82 



NATURE 



[May 24, 1888 



4. All bands markedly wide and strong. 



6. Bands thin and pale. 



A glance at the above tables will show that the kind of 

 variability presented by these objects is a very special one, 

 and is remarkable for its great range. The light may be 

 stated in themost general terms to vary about six magni- 

 tudes, from the sixth to the twelfth. This I think is a fair 

 average ; the small number of cases with a smaller varia- 

 tion I shall refer to afterwards. A variation of six 

 magnitudes means roughly that the variable at its 

 maximum is somewhere about 250 times brighter than at 

 its minimum. 1 



I have already indicated that, with regard to the various 

 origins of the variability of stars which have been sug- 

 gested, those which have been always most in vogue 

 consider the maximum luminosity of the star as the 

 normal one ; and indeed with regard to the Algol type of 

 stars of short periods, which obviously are not here in 

 question, there can be no reasonable doubt, that the 

 eclipse explanation is a valid one ; but in cases such as 

 we are now considering, when we may say that the 

 ordinary period is a year, this, explanation is as much out 

 of place on account of period, as are such suggested 

 causes as stellar rotation and varying amount of spotted 

 area on a stellar surface, on account of range. 



1 Obtained by the formula l m = (2\5i2)« . L w + „. For differences of 

 5, 6, 7 and 8 mag. we get 



\, m = ioo-o2 . L m + 5 



= 251-24 • L w+6 



= 631-11. L m + J 



= 1585-35. L w + 8 



1- m = light of a star if magnitude m. 



L //( _|_„= ,, ,, tt'majnitudes fainter 



We are driven, then, to consider a condition of things in 

 which the minimum represents the constant condition, and 

 the maximum a condition imposed by some cause which 

 produces an excess of light ; so far as I know the only 

 explanation on such a basis as this that has been 

 previously offered is the one we owe to Newton, who 

 suggested such stellar variability as that we are now con- 

 sidering was due to conflagrations brought about at the 

 maximum by the appulse of comets. 



How the Difficulty of Regular Variability on Newton's 

 View is got over in mine. 



It will have been noticed that the suggestion put forward 

 by myself is obviously very near akin to the one put forward 

 by Newton, and no doubt his would have been more 

 thoroughly considered than it has been hitherto, if for a 

 moment the true nature of the special class of bodies we 

 are now considering had been en Evidence. We know that 

 at their minimum they put on a special appearance of their 

 own in that haziness to which I have before referred as hav- 

 ing been observed by Mr. Hind. My researches show that 

 they are probably nebulous, if indeed they are not all of 

 them planetary nebulae in a further stage of condensation, 

 and such a disturbance as the one I have suggested would 

 be certain to be competent to increase the luminous 

 radiations of such a congeries to the extent indicated. 



Some writers have objected to Newton's hypothesis 

 on the ground that such a conflagration as he pictured 

 could not occur periodically, but this objection I imagine 

 chiefly depended upon the idea that the conflagra- 

 tion brought about by one impact of this kind would 

 be quite sufficient to destroy one or both bodies, and 

 thus put an end to any possibilities of rhythmically re- 

 current action. It was understood that the body con- 

 flagrated was solid like our earth. However valid this 

 objection might be as urged against Newton's view, it 

 cannot apply to mine, because in such a swarm as I have 

 suggested, an increase of light to the extent required might 

 easily be produced by the incandescence of a few hundred 

 tons of meteorites. 



I have already referred to the fact that the initial species 

 of the stars we are now considering have spectra almost 

 cometary, and this leads us to the view that we may have 

 among them in some cases swarms with double nuclei — 

 incipient double stars, a smaller swarm revolving round 

 the larger condensation, or rather round their common 

 centre of gravity. In such a condition of things as this, 

 it is obvious that, as before stated, in the swarms having 

 a mean condensation this action is the more likely to take 

 place, for the reason that the more the outliers of the 

 major swarm are drawn in, the more likely is the orbit of 

 the smaller one to pass clear. The tables show that this 

 view is entirely consistent with the facts observed, for 

 the greater number of instances of variability occur in 

 the case of those stars in which, on other grounds, mean 

 spacing seems probable. 



The Cases of Small Range. 



So far, to account for the greatest difference in 

 luminosity at periastron passage, we have supposed the 

 minor swarm to be only involved in the larger one during 

 a part of its revolution, but we can easily conceive a con- 

 dition of things in which its orbit is so nearly circular that 

 it is almost entirely involved in the larger swarm. Under 

 these conditions, collisions would occur in every part of 

 the orbit, and they would only be more numerous at 

 the periastron in the more condensed central part of the 

 swarm, and it is to this that I ascribe the origin of the 

 phenomena in those objects — a very small number — in 

 which the variation of light is very far below the normal 

 range, one or two magnitudes instead of six or seven. . Of 

 course, if we imagine two subsidiary swarms, the kind of 

 variability displayed by such objects as /3 Lyras is easily 

 explained. 



