most significant discovery that any astronomy has yet given to the human race. It was and continues to be the discovery of a wide range of diverse molecules scattered throughout interstellar space. These molecules were discovered in the same way that hydrogen atoms were discovered. That is, by the discrete but different spectral frequencies they emit when undergoing some change. Each molecule has its own unique spectral signature from which it can be identified. The first interstellar molecule discovered was the hydroxyl radical (OH) in 1963. Since then an incredible range of inorganic and organic molecules have been detected. An Australian team from Monash University was very active in this research. The molecules discovered include hydrogen sulphide, carbon monoxide, hydrogen cyanide, acetaldehyde and vast amounts of ethyl alcohol. The last item has caused eminent radio astronomer John Kraus to joke that "even if there is no life in space, there is at least one of the amenities of the good life in abundance." However, the range of molecules now seems ot include all the precursors necessary to construct proteins. The seeds of life in space may yet outweigh all the celestial vodka. The presence of either entity throughout our galaxy still remains a complete mystery.
The sixth major contribution of radio astronomy gained its discoverers the 1978 Nobel prize in physics. The discovery was made in the course of an unrelated investigation, much like that of Jansky. The coincidence however was much stronger, for Arno Penzias and Robert Wilson were working for Bell Telephone Laboratories in New Jersey in a part of the same laboratory where Jansky made his discovery. Penzias and Wilson discovered, after very careful experimentation, that the sky is filled with a background microwave radiation that indicates that the universe has an overall temperature of 3o Kelvin (ie 3 degrees above absolute zero or -270o C). It took some time to recognise the significance of this, but eventually it became very important in speculations about the origin of the universe. It provided decisive evidence in favour of the big bang theory. This theory states that some 14,000 million years ago (it was believed to be 20,000 million in 1983) the universe started as an extremely dense and compact primordial fireball at a temperature of 10,000 million degrees Kelvin. By our present age, the expanded (and expanding) universe should have cooled to 3o K, precisely that measured in 1963.
The seventh and most recent contribution has ramifications that encompass almost everything from fundamental astrophysics through the feminist movement and into Hollywood. The discovery was again serendipitious, and was made by a young lady Jocelyn Bell, working at the University of Cambridge. It was, of course, the discovery of pulsars - those strange objects that emit radio energy in rapid bursts. For months after the initial discovery, unscrupulous tabloids were running stories about LGM's (little green men) in space. It turned out however, that the reality of pulsars was just as interesting, but in a purely physical sense. They form a vital link in the process of star development. When a large star is nearing the end of its life it may suddenly explode in a most spectacular way. The visual outburst may be seen on Earth as a supernova, or bright new star. Most of the mass of the star is blown off into the surrounding space, but a small dense core may remain. The core contracts under the influence of enormous gravitational forces until the parts of each atom are packed so closely together that everythng turns into neutrons. We then have a neutron star. If, as is most probable, the neutron star is rotating, it sends out a directional radio beam once per rotation much like a lighthouse. If the neutron star is dense enough it may continue to shrink, and eventually become, a black hole. At this stage, the escape velocity of the object is greater than the velocity of light and thus nothing, including light, can reach the rest of the universe.
In spite of its rather slow beginnings, radio astronomy has now progressed fast and far. Jansky could never have even dreamed, in 1933, that over the next 50 years fantastically sensitive radio ears would quite literally 'spring' from the Earth to probe the cosmos almost to its visible limits - and yet much remains to be done. Limited resolution, the ability to separate close objects, has until recently been a restricting factor in radio observations of the sky. However, the development of 'aperture-synthesis' techniques has recently enabled radio telescope to locate celestial sources with considerably more precision than any optical telescope now in existence. These techniques allow a number of small, widely spaced antennas to simulate the resolution of one much larger single antenna. The Australia Telescope, a CSIRO project to come on-line in 1988, will make extensive use of aperture-synthesis to keep Australia in the fore-front of radio astronomical research.
Seven Major Contributions to Knowledge
The ability of a telescope to separate two closely spaced objects is termed its resolving power and depends upon the diameter of the telescope and the wavelength at which it operates.
The smallest angle R (in radians, where 1 radian = 57.3 degrees) that a telescope of effective diameter D metres can separate at a wavelength of L metres is given by the formula:
For the 200 inch (5 metre) Mt Palomar optical telescope, D=5m and L=5x10-7m giving R=10-7radians or 0.02 seconds of arc.
In actual practice atmospheric "seeing" conditions usually limit this to around 1 second of arc.
Typical maximum resolution for large optical telescope
For a radio telescope operating at 300 MHz (L = 1m) to have a resolution of 1 arc second, it would need to have a diameter of 200 km!
Obviously it is impossible to construct a parabolic dish of such a size. However, a telescope with equivalent resolution (but not collecting power) could be made by using a number of much smaller dishes whose maximum separation was 200 km. This technique is called 'aperture synthesis'.
AN HISTORICAL CALENDAR OF RADIO ASTRONOMY
A short history of the early years of radio astronomy has been published by one of the pioneers, J S Hey as "The Evolution of Radio Astronomy". Published by Science History Publications as a paperback in 1975 with the ISBN 0-88202-030-7.