There are about a hundred known supernova remnants in Galaxy. Although visible, X-ray and gamma radiation have been detected from a few of these, most are seen only at radio wave¬lengths. The radio emission is caused by high-speed electrons, produced within the supernova, spiralling around magnetic field lines. This process, called synchrotron radiation, is also the origin of the radio emission from quasars, radio galaxies and the general galactic radio background, but is quite different from that which causes the emission from H+ regions. It is therefore easy to distinguish between H+ regions and supernova remnants purely on the basis of their radio emission, despite the fact that they are often found in fairly close proximity to each other. In particular the observable differences are:

(a) The emission from an SNR declines with increasing radio frequency while that from an H+ region in¬creases or stays constant.
(b) Emission from SNR’s is often polarized; that from an H+ region never is.
( c) H+ regions show hydrogen radio recombination lines while SNR’s do not.

Supernova remnants may also be distinguished from H+ regions by their shape. H+ regions are usually composed of a group of compact sources: supernova remnants, on the other hand, often have a very distinctive roughly circular shell shape several arc minutes in diameter. A classic example of a young shell-shaped supernova remnant is provided by CASSIOPEIA A – the object which, after the Sun, is the strongest source of radio emission hi the
sky as seen from the Earth There is no historical record of the supernova explosion that gave rise to Gas A, but from the measured rate at which the supernova is expanding we know that the event must have taken place in the late seventeenth century. At some positions around the shell, nebulous filaments can now be seen on sensitive red photographs. These filaments have an emission-line spectrum, which has allowed their velocities to be measured via the Doppler effect; they are found to be moving at speeds of up to 9000 km s”1 from the Earth, and to have proper motions of up to 0.5 arc sec per year-. These velocities are consistent with the idea that they were produced in the original explosion, 3000 pc away, 300 years ago.

The evolution of Gas A is rapid enough for changes to be seen in both its radio and optical appearance in a few years. Some of the radio knots seen in altered their positions and strengths between 1969 and 1974, while brightness variations in the optical knots have typical timescales of 20 years. The total emission of Gas A at radio wavelengths decreases steadily with time at the rate of about 1 per cent per year. This decrease is equivalent to a cooling down of the relativistic gas as the SNR expands. In 1975, however, it was reported that the flux density of Gas A at 38 MHz (one of the lowest frequencies used in radio astronomy) had started to increase again. As yet we do not know whether or not the increase will continue or whether it comes from a part or from the whole of the SNR. The reason for the increase is also a mystery.

There are two other supernova remnants that are known to date from explosions seen at about the same time as Gas A originated. The Danish astronomer Tycho Brahe witnessed a supernova in 1572, and Kepler saw one in 1604. Both of these objects now appear as radio shells looking very much like . Their diameters are about 10 pc – larger than Gas A – but they are considerably fainter than it. The most famous supernova remnant, the Crab Nebula, does not have a shell structure and is, in many ways, different from almost all the other SNRs in the Galaxy. Since it is the best studied of all the SNRs, it is worth devoting the whole of the next section to a description of it.

Comments are closed.