Febhuart 1, 1896.] 



KNOWLEDGE. 



39 



nebula has no slight luminosity, and the extension of its 

 spectrum far into the ultra-violet points to a considerable 

 elevation of temperature. Yet, on the other hand, the 

 presence of the yellow line of helium would indicate that, 

 in those regions which give this line, the gas is at a far 

 higher pressure than that just indicated. Lastly (and 

 perhaps the most difficult feature of all), whilst we should 

 expect a freely expanding gas to diffuse itself equally 

 and indefinitely in all directions, we find nebuhe taking 

 strange and complicate shapes, and showing here and there 

 strongly-marked outlines. 



If we think of nebulse as merely vast extensions of rarefied 

 gas, it is exceedingly diiiicult to understand this last-named 

 peculiarity. To take the Orion nebula, for example: how can 

 we explain the sharpness which some of its brightest edges 

 show — the Fish's Month, the Great Proboscis : how regard 

 its delicate complexity of detail, if we think of it as merely 

 a vast mass of feebly glowing gas '.' Look for a moment 

 at Dr. Eoberts's striking photograph of Messier 78 in the 

 November number of Knowledge. Can it be supposed 

 that this is a purely gaseous object, with so well-defined 

 and yet so irregular a margin as that on the n.p. side '? 

 Or that the curious conical dark area shown in Dr. 

 Roberts's present plate of the nebula near 15 Monocerotis 

 is formed by the retreat from it, through some unexplained 

 force, of the glowing gases which are in such evidence 

 surrounding it ? But if we follow out the idea already 

 suggested, that there are in sidereal space systems wherein 

 the arrangement of matter differs from that in the solar 

 system in two directions — first, instead of being concen- 

 trated into one sun it is distributed amongst many ; and, 

 next, instead of the chief bulk of each of these suns lying 

 below the photosphere, a disproportionate amount exists 

 in the form of chromosphere and corona — it is easy 

 to see that an appearance might be created not different 

 from that which we recognize in many nebulfe. 



The aspect of such a system, as viewed from our stand- 

 point, would vary according to the arrangement of the 

 stars, and the relative importance of the actual stars them- 

 selves and of their appendages. The closer the stars were 

 together the greater would be the tendency for the 

 hydrogen and coronal streamers to be drawn out to an 

 enormously exaggerated extent, and these might, in the 

 mass, be more apparent to us than the stars themselves ; 

 whilst the character of the spectrum of such a group, whether 

 continuous or gaseous, would depend on the relative 

 brightness and surface of the stellar photospheres and of 

 their prominences. In other words, whether it appeared 

 to us as a star-cluster or a nebula would depend largely upon 

 whether its substance was aggregated into comparatively 

 few bodies or distributed over a vast number of inferior size. 



If we imagined such a transformation to take place in 

 our own system, the sun being degraded to the rank of a 

 eelf-luminous Jupiter and the various planets raised to 

 the rank of miniature suns, all with extended chromo- 

 spheres and coronffi, and we were to view the whole 

 from a great distance, it would appear to us as a spiral 

 nebula ; irregular and broken, it may be, but still 

 approximating to the spiral form. 



The example of Saturn's rings, where we have a vast 

 number of small bodies so evenly distributed as to appear 

 like a series of solid concentric rings, and the usual 

 diagrams of the solar system, may suggest that a similar 

 target-like appearance would result. But this would not 

 be the case unless the subdivision were carried to the same 

 extreme extent as in the Saturnian annuli. However 

 complicated the orbits of the various little suns might be, 

 each body would only occupy one part of its orbit at any 

 given time, and there would be no other bodies, except by 



accident, to mark out the rest of its course. At any given time 

 the distribution of these sunlets would be unsymmetrical ; 

 but the general tendency, however irregular and broken their 

 arrangement, would usually be towards the spiral form. 



Such an object as the great spiral nebula in Canes 

 Venatici need not, therefore, be looked upon as rotating 

 gases, subject to no control but that of the general mass. 

 It is difficult, indeed, to see how it could be conceived as 

 such. But the gases which make their presence evident 

 in it are probably under the control of a great number of 

 somewhat small suns, which form the bright knots that 

 trace out its remarkable spirals. They form, in effect, the 

 chromospheres of these little orbs. 



We are not to suppose that these are true stellar 

 atmospheres. As I pointed out in the case of the sun, 

 the molecules of hydrogen and helium in the prominence 

 region round the sun are probably moving in free paths, 

 and do not build up an atmosphere in which one layer 

 presses on the layer below it. In nebulas of the kind 

 referred to, but giving a continuous spectrum, we probably 

 have cases where the chromospheric element is less 

 important than the coronal. It is noteworthy — it is a 

 point on which Mr. Ranyard insisted again and again — 

 that the nearest approach to the coronal forms is to be 

 found in the study of nebular structure. 



One great objection to the foregoing suggestions lies in 

 the peculiarity of the nebular spectrum. If the sim showed 

 the two lines at wave-lengths 5007 and 4059 the difficulty 

 would be solved. But though the sun fails to give us 

 these two typical lines, yet the balance of evidence appears 

 to show that these are just the lines which yova give to 

 us in their second or fainter aspect. At their first out- 

 burst we get a spectrum which is practically chromospheric : 

 later, when they have faded down, a spectrum which is 

 practically nebular. It may well be that, could we examine 

 the spectrum of the outermost regions of the corona under 

 favourable conditions, we might find these two enigmatical 

 lines ; but in the meantime Nova Cygni and Nova Aurig* 

 are sufficient to form a connecting link between bodies at 

 such opposite ends of the series as our densely concen- 

 trated sun and the indefinitely diffused nebula'. 



To sum up, I would wish to urge that our best and 

 safest way to understand the nature of the sidereal 

 structures is to argue from the one system which is 

 sufficiently near us to reveal something of its character — 

 that is to say, our own. But that, whilst we may 

 reasonably take its constitution as a type, just as the 

 structure of one vertebrate may be taken as typical of all, 

 we must be prepared to find the largest differences in the 

 scale upon which other systems are built, and in the 

 proportions which their several parts bear to each other. 

 And a system in which the total mass was distributed 

 amongst very many small members, and in which the 

 chromospheric and coronal element was in large excess of 

 the truly stellar, would undoubtedly appear to us as a 

 nebula. Whether there are nebube of an altogether 

 different typa is a question beyond my present purpose. 



-♦- - - 



NEBULA NEAR 15 MONOCEROTIS. 



By Isaac Roherts, D.Sc, F.R.S. 

 R.A. Gh. Sum., Decl. N. 10^ 0'. 



THE photograph covers the region between R.A. 

 Oh. 32m. 5(;-8s. and R.A. Gh. 37m. 53 8s.; Deol., 

 between !)' !l-6 and 10 lo-3' North. 

 Scale, 1 millimetre to twenty-four seconds of arc. 

 Co-ordinates of the Fiducial stars marked with 

 dots for the epoch a.d. 1900. 



• Knowledge, llaivh, ISO-t. 



