I3 PLEASANT WA YS IN SCIENCE. 



nebulae just described are bodies which once were suns, but 

 whose crust has now become non-luminous, while around 

 the crust masses of gas shine with a faint luminosity, so 

 are we precluded from believing that this smaller membei 

 of the same family is in that condition. 



It is conceivable (and the possibility must be taken into 

 account in any attempt to interpret the phenomena of the 

 new star) that when shining as a star, the new orb, so far as 

 this unusual lustre was concerned, was of sunlike dimensions. 

 For we cannot tell whether the surface which gave the strong 

 light was less or greater than, or equal to, that which is now 

 shining with monochromatic light Very likely, if we had 

 been placed where we could have seen the full dimensions of 

 ihe planetary nebula as it at present exists, we should have 

 found only its nuclear part glowing suddenly with increased 

 lustre, which, after very rapidly attaining its maximum, 

 gradually died out again, leaving the nebula as it had been 

 before. But that the mass now shining with monochromatic 

 light is, I will not say enormously large, but of exceedingly 

 small mean density, so that it is enormously large compared 

 with the dimensions it would have if its entire substance 

 were compressed till it had the same mean density as our 

 own sun, must be regarded as, to all intents and purposes, 

 certain. 



We certainly have not here, then, the case of a sun 

 which has grown old and dead and dark save at the surface, 

 but within whose interior fire has still remained, only waiting 

 some disturbing cause to enable it for a while to rush forth. 

 If we could suppose that in such a case there could be such 

 changes as the spectroscope has indicated that the bright 

 lines of the gaseous outbursting matter would, during the 

 earlier period of the outburst, show on a bright continuous 

 background, due to the glowing lips of the opening through 

 which the matter had rushed, but later would shine alone, 

 becoming also fewer in number, till at last only one was left, 

 we should find ourselves confronted with the stupendous 

 difficulty that that single remaining line is the bright line of 



