LIFE IN SPACE AND TIME 29 



snuiU IVaction ol" that which, barring accidents, it may in 

 future enjoy. 



Only one type of physical catastrophe appears to threaten 

 any premature termination of this long forecast. Every 

 now and then a star, which, so far as we can tell, has pre- 

 viously been normal, suddenly blazes out, and in a few 

 days becomes ten thousand times brighter than the Sun, or 

 more, appearing, even to the naked eye, as a conspicuous 

 "new star," or nova. Within a few weeks the brightness 

 fades, and in a decade or two it is back almost where it was 

 before. Such an outburst by the Sun would unquestionably 

 destroy all earthly Hfe, except possibly that in the cold waters 

 of the deep sea, and even this would probably soon perish 

 for lack of food; and we may be sure that none has occurred 

 since Pre-Cambrian times. 



Novae, however, are by no means rare phenomena; 

 one is seen every year or two, and many more must escape 

 discovery. Allowing one nova per year, which is too few, 

 and ten biUions of stars, visible and invisible (which is 

 a great many) to share the risks of catastrophe, we find 

 that an average star is likely to blow up once in ten billion 

 years, at the longest. An interval ten times shorter would 

 be better in accordance with the evidence. 



We do not know, however, whether such calamities 

 happen to stars of all sorts, or only to those of certain 

 special characteristics, nor, in the latter case, whether 

 the Sun belongs to a susceptible species. In any case, the 

 possibility is remote enough to be eliminated from practical 

 human consideration. 



THE POSSIBLE EXISTENCE OF LIFE IN UNKNOWN WORLDS 



Whether other stars, as well as the Sun, have attendant 

 planetary systems is not a question which can be settled 

 by observation, unless our present powers of investigation 

 should be increased to an extent which now appears improb- 

 able. To assess the probability that such systems may 

 exist is not, however, beyond the sphere of legitimate 

 inference. 



If planetary systems arise from close encounters between 

 stars, the chance of their birth is capable of calculation. 



