14. EXPLODING STARS

It is now known that a significant part of the considerable radio energy which impinges upon the earth from outer space is due to emission from supernovae' remnants, the violently expanding gaseous shells of long-since exploded stars. Since the last recorded supernova in our Galaxy was Kepler's Star. of 1604, and since supernova activity declines quite rapidly after the initial cataclysmic phase, the full impact of these titanic stellar explosions is quite unknown; but a few simple calculations show that the effects could be serious, even if the outburst occurred at hundreds of millions of times the distance of the Sun.

Novae are variable stars which differ from other types of variable star in that the outburst is non-recurrent, or occurs only at very long intervals. For reasons which are only now becoming understood by astrophysicists, a tremendous explosion occurs: within a few hours the brightness of the star increases by a factor of 10,000 or 100,000 (ten to twelve magnitudes), and simultaneously a gaseous shell is ejected with a velocity of the order of 1,000 miles per second. The power output from a nova at the peak of the explosion, is of the order of 10³¹watts, or about 100,000 times the power output of the Sun. Supernovae occur less often than novae, and although the observed phenomena are similar, the awesome explosion is of enormously greater violence. The peak power is of the same order as the total power output from a galaxy of one hundred thousand million stars. The velocity with which the gaseous shell is expelled is usually of the order of 2,000 to 3,000 miles per second; so high that much of the ejected material escapes from the gravitational field of the core and continues to rush outwards indefinitely. Astrophysicists consider the supernova explosion to be the result of the sudden collapse of a supergiant star which has exhausted its supplies of hydrogen and can no longer maintain its internal radiation pressure by means of the energy released from the fusion of hydrogen isotopes to form alpha particles. The star continues to radiate energy from its blue-hot surface, and therefore it shrinks under the pull of its own gravitational field.

Within the last five years the Academia Sinica, Peking, has made a complete survey of historical Chinese and Japanese documents (which go back much farther than any European records of astronomical events) and has published a new catalogue of 90 novae and supernovae observed up to the end of the seventeenth century. As a result of this exhaustive study the Academica Sinica came to the conclusion that the frequency of occurrence of supernovae within the Milky Way system is about one every 150 years.

A new supernova, a real "radio star", arising in the solar neighbourhood could have disastrous, though temporary and intermittent, effects upon electrical communications. Broadcasting services would be affected to a more or less severe extent depending upon the wavelength and bandwidth of the transmission. In general, sound transmission would not be badly affected except at very short wavelengths, but television transmissions could be completely obliterated.

(from Discovery, August 1961)