THE GREAT STAR MAP 563 



nebulous appearance was expanding in all directions, just as 

 though there had been an explosion and the fragments were 

 still flying outwards. The phenomenon aroused the greatest 

 possible interest, for a rapid change — that is to say, any change 

 which is perceptible in a few weeks — is almost unprecedented 

 in the case of the stars and could have only one of two explana- 

 tions : either the star is specially close to us so that the 

 changes appear larger than usual or, if the star be at a distance 

 similar to those of other stars, the changes themselves must be 

 on a gigantic scale. It was soon seen that the latter was the 

 right alternative and it was inferred that the velocities of the 

 flying fragments must be comparable with the velocity of light 

 (nearly 200,000 miles a second). 



Now there is an interesting physical question, whether it 

 is possible for gross matter to move through the ether with 

 a velocity greater than or even as great as that of light. At 

 first the hope was entertained that we were going to get some 

 information on this interesting question but a more practical 

 alternative was suggested, viz. that the velocity exhibited was 

 not that of matter but actually that of light itself The observed 

 facts would be explained if the nebula had been in existence 

 previously but had been without illumination, so that we were 

 unaware of it ; just as we are unaware of an object in a dark 

 room until a flash of lightning illuminates the room. In such a 

 case the illumination appears to be instantaneous, but since light 

 does actually take time to travel, it cannot be quite instantaneous, 

 which we should realise were the room billions of miles in size. 

 The room taken up by a nebula is of this size and the flare-up of 

 the new star therefore illuminated it gradually, beginning with 

 the nearer portions and spreading to those more distant as time 

 went on. This explanation of the facts was confirmed by a 

 remarkable experiment. The light of the nebula was analysed 

 by means of the spectroscope and found to correspond with that 

 of the original flare. A spectrum is, after all, only a glorified 

 name for a colour ; we may represent the facts in simple 

 language by substituting names of colours. The events would 

 then be as follows : the star rose to its greatest brightness with 

 a blue light, which afterwards turned to red and remained red 

 as the light died away. Now the light of the nebula was not 

 found to be red, as it would have been if it belonged to the star 

 in its later stages, but was found to be blue, and must therefore 



