108 SESSION I. DISCUSSION 



residues of a supernova which burst out several thousand years ago [5]. In the Crab 

 nebula [6], the mean concentration of relativistic electrons with energies li > 10'' eV, 

 N{E > 10-') '^3 10 " cm ■'. It might be expected that the concentration of relativistic 

 protons would be of the same order. When the radius of the Crab nebula in its expan- 

 sion reaches 5 parsecs the concentration of relativistic particles will reach 3 ■ 10"'' cm~^, 

 i.e., it will still be about 30 times as great as the concentration of primary' cosmic rays in 

 the neighbourhood of the Earth. 



The question arises as to whether the Sun, moving within the Galaxy, w ith the planets 

 revolving round it, might have entered a region in which the abundance of cosmic rays 

 was tens, or even hundreds of times greater than it is now. We can answer this question 

 in the affirmative. It happened when supernovae burst out in the immediate vicinity of 

 the Sun. 



The frequency with which Supernovae burst out in the Galaxy was, until recently, 

 estimated by means of indirect evidence from other galaxies. It has been shown [6] that 

 this does not give a correct estimate, such outbursts being of at least one order higher in 

 our own Galaxy. In this respect our Galaxy resembles the galaxies NGC 3184, 6946 

 and 4321, in which the frequencies of outbursts of supernovae are anomalously large. 

 In fact, during the past thousand years, at least 5 outbursts of supernovae have been 

 observed in our Galaxy (in 1006, 1054, 1572, 1604 and 1843); the last being A^ora Caritiae 

 [7]. These were all very bright so they cannot have been more than 2000-2500 parsecs 

 away, especially if we allow for interstellar absorption of Light. Hence it follows that 

 about once in every thousand years there occurs an outburst of a supernova at a distance 

 of not more than 1000 parsecs from us. Assuming that the supernovae form a very flat- 

 tened system within the Galaxy, being 100 parsecs thick, we find that in each 500 million 

 years a supernova will burst out at a distance of less than 5 parsecs from the Sun. This 

 means that during the existence of the Earth stars near to the Sun have burst out into 

 supernovae on several occasions (about 10). 



What sort of things could have happened under such unusual circumstances? The 

 brightness of the stars bursting could hardly exceed —20™, i.e. the amount of radiation 

 reaching the Earth from the star was 1000 times less than that from the Sun. That, how- 

 ever, does not necessarily mean that the amount of hard radiation (e.g., X-rays) reaching 

 the Earth from the star and from the nebula in the very earliest stage of its development, 

 could not be considerably greater than that from the Sun. This could have had serious 

 consequences (see below). The gaseous envelope formed by the flare-up of the supernova, 

 which was expanding at 10^ cm/sec, passed through the solar system. However, con- 

 sidering the negligible density of the gas in this envelope (about io~'-'- g cm^) this would 

 hardly have left any traces. 



What is important is that at such times the amount of cosmic radiation reaching the 

 Earth must have been several tens of times greater than the normal value for a period 

 of some thousands of years. It should also be noted that the relativistic particles in the 

 expanding nebula were very irregularly distributed. Thus there might be periods, lasting 

 many hundreds of years when the amount of cosmic radiation reaching the Earth was 

 hundreds of times what it is now. 



This could well have entailed serious biological, and above all, genetical consequences. 

 Until recently it was held that the process of evolution of living things was brought 

 about by the influence of natural selection and the various physical conditions of the 

 surrounding medium. Among these, however, no consideration has ever been given to 

 the important factor of the general level of hard radiations. On the strength of what has 

 just been said, we must recognize that, at some stages of evolution, this latter factor must 

 have been of great, if not of decisive significance. 



According to the evidence available (e.g., 8), the amount of irradiation due to cosmic 

 rays in the lowest layer of the atmosphere is now 004 r yr, i.e. about one-third of the 

 general level of natural radiation. Nevertheless, an increase in the dose of radiation to as 

 little as twice the normal level may have serious genetical consequences [8]. Hence it is 

 clear that if the intensity of cosmic radiation were increased ten- to a hundredfold for 

 thousands of years, this might have catastrophic results for many forms of animals and 

 plants. For example, one may put forward the hypothesis that the reptiles of the Mesozoic 

 age might have become extinct within a relatively short period for this reason. On the 

 other hand, speaking generally, it is quite possible that for other forms, this factor might 

 have favoured their further evolution. 



