1903.] on the New Star in Gemini. 379 



remarkable series of pliotograplis which materially strengthen the 

 case for the existence of such objects. 



Nova Persei, like all new stars, rose to maximum brightness very 

 suddenly and then faded away. The point which chiefly concerns 

 us is that the star was only very bright, i.e. brighter than the second 

 magnitude, for about a week. It remained visible to the naked eye 

 for about two months, and there was a secondary outburst on 

 March 26, but it is to the first week during which the star was 

 conspicuously bright that I wish to direct your attention, for this 

 great sudden flare had important consequences later in the star's 

 history. 



About six months after this flare, when the star had faded and 

 became invisible to the naked eye, photographs taken of it with 

 powerful instruments showed that it was surrounded by a nebula. 

 As successive pictures were taken it was seen that this nebula was 

 rapidly expanding in all directions, and the thought suggested was 

 that of a great explosion driving fragments outwards from the centre. 

 "In comparing the phenomenon with any terrestrial explosion, let us 

 say that of a rocket in mid air, allowance must of course be made for 

 great differences of space and time. The fiery sparks from a rocket 

 are scattered a few yards only within a second or two ; the fragments 

 of Nova Persei were being projected for millions of miles and the 

 time was months instead of seconds. How many millions of miles 

 we could not tell until we knew the distance from which we were 

 viewing this magnificent explosion ; and it presently became clear, 

 by the method of parallax, that the star must be at least 10 billions 

 of miles away, and might be at any greater distance. Thence it could 

 be readily inferred that the velocity of the fragments must be at 

 least 10,000 miles per second, and might have any value greater 

 than this. 



Now, although such a speed is too great, according to our 

 experience, for material bodies, there is a well-known speed which is 

 greater still — the velocity of I'ght, which is nearly 200,000 miles 

 per second. There was no reason y, hatever, why Nova Persei should 

 not be 200 billion miles away from us, in which case the observed 

 velocity of expansion would be that of light ; and now a new inter- 

 pretation of the facts was suggested, which perhaps an analogy from 

 our solar system will help to elucidate. 



The planets which circle round the Sun do not shine with light 

 of their own, but with light received from the Sun and merely 

 reflected by them ; and the distances of the planets from the Sun are 

 such that light, although travelling with enormous speed, takes an 

 appreciable time to reach them. Thus the light we receive on the 

 Earth takes eight minutes to reach us, and the light received by 

 Saturn, which is ten times as far away as we are, takes 80 minutes. 

 If the Sun were suddenly put out we should not know it for eight 

 minutes, since we should still receive light during that time which 

 Lad started on its journey before the extinction ; and Saturn would 



