1903.] on the New Star in Gemini. 383 



helium and other gases which had been absorbed in the star, on this 

 accession of temperature, are blown out of it in all directions with 

 a velocity of 1000 or 2000 miles per second. The supply of them 

 within the star is sufficient to keep up this eruption for months ;' and 

 the velocity of the outer layers of gas, which might tend to suffer 

 diminution from expansion or gravitational attraction, is maintained 

 by the pressure of the new gas just issuing from within, until a huge 

 volume of gas is produced which nevertheless at that immense dis- 

 tance would only subtend a few seconds of arc to our view. The 

 eruption is moreover so violent that it rapidly disperses the whole 

 materials of the star, the light dies down, and the mass of gas is left 

 in that condition, not yet precisely understood, which we call a 

 planetary nebula. To this must be added that the fierce light of the 

 original outburst in some way illuminates the vast dark nebula 

 which caused the catastrophe. Of the explanation thus sketched it 

 may I think be said that there is no fact known as yet which is 

 clearly irreconcilable with it. I cannot defend it in detail here for 

 lack of time, and can only refer to one or two points of the general 

 question which have not yet been noticed. 



Firstly no special peculiarity has been assigned to the star which, 

 wandering through space, meets the dark nebula ; and the evidence 

 collected both positively and negatively supports the view that the 

 star may be just an ordinary star like the vast majority of those we 

 know, including our own sun. When Nova Persei first blazed up, 

 and before it exploded, its spectrum was just that of an ordinary star. 



The name " new star " suggests that there was nothing seen in 

 the place before, and this is generally true, but it only means that 

 thero was nothing bright enough to attract attention. Prior to the 

 last 10 years there were no maps of stars fainter than those of magni- 

 tude 9 J ; so that all that could be said of the previous history of a 

 *' new star " was that it must have been at least of the tenth magni- 

 tude. But it may have been precisely similar to our sun nevertheless. 

 I drew your attention specially to the evidence that new stars were 

 at a great distance, say 100 times or even 1000 times the distance of 

 Sirius. A tenth magnitude star brought from 100 times the distance 

 of Sirius and placed alongside him would not be much inferior in 

 brightness, and if the distance were 1000 times as great would far 

 outshine him. 



Kecently we have learnt a little more of the possibilities of prior 

 existence from the magnificent records accumulated at Harvard ; but 

 even these only take us to magnitude twelve or thirteen. Nova 

 Persei may have been shining before the outburst in 1901 as a star 

 of the fourteenth magnitude and no one would have known it ; in 

 which case, if it had been brought up alongside Sirius, it would 

 have been easily visible to the naked eye. We must make our 

 surveys far more exhaustive before we can say with any certainty 

 that the bodies which blaze up as new stars were previously " dark," 

 or inferior in intrinsic lustre to other stare. In one case, that of 



