ANOTHER DISCOVERY BY RADIO ASTRONOMY

At Cambridge, England a large arrangement of 2048 erect-standing di-poles, more than four acres in area was erected. It was very sensitive to radiation of wavelength 3,7 metres.

In 1967 Miss Jocelyn Bell and Anthony Hewish used this system and became aware of queer, repeating radio signals having a periodicity of just more than one second. For all the orld it appeared as if there was a radio transmitter in space transmitting very regular signals. Could these signals be coming from some intelligent source? Bell and Hewish spoke under their breath about the L G M's (little green men). But no, there was no code in the signals and no Doppler-shift due to the rotation of a planet housing the L G M's which there would be on account of the spin of the planet. They called this source of pulsating signals a "pulsar" and it was designated CP 1919 +21 - Cambridge pulsar, situated at right ascension 19h 19m and declination + 21°.

Early in 1968 another three pulsating objects were found and by 1970 another 50 were known. Their periods ranged from 3,74 down to 0,00155 seconds. T Gold suggested that the pulsations were due to the rapid spinning of very small bodies consisting of neutrons, which astrophysical theory had shown to be the possible remnants of supernova explosions. The diameters of these "stars" would be no more than 10 to 20 km. The pulses would be caused by rapidly moving electrons that are constrained by a very strong magnetic field. Every time that the axis of the magnetic field points towards the Earth we receive a pulse of radiation. With a period of 0,00155 seconds, the number of rotations per second must be 1 ÷ 0,00155 = 645 rotations per sec! These neutron stars can only be the residues of supernovae - stars that had exploded and left a tiny residue no more than 20 km in diameter.

When the Crab nebula was examined at X-ray wavelengths by the Einstein satellite HEAO-2, launched in 1978, it was found that 96% of the X-rays of the nebula were due to synchrotron radiation and that the remaining 4% of the X-rays come from the star in the centre of the nebula. These X-rays are caused by synchrotron radiation which is due to electrons moving at speeds close to the speed of light in a very strong magnetic field. The energy of the X-rays from the pulsar comprise at least 100 milliard electron volts. This proved that the residue of the Chinese "visiting star" of 1054 is a pulsar. Its period of 0,033134 seconds, means that it spins 1 ÷ 0,033134 = 30 times per second. The neutron stars or pulsars were thus found to be the residues of supernovae. The rate of spin of a pulsar gradually decreases with time. Those with the shortest periods are the youngest.

HEAO-2 received X-rays from the remnant of Tycho's star of 1597 from a globular volume but no radiation from a central star. This supernova must therefore have belonged to class la, namely a white dwarf whose mass increased by accretion from a companion star to more than the Chandrasekhar limit of 1,44 solar masses. It then blew itself to smithereens and left no central residue. The area of Tycho's remnant fits closely the area of the radiation from the radio waves which show that the mass of that star must have been 1,5 solar masses so that it exceeded the Chandrasekhar limit.

In the Gum nebula a pulsar has been found in the Vela portion and it is belting it out at 200 km per sec. This shows that the supernova explosion is not spherically symmetrical. A supernova is now pictured as having two diametrically opposite sets of jets from the poles of its magnetic field. This axis is close to the axis of spin so that the jets describe cones in space. Around the equator of the neutron star a "collar" of material expands at hundreds of kilometres per second. Because the speeds of the jets are not equal the neutron star gets a kick which sends it off at great speed.

Jan Eben van Zyl