May 24, 1888] 



NATURE 



79 



SUGGESTIONS ON THE CLASSIFICATION OF 

 THE VARIOUS SPECIES OF HEAVENLY 

 BODIES. 1 



VI. 



ON THE CAUSE OF VARIATION IN THE LIGHT OF 

 BODIES OF GROUPS I. AND II. 



I. General Views on Variability. 



T N my former paper I referred to the collision of 

 -*- meteor-swarms as producing " new stars," and to the 

 periastron passage of one swarm through another as 

 producing the more or less regular variability observed 

 in the case of some stars of the class under consideration. 

 I propose now to consider this question of variability 

 at somewhat greater length, but only that part of it which 

 touches non- condensed swarms ; i.e. I shall for the pre- 

 sent leave the phenomena of new stars, and of those 

 whose variability is caused by eclipses, aside. 



It is not necessary that I should pause here to state at 

 length the causes of stellar variability which have been 

 suggested from time to time. It will suffice, perhaps, 

 that I should refer to one of the first suggestions which 

 we owe to Sir I. Newton, and to the last general discus- 

 sion of the matter, which we owe to Zollner (" Photo- 

 metrische Untersuchungen," 76 and JJ, p. 252). 



Newton ascribed that special class of variability, to 

 which I shall have most to refer in the sequel, as due to 

 the appulse of comets. 



" Sic etiam stellae fixae, quae paulatim expirant in lucem 

 et vapores, cometis in ipsas incidentibus refici possunt, et 

 novo alimento accerisce pro stellis novis haberi. Hujus 

 generis sunt Stellas fixae, quae subito apparent, et sub 

 initio quam maxime splendent, et subinde paulatim evan- 

 escunt. Talis fuit Stella in cathedra Cassiopeia? quam 

 Cornelius Gemma octavo Novembris 1572 lustrando 

 illam cceli partem nocte serena minime vidit ; at nocte 

 proxima (Novem. 9) vidit fixis omnibus splendidiorem, et 

 luce sua vix cedentem Veneri. Hanc Tycho Brahaeus 

 vidit undecimo ejusdem mensis ubi maxime splenduit ; 

 et ex eo tempore paulatim decrescentem et spatio men- 

 sium sexdecim evanescentem observavit " (" Principia," 

 p. 525, Glasgow, 1 871). 



With regard to another class of variables he makes a 

 suggestion which has generally been accepted since. 



" Sed fixae, quae per vices apparent et evanescunt, 

 quaeque paulatim crescunt, et luce sua fixas tertiae 

 magnitudinis vix unquam superant, videntur esse generis 

 alterius, et revolvendo partem lucidam et partem obscu- 

 ram per vices ostendere. Vapores autem, qui ex sole et 

 stellis fixis et caudis cometarum oriuntur, incidere 

 possunt per gravitatem suam in atmosphaeras planetarum 

 et ibi condensari et converti in aquam et spiritus 

 humidos, et subinde per lentum calorem in sales et 

 sulphura et tincturas et limum et lutum et argillam et 

 arenam et lapides et coralla et substantias alias terrestres 

 paulatim migrare." 



Zollner, in point of fact advancing very little beyond 

 the views advocated by Newton and Sir W. Herschel, 

 considers the main causes of variability to be as 

 follows. He lays the greatest stress upon an advanced 

 stage of cooling, and the consequent formation of scoriae 

 which float about on the molten mass. Those formed at 

 the poles are driven towards the equator by the centri- 

 fugal force, and by the increasing rapidity of rotation 

 they are compelled to deviate from their course. These 

 facts, and the meeting which takes place between the 

 molten matter, flowing in an opposite direction, influence 

 the form and position of the cold non-luminous matter, 

 and hence vary the rotational effects, and therefore the 



1 The Bakerian Lecture, delivered at the Royal Society on April 12, by 

 J. Norman Lockyer, F.R.S. Continued from p. 60. 



luminous or non-luminous appearance of the body to 

 distant observers. 



This general theory, however, does not exclude other 

 causes, such as, for instance, the sudden illumination of a 

 star by the heat produced by a collision of two dark 

 bodies, variability produced by the revolution of a dark 

 body, or by the passage of the light through nebulous 

 light-absorbing masses. 



If the views I have put forward arc true, the objects 

 now under consideration are those in the heavens which 

 are least condensed. la this point, then, they differ 

 essentially from all true stars like the sun. 



This fundamental difference of structure should be re- 

 vealed in the phenomena of variability ; that is to say, 

 the variability of the bodies we are now considering 

 should be different in kind as well as in degree from that 

 observed in bodies like the sun or a Lyrae, taken as 

 representing highly condensed types. There is also little 

 doubt, I think, that future research will show that, 

 when we get short-period variability in bodies like these, 

 we are really dealing with the variability of a close 

 companion. 



II. On the Variability in Group I. 



That many of the nebulae are variable is well known, 

 though so far as I am aware there are no complete re- 

 cords of the spectroscopic result of the variability. But 

 bearing in mind that in some of these bodies we have the 

 olivine line by itself, and in others, which are usually 

 brighter, we have the lines of hydrogen added, it does not 

 seem unreasonable to suppose that any increase of tem- 

 perature brought about by the increased number of col- 

 lisions should add the lines of hydrogen to a nebula in 

 which they were not previously visible. 



The explanation of the hydrogen in the variable stars 

 is not at first so obvious, but a little consideration will 

 show that this must happen if my theory be true. 



Since the stars with bright lines are, as I have attempted 

 to show, very akin to nebulae in their structure, we might, 

 reasoning by analogy, suppose that any marked variability 

 in their case also would be accompanied by the coming 

 out of the bright hydrogen lines. 



This is really exactly what happens both in /3 Lyrae 

 and in y Cassiopeiae. In j!i Lyrae the appearance of the 

 lines of hydrogen has a period of between six and seven 

 days, and in y Cassiopeiae they appear from time to time, 

 although the period has not yet been determined. 



III. On the Variability in Group II. 



This same kind of variability takes place in stars with 

 the bright flutings of carbon indicated in their spectra, 

 o Ceti being a marvellous case in point. In a Orionis, 

 one of the most highly-developed of these stars, the 

 hydrogen lines are invisible ; the simple and sufficient 

 explanation of this being that, as I have already sug- 

 gested, the bright lines from the interspaces now at 

 their minimum and containing vapours at a very high 

 temperature — teste the line-absorption spectrum now be- 

 ginning to replace the flutings— balance the absorption of 

 the meteoritic nuclei. 



Anything which in this 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 as bright ones. The thing above all 

 things most capable of doing this in a most transcend- 

 ental 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 should 

 not add itself to the bright carbon, not only in bright- 

 line stars, but in those the spectrum of which consists of 

 mixed flutings, bright carbon representing the radiation. 



