THE ASTRONOMY OF THE INVISIBLE 381 



served that there were in the nebula well-defined knots and 

 streaks of condensation. These were estimated to be moving 

 away from the star at such a speed that, if it were as near as 

 alpha Centauri, would mean a velocity of more than two thou- 

 sand miles per second. If the distance is fifty or a hundred 

 times as great, the speed becomes comparable with that of light 

 itself. The most plausible explanation of the phenomenon 

 offered was that suggested by Kapteyn, that this motion of the 

 knots and streaks was merely apparent and not an actual rush 

 of masses of star matter. It represented simply a progressive 

 illumination of spiral streams of nebulosity, advancing outwards 

 with the speed of light. If this explanation is correct, the 

 distance of the star would be about three hundred light-years, 

 and its actual outburst occurred about the time that Columbus 

 was discovering America. It follows from this estimate that 

 the diameter of the resulting nebula was perhaps two or three 

 hundred times that of the distance of Neptune from the sun. 



If our sun had been the body or one of the bodies involved, 

 our earth would have been enveloped in a vast mass of nebulous 

 matter, probably hot, irradiating the entire sky and abolishing 

 night and day. Probably the heat would have been so great 

 that every vestige of life would have been instantly shrivelled 

 at the first onset of this flaming deluge ; the crust of the globe 

 itself might have been melted, and the earth have returned 

 to the primitive condition from which it sprang. 



It is by no means certain as yet that these stupendous out- 

 bursts really represent starry collisions. For a little time the 

 behaviour of their spectra seemed to negative this view. But 

 the researches of Ebert of Munich made clear that the per- 

 plexing contrariety of apparent motions was readily explicable 

 on the theory of anomalous refraction. On the other hand, it 

 is evident that the swift rush of a sun or dark body through 

 a dense mass of nebula would produce the same spectacle. 

 The number of nebulae we now know to be enormous. They 

 may be counted by the hundreds of thousands, and their extent, 

 even measured by sidereal standards, can only be regarded as 

 something monstrous. 



Both explanations may turn out to be true. A variety of 

 circumstances make it probable that actual collisions do take 

 place, and with such frequency as to be quite unaccountable 

 from any calculus of probability based upon our present ideas 



