April 19, 1888] 



NATURE 



589 



in some of the "stars" to which reference has been 

 made, the analogy on which they were based was 

 founded on solar phenomena ; the " stars " in question 

 being supposed to be represented in structure by our 

 central luminary. The main constituent of the solar 

 atmosphere outside the photosphere is hydrogen, and it 

 was precisely this substance which was chiefly revealed by 

 these stellar observations and in the Novas, in which cases 

 it was sometimes predominant. A tremendous develop- 

 ment of an atmosphere like that of the sun seemed to 

 supply the explanation of the phenomena. 



Acting on this view in 1878,1 I attempted to catch these 

 chromospheric lines in a Lyrae, abandoning the use of a 

 cylindrical lens in front of the slit with this object in 

 view. 



Further, it was quite clear that if such gigantic supra- 

 photospheric atmospheres existed, their bright lines 

 might much modify their real absorption-spectra ; even 

 "worlds without hydrogen" might be thus explained 

 without supposing a histts 7iatiira, and so I explained 

 them. 



That this view is untenable, as I now believe, and that 

 it is unnecessary, will, I think, be seen from what follows. 

 A long series of newly described phenomena, which 

 are absolutely incomprehensible while it is applied to 

 them, find, 1 think, a simple and sufficient explanation. 

 I must hold that the view is untenable, because how a 

 body constituted in any way like the sun could change 

 its magnitude from the thirteenth to the sixth every 

 year or so, or change its hydrogen lines from bright to 

 dark once a week, passes comprehension ; and the more 

 closely a " star " resembles the sun the less likely are 

 such changes to happen. Even the minor evolutionary 

 changes are inexplicable on this hypothesis, chiefly be- 

 cause in a completely condensed mass the temperature 

 must be very high and constant, while I have shown that 

 the spectrocopic phenomena are those of a specially low 

 temperature ; and I may now add that many of the ob- 

 jects are extremely variable in the quantity and quality 

 of the light they emit. 



Another cause of the appearance of the hydrogen lines 

 has been suggested by Mr. Johnstone Stoney (Proc. Roy. 

 Soc, vol. xvii. p. 54). He considers it due to the clash- 

 ing together of the atmospheres of two stars, the outer 

 constituent of the atmosphere — hydrogen — alone being 

 raised by the friction to brilliant incandescence. 



Another objection we can urge against the old view 

 is that all bodies in the universe cannot be finished 

 suns in the ordinary sense, and that it leaves out of ac- 

 count all possible processes of manufacture, not only of 

 single stars, but of double and multiple systems, at all 

 stages between nebula and sun ; while the new one, by 

 simply changing the unit from the star to each individual 

 constituent, it is hardly too much to say, explains every- 

 thing, though it is perfectly true that in some of the 

 steps a considerable acquaintance with spectroscopic 

 phenomena is necessary to realize the beauty and the 

 stringency of the solutions. 



' ". . . The sun which we see, the sun vhich sends us the majority 

 of the light we receive, is but a small kernel in a gigantic nut, so that 

 the diameter of the real sun may be, say, two million miles. Suppose 

 then that s me stars have very large coronal atmospheres ; if the area of 

 the coronal atmosphere is small compared with the area of the section of the 

 true disk of the .sun, ofcour.se we shall get an ordinary spectrum of the star ; 

 that is to say, we shall pet the indications of absorption which make us class 

 the stars apart ; we shall get a continuous spectrum barred by dark lines. 

 ISiit suppose that 'he area of the coronal atmosphere is something very con- 

 siderable indeed, let us .assume that it has an area, say fifty times greater 

 than the section of the kernel of the star itself; now, although each unit of 

 surface of that coronal atmosphere may be much less luminous than an equal 

 unit of surface of the true star at the centre, yet, if the area be very large, 

 the spectroscopic writing of that large area will become visible side by side 

 with the dark lines due to the brilliant reg'on in the centre where we can 

 study absorption ; other lines (bright ones) proceeding from the exterior 

 portion of that star will be visible in the spectrum of the apparent point we 

 call a star. Now it is difficult to say whether such a body as that is a star or 

 a nebula. We may look upon it as a nebula in a certain staga of conden- 

 sation ; we may look upon it as a star at a certain stage of growth." — Proc, 

 R.S. 1878, No. 185, p. 49. 



