Apart from the handful of remnants in our Galaxy which can be identified with particular explosive events, the ages of Supernova remnants must be determined by indirect means. In Some Cases, such as cas A, this may be; done by comparing the diameter of the remnant with its expansion velocity; in general, however, one must calculate models of interstellar explosions in order to find a theoretical explanation for the variations that are seen among SNRs. We now believe that the, evolution of a supernova remnant takes place in several fairly distinct phases. The initial stage comprises the unimpeded expansion of the hot gas away from the central collapsed object. After about 100 years the original expansion velocity of-10-20000kms-1 will start to decrease as a result of the ram pressure of the surrounding interstellar medium, with the result that the interstellar gas swept up by the SNR is compressed into a shell. Hydrodynamic calculations show that it is instabilities in this shell that lead to the uneven ring of strong synchrotron radiation that is characteristic of remnants in the second stage of expansion, such as Gas A .The shell expands steadily over the next 20 000 years, cooling and slowing down as it does so. The third phase begins when the shell becomes cool enough (about 106K) that radiation by line emission becomes important, cooling the gas even more. The Cygnus Loop is near the be¬ginning of this stage, and is currently expanding at 100-200 kms-1.Finally, after several hundred thousand years the shell slows down to a velocity of a few kilometers a second, and loses its identity in the general interstellar medium.

The lifetimes of supernova remnants are therefore less than the lifetimes of even the shortest-lived main-sequence stars. There are however ,longer-term effects from supernovae. Pulsars are probably visible for about 107 years – long after the remnant has disappeared. The galactic spurs and the general galactic-background radio emission have their origins in supernova remnants, as do some, or perhaps all, of the galactic cosmic rays. Although the total rate of mass return to the interstellar medium is smaller for super–novae than for certain other types of evolved stars, the material in the supernova shells is unique in that it is enriched in the elements heavier than iron. As important as the mass is the energy that the SNRs put into the interstellar medium; the turbulence produced by the passage of shock waves may provide at least 10 per cent of the total kinetic energy of the interstellar gas. The very hot (105K) regions of interstellar gas discovered by ultraviolet satellites may also have their origins in supernova remnants.

With the estimates we now have of the ages of SNRs in our Galaxy we can calculate the rate at which supernova explosions must have been taking place in order to produce the number of remnants that are now seen. This calculation is very uncertain, but indicates that there is something like one supernova explosion somewhere in our Galaxy every 50-150 years. This number is broadly consistent with both the observations of supernovae in other galaxies and with the expected rate of star deaths in our own. The fact that only four of the five explosions in the last millennium have actually been seen from the Earth can be explained by the presence of interstellar extinction. The frequency of Galactic supernova explosions is just high enough to give every astronomer the hope that he will witness one during his own lifetime – prefer¬ably on a night when he himself is at a telescope!

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