ANOTHER TURNING POINT IN ASTRONOMY

The second turning point in astronomy during the twentieth century was the advent of radio-astronomy which came about almost by accident. During 1932 Karl Jansky of Bell Telephone Laboratories, while trying to overcome background noise in a radio receiver, discovered that a background noise repeated with a periodicity of 24 hours, thus a stellar day. The direction from which the noise came was the constellation of Sagittarius, the densest part of the Milky Way. Subsequently this point became known as Sagittarius A and it turned out to be the centre of the Milky Way galaxy.

The years passed. By 1937 radiowaves were detected coming from nebulae. These waves were emitted by the methylidene molecule CH and in 1940 cyanogen CN, was discovered in a nebula. By 1940 Grote Reber had built the first radio telescope. The antenna consisted of 45 pieces of sheet iron nailed to a parabolically shaped wooden frame. At the focus he used a crystal detector. An audio-triode intensified the signal of the incoming waves which produced an audible rumbling noise. When this rumble was traced by an oscillating needle on to a strip of moving paper a spectrum was obtained showing peaks and valleys in the tracing caused by the varying intensities of the various wavelengths. The peaks were found to correspond to emissions of various molecules.

By 1942 J S Hey discovered that the Sun emits radio waves. So too, does Jupiter. A very important discovery was that of radio waves at wavelength of 21 centimetres emitted by atomic hydrogen. That this radiation should exist was proved theoretically in 1944 by H C van de Hulst, 26 year old assistant at the Leiden Observatory in the Netherlands. And it was found experimentally in 1951 !

By 1948 strong radio waves were found by M Ryle and F G Smith to come from Taurus A, Virgo A and Centaurus A, corresponding to the optical sources, the Crab nebula M1 in Taurus, the "nebula" M87 in Virgo and the galaxy NGC 5128 in Centaurus. Nebulae such as M87 subsequently became resolved into galaxies. The field was wide open and an altogether new branch of Astronomy came into existence. After World War II dish-shaped antennae popped up all over the world.

Radio waves are a million times, 10⁶, longer than optical waves. Therefore the power of resolution of radio telescopes are a million times weaker than optical telescopes. The aperture must therefore be so much greater. The lobe width, i.e. the angle over which the radio signal is spread is equal to ? ra, or degrees where is the wavelength of the radiation and D is the diameter of the dish antenna. When a radio telescope receives waves of wavelength 1 metre by means of an antenna 100 metres in diameter its bandwidth is equal to 60 x 1 x 100 = 0,6° as compared with the resolving power of the 5 metre Mt Palomar telescope which is 1 arc second. This is 2160 times better than the resolving power of the radio telescope.

To overcome this problem two dish antennae, separated by some distance are connected to act as an interferometer. Instead of D, the diameter of the dish as divisor in the expression , the distance between the two dishes becomes the divisor. To obtain a resolving power of 1 arc second the two dishes must be 2160 metres apart (60 x 3600 ÷ 100). Theoretically the separation between the discs can be made equal to the diameter of the Earth or the distance between the Earth and a distant orbiting antenna.

The Very Large Array in New Mexico ( USA ) has 27 antennae, each of 25 metres width, mounted on a Y-shaped railway track. Each leg of the Y is 21 kilometres long. In their most effective positioning the array has an equivalent width of 27 kilometres. In the late 1950's Martin Ryle developed the method of aperture synthesis whereby the separated antennae are synthesised. For this advance, Ryle and his colleague Anthony Hewish were granted the Nobel Prize for Physics. On a wavelength of 21 cm, the resolving power of the Very Large Array is 2,1 arc seconds.

The largest single dish was built on the ground in a hollow between hillocks at Arecibo on Puerto Rico. This dish has a diameter of 300 metres. Being fixed to the ground, it is non-steerable and is dependent on the spinning of the Earth whereby a strip of the sky moves across the field of view of the telescope. To broaden the width of this strip, the cables by which the receptor hangs, can be slewed.

The findings of radio astronomy revealed facts that are fantastic and undreamed of. There are whole galaxies that radiate very strongly in radio wavelengths. Other sources emit very rapid pulses. These pulsars were found to be the compressed remains of supernovae. They have rapidly moving electrons which spin around magnetic beams. Each time that the beam points towards the Earth we receive a pulse of radio, light and X-ray radiation. The pulsars have diameters of no more than 30km. In the centre of the Crab nebula there is a pulsar which emits 30 pulses per second. It is the remains of the supernova of 1054 which was observed by Chinese astronomers. Its period is 1/30, or 0,033134... seconds. The shortest period of any pulsar is that of the pulsar 1937 + 21, situated at 19h37,6m and with declination +21,55°. Its period is 0,001557810049 seconds so that it spins 642 times per second. The source X1 in Cygnus has been found to radiate strongly in X-rays which emanate from rapidly moving gases being absorbed by a gravitational vortex, commonly called a black hole.

The radio telescopes discovered the quasars (quasi-stellar radio sources). They are star-like objects of low colour indices and are therefore typified as "blue". They pour out as much power as a whole galaxy, emitting strongly in the radio frequencies as well as in the ultraviolet. They are very distant, having red shifts as great as 6 and they have recession velocities of as much as 96% of the velocity of light. They are seen by the radiation which left them when the universe was very young, no more than 4% of the age of the universe and therefore reside on the edge of the observable universe.

Radio astronomy also discovered the 2,7° background radiation which is the remnant of the radiation that was emitted from the cosmo-genesis (commonly but erroneously referred to as the big bang). This all-pervading radiation is to be found everywhere and is the same in all directions (isotropic) and is similar to all observers (homogeneous).

Radio astronomy has become a new science in its own right.

Jan Eben van Zyl