January 23, 1903.] 



SCIENCE. 



127 



able extent of such a nebula. To do this 

 requires that we know something of the dis- 

 tance of the star. This can not be deter- 

 mined by any absolute method, so that our 

 conclusions as to the distance must in part 

 be conjectural. Yet we can say with a 

 high degree of probability that the annual 

 parallax of these new stars can scarcely 

 be much greater than the thousandth of a 

 second. We have two independent bases 

 for this conclusion. 



One is that such stars have never blazed 

 forth except in the regions of the Milky 

 "Way. We are, therefore, justified in be- 

 lieving them as distant as the Milky Way. 

 Now one of the results of stellar statistics 

 which we need not stop to reason out at the 

 present time is that the distance of the 

 Milky Way can scarcely be much less than 

 that corresponding to the parallax I have 

 indicated. Even this distance falls far 

 short of the estimates of Sir William Her- 

 sehel, who is stated to have placed the 

 outermost visible stars of our system at a 

 distance which light would require many 

 thousand years to traverse. He supposed 

 us to see all the stars of the Millty Way 

 by pre-Adamite light. But the distance 

 which I have indicated is that over which 

 light would travel in about 3,400 years. 



The other argu"ment on the subject may 

 be briefly stated in this form. From what 

 we know of the thickness of the stars in our 

 immediate neighborhood, there is every rea- 

 son to believe that, out of several hundreds 

 of million of stare in the universe, not more 

 than twenty thousand are within the dis- 

 tance corresponding to a parallax of 0".02. 

 The chances are, therefore, more than ten 

 thousand to one that any star in the uni- 

 verse, taken at random, would lie within 

 this range of distance from us. 



Another reason for placing the Milky 

 Way, and with it the new stars, at this dis- 

 tance is found in t'^e absence of proper 

 motions from such stars. Most careful and 



refined measures made by Barnard on Nova 

 Persei show a motion of only 0".01 in the 

 course of a year, which is only saying that 

 no motion has been seen. Although this 

 result is not conclusive, it affords additional 

 very strong evidence in favor of the view 

 that this star was reallj^ in the region of the 

 Milky Way. 



Assuming, then, that the distance is of 

 this order of magnitude, let us ask at what 

 speed a nebula must rise in order that it 

 may expand as rapidly as observation seems 

 to show the matter around Nova Persei to 

 have flown outward. Calculations would 

 show this speed to beggar all our concep- 

 tions. The highest speed which matter has 

 been known to reach is that attained by the 

 eruption of hydrogen and other gases from 

 the sun, which sometimes amounts to sev- 

 eral hundred miles a second. But matter 

 moving only with such a speed as this 

 would require centuries to form a nebula 

 of appreciable size at the distance we have 

 assigned to the new stars. 



The application of this principle to the 

 case of Nova Persei led to an ingenious 

 suggestion by Kapteyn that the seemingly 

 sloAv expansion of the nebula which sur- 

 rounds Nova Persei was not a motion of 

 matter at all, but only an illumination of 

 nebulous matter already existing by the 

 wave of light thrown out from the ex- 

 ploded star. At first sight the reply to 

 this suggestion might be that the observed 

 expansion can not come up to light in 

 speed. One might be astonished to hear 

 that, inconceivably swift as is the motion 

 of light, it might be well that, at such a 

 distance, it would seem to us as slow as 

 the apparent expansion of the nebula in 

 question. But when we put the matter 

 into cold figures, we find that the great 

 difficulty in the way of accepting Kap- 

 teyn 's explanation is the opposite of this. 

 "V\Tiat we have to deal with is not the ap- 

 parent slowness of the motion, but the in- 