The Question of Hydrogen in the Case of Bright-Line 

 Stars. 



It may be convenient also that I should summarize the 

 various conditions under which the lines of hydrogen are 

 observed in the meteorite swarms we are now considering. 



In the "nebul?e" we begin with the widest interspaces. 

 Future investigation may show that, as I have suggested, 

 those in which the hydrogen lines are absent are the most 

 widely spaced of all. Be this as it may, it is a matter of 

 common knowledge that with the brighter nebulae, such 

 as that of Orion, to take an instance, we have hydrogen 

 associated with the low-temperature radiation of olivine. 

 That the hydrogen is electrically excited to produce this 

 glow is proved by the fact that the temperature of the 

 meteorites themselves must be very low ; otherwise the 

 magnesium would not show itself without the manganese 

 and iron constituents, and the continuous spectrum would 

 be much brighter and longer than it is. 



In the former paper I showed that in my laboratory 

 experiments, when the pressure was slightly increased in 

 a tube containing gases obtained from meteorites, the 

 carbon bands began to be visible. We should expect this 

 to happen therefore in a meteor swarm at some point at 

 which the mean interstitial space was smaller than that ac- 

 companied by the appearance of the hydrogen lines ; and it 

 would be natural that both should be seen together at an 

 early stage and both feeble, by which I mean not strongly 

 developed, as hydrogen is not strongly developed even 

 in the nebula of Orion, none of the ultra-violet lines being 

 visible in a photograph, while the magnesium line is. 



The association of the lov/-temperature lines of hydro- 

 gen with the flutings of carbon is therefore to be expected, 

 and I shall subsequently show that we have such an asso- 

 ciation in the so-called bright-line stars ; and even at a 

 further stage of development, in stars like a Orionis, the 

 hydrogen is still associated with the carbon. 



The Cometic Nature of Stars with Bright Lines in their 

 Spectra. 



Seeing that the hypothesis I am working on demands 

 that the luminosity in stars and the bright lines in their 

 spectra are produced by the collisions of meteorites, the 

 spectra of those bodies must in part resemble those of 

 comets, in which bodies by common consent the luminosity 

 is now acknowledged to be produced by collisions of 

 meteorites. 



We must, however, first consider the vast difference in 

 the way in which the phenomena of distant and near 

 meteoric groups are necessarily presented to us ; and, 

 further, we must bear in mind that in the case of comets, 

 however it may arise, there is an action which drives the 

 vapours produced by impacts outward from the swarm in 

 a direction opposite to that of the sun. 



It must be a very small comet which, when examined 

 spectroscopically in the usual manner, does not in con- 

 sequence of the size of the image on the slit enable us to 

 differentiate between the spectra of the nucleus and 

 envelopes. The spectrum of the latter is usually so 

 obvious, and the importance of observing it so great, 

 that the details of the continuous spectrum of the nucleus, 

 however bright it may be, are almost overlooked. 



A moment's consideration, however, will show that if 

 the same comet were so far away that its whole image 

 would be reduced to a point on the slit-plate of the instru- 

 ment, the differentiation of the spectra would be lost ; we 

 should have an integrated spectrum in which the brightest 

 edges of the carbon bands, or some of them, would or 

 would not be seen superposed on a continuous spectrum. 



The conditions of observations of comets and stars 

 being so different, any comparison is really very difficult ; 

 but the best way of proceeding is to begin with the 

 spectrum of comets in which, in most cases, for the reason 

 given, the phenomena are much more easily and accurately 

 recorded. 



