4i8 



NATURE 



[August 28, 1890 



idea 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 dealing with had 

 been en evidence. We know that some of them at their 

 minimum put on a special appearance of their own in 

 that haziness to which I have before referred as having 

 been observed by Mr. Hind. My researches show that 

 they are all 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 radia- 

 tions 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 conflagration 

 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 recurrent action. 

 It was understood that the body conflagrated 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 naturally 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 both 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 at first the meteorites 

 are too sparse for many collisions to occur, and that, 

 finally, the outliers of the major swarm are drawn 

 within the orbit of the smaller one, so that it passes 

 clear. The tables, which shall be given hereafter, show 

 that this view is entirely consistent with the facts ob- 

 served, for the greater number of instances of variability 

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

 mean spacing seems probable. 



I propose here to consider the suggested cause of 

 variability somewhat in detail. I will begin with Groups 

 I. and II. 



In these groups the variability is produced by the re- 

 volution of one or more smaller swarms round a central 

 swarm, the maximum luminosity occurring at periastron, 

 when the revolving swarms are most involved in the 

 central one. 



According to the theory, the normal condition is that 

 which exists at minimum, and in this respect it resembles 

 that suggested by Newton— namely, that the increase of 

 luminosity at maximum was caused by the appulse of 

 comets. All other theories take the maximum as the 

 normal condition, and the minimum as a reduction of the 

 light by some cause— large proportion of spotted surface, 

 or what not. 



Anything which in the normal minimum condition of 

 light-equilibrium will increase the amount of incandescent 

 gas and vapour in the interspaces will bring about the 

 appearance of the hydrogen lines and carbon flutings as 

 bright ones. The thing above all things most capable of 

 doing this in a most transcendental fashion is the invasion 

 of one part of the swarm by another one moving with a 

 high velocity. This is exactly what I postulate. The 

 wonderful thing under these circumstances then would be 

 that bright hydrogen and carbon should not become more 

 luminous, not only in bright-line stars, but in those the 

 spectrum of which consists of mixed flutings, bright 

 carbon representing the radiation. 



We may consider three cases of revolution. Taking 

 that first in order which will give us the greatest light 

 range, we find that this obviously will occur in those 



NO. 1087, VOL. 42] 



systems in which the orbits are most elliptic and the 

 periastric distances least. 



On the other hand, a mean ellipticity will give us a 

 mean range. 



In these two cases, 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 condition of things in which the orbit is so nearly 

 circular that it is almost entirely involved in a larger 

 swarm. Under these conditions, collisions would occur 

 in every part of the orbit, and they would only be more 

 numerous at 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 small 

 number — in which the variation of light is very far below 

 the normal range, one or two magnitudes instead of six or 

 seven. 



Now it is at once obvious that we should get more 

 variability in these early groups than in any of the more 

 condensed ones, for the reason that in the latter we 

 require the conditions either that the plane of revolution 

 should pass through the earth, or that the light of the 

 central star shall be relatively dim. 



This point is best studied in relation to Group II. 



The total number of stars included in Argelander's 

 Catalogue, which deals generally with stars down to the 

 ninth magnitude, but in which, however, are many stars 

 between the ninth and tenth, is 324,118. The most com- 

 plete catalogue of variables (without distinction) that we 

 have has been compiled by Mr. Gore, and published in 

 the Proceedings of the Royal Irish Academy (Series II., 

 vol. iv.. No. 2, July 1884, pp. 150-163). I find 191 known 

 variables are given ; of these 1 1 1 are in the northern 

 hemisphere and 80 in the southern hemisphere. 



In the catalogue of sicspected variable stars given in 

 No. 3 of the same volume (January 1885, pp. 271-310), I 

 find 736 stars, of which 381 are in the northern and 355 

 in the southern hemisphere. Taking, then, those in the 

 northern hemisphere, both known and suspected, we have 

 the number 492. We have, then, as a rough estimate 

 for the northern heavens one variable to 659 stars taken 

 generally. 



The number of objects of Group II. observed by 

 Duner, and recorded in his admirable memoir, is 297 ; of 

 these 44 are variable. So that here we pass from i in 

 657 to I in 7. Of the great development of variability 

 conditions in this group then there can be, therefore, no 

 question. 



Further, while by the hypothesis there is no limit to the 

 increase of luminosity, the variability presented by these 

 objects is remarkable for its great range. The light may 

 be stated in most general terms to vary about six magni- 

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

 fair average ; sometimes a difference of eight magnitudes 

 has been observed ; the small number of cases with a 

 smaller variation 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 ; a variation of eight magnitudes means that 

 it is 1600 times brighter at maximum than minimum. 



These values alone would indicate a condition of things 

 in which the minimum represents the constant condition, 

 and the maximum, one imposed by some cause which 

 produces an excess of light. These various conditions 

 having been premised in considering these groups, I will 

 first deal with the nebulas. 



That many of the nebulcC are variable is well known, 

 though, so far as I am aware, there are no complete 

 records of the spectroscopic result of the variability. But 

 bearing in mind that in some of these bodies, such as the 

 Dumb-bell Nebula, we have the olivine line almostby itself ; 

 and in others, which are usually brighter, we have the 

 lines of hydrogen intensified, as in Orion ; and in others. 



