A\ I TL RH 



[May 2, 1895 



A^ 



THE SUN'S PLACE /.V XATLKE} 

 V. 

 T the end of the last lecture »e arriveti at that point of our 

 inquiry which is connected with the possible first stage of 

 all ciisniicil bodies, and we saw thai there was a considerable 

 amount of evidence in favour of the idea that in this first stajje 

 all cosmical bodies are not masses of hot gas, but that their 

 temperature is low. 



Continuing this inquiry in (he light of the suggestion that 

 the first stage might Iw connected with swarms of meteorites, 

 we found the great proliability that, in swamis or streams of 

 meteorites, or m'eteoritic dust, we had to deal with the rea basis 

 of all cosmical Ixxlies. 



Now, if we take the he-avens as we find them, whether we 

 deal with stars, seeondar)' bodies, or satellites, we find that they 

 are all in movement, and it therefore follows that in these 



t t. . 2;, - T he <'.ri.;il Nclml.-i in Andr.tnit-tla, from ;i ['liuioKr.-ipU 1»> 

 I>r. Roberts. 



tariie^i ^i.igcv Hiih which we have now to ileal, whelher ihey 

 were meli iiritir swarms or streams, ihey were also in movement. 

 I have already laken an opjiortunily of [HMnling out l<i you how 

 very im|Xirtant these ronsideraliims are when we come to inquire 

 into the constiliiti'm of each nebula. I showed you in the last 

 lecture a licauliful photograph (Kig. 7, vol. li. p. 397), laken by Dr. 

 Roberts, of the spiral nelnila in one of our northern constellalicms, 

 and I now pro|».s< |.i shnw you another very similar toil, in or<ler, 

 if I can, to brin;; up. re r Ic^ily before you rerlain of the facls which 

 were then indii.iieil. In this wonderful photograph of the nebula 

 in Androme<la we are umloubledly dealing with streams, and the 

 movements towards the centre are all along spirals. In the case 



> Reviaed from nhorlli.ind notes of .t toiirsv of (ycctures to Working Men 

 .-u tbc Mu«cuin of Hracticiil (reoloKy (luriiiu XovemlHrT ;intl Ileeunilicr, 

 1^.94. iConlinuc<) from vol, li. paK^ S'^-'-) 



NO. 1331, VOL. 52] 



of the other nebula we were in a better ct>ndition for itbserving 

 the actual direction of motion because we were locking down on 

 the system, we got a section in llie plane c»f movement ; but wc 

 are looking at this nebula in an inclined direction, though I think 

 you will still h.ave no difficulty in seeing that the various streams 

 round the centre of coiulens;ilion are all of theni of a spiral 

 form, with certain condens;itions intcrs|iersed here and there 

 along them. 



We have a condensation in the prolongation of one of the 

 spirals, antl there is consitlemble clustering of apjxirenl stars 

 along those lines, which I ventured in my last lecture to call 

 stream lines, f^>r the reason that I was anxious to indicate that 

 we had in these appearances, not signs which toKl us of the 

 existence of matter — so that wheit you have not the appearances 

 you would be justified in supposing that there was no matter- 

 but an indication of movement in matter, so that we ma) 

 imagine that this nebula and others like it do probably consist 

 of a swarm of meteorites, extending enormously in |)ace 

 beyond the iiKlicalions which y»-iu see. fv)r the reason that 

 towards the centre the movements will be more violent than 

 they are towards the outside. We are tliere face to face with 

 the idea that we have to deal with orderly movements of 

 meteoritic masses. Now, let me call your attention to this 

 fact. If the movements are orderly, it means that the move- 

 ments of the constituent particles of the swarm, all of them, 

 or most of them, will be in the same tlirection ; that in that 

 case you have the condition of minimum disturbance, and 

 therefore the condition of minimum tem]ierature. 



We can pass from such a nebula as this to tl>e well-known ■ 

 planetary tiebuUe. .Mmosl all the knowledge which we have 

 of these nebuUv we owe to the labours of .Sir William and 

 Sir John Herschel. You will ;see that so far as appearance 

 goes, we have in these 'jTlanetary nebuUe alnu)sl to deal 

 with. a planet like Jupiter, except tli.at we do not see the 

 l)ells. That is why these lM«|ies are called planetary nebula; ; 

 they give us the idea that we are dealing with discs. If 

 we jwss for a moment from a nebula whch is simply discoidal 

 to one such as is represented in another (xirt of the diagram, 

 you find there that we get a very faint disc, including much 

 brighter condensation at the centre. If you look at the 

 others, you will find that we get a very obvious condens;ttion 

 towards the centre ; there is a very considerable difierence 

 in the intensity of the light given out as the centre is 

 ajjproached. 



Of course we understand that if in these, also, the move- 

 ments are tjuite orderly, we must not expect to get any very 

 great disturlKince, and therefore — if these disturlwnces pro- 

 (luce high temperatures -we shall not exiiect to get indica- 

 tions of any |iarticularly high temperatures from their external 

 portions. 



Dealing with nebula-, then, as a whole, it does not seem 

 too much to say that we are justified in supposini; that they 

 may advance towards condensation along two perfectly dis- 

 tinct roads. If we consider a regular spiral nebula, like the 

 otie in -Andromeda, or a planetary nebula, we may imagine 

 them living their life as nebuhv without very nuich disturb- 

 ance ; there is not much lighting to be dime, ihey progress in 

 orilerly fashion towards the conililion c.f complete coiulensi- 

 linn at tile centre. 



Hul there is another way. 



In the nebula of Orion we gel absolute absence of any • 

 thing like regularity. In any part where the structure can 

 be studied, we finti il consists of whirls .and streams cros.s 

 ing each other, some of them straight, s(mie of them 

 curved, the whole thing an irregular complicated mixtun 

 of ilivergeiil movements, so far as the pliulographs, which 

 are absolutely untouched, can give us any idea of what is 

 going on. i'ake. for instance, the magnificent slreamci 

 trending upwards. It gradually becomes brighter until il 

 reaches one of the brightest parts of the nebula ; and obsc'r\e. 

 also, the stars which seem dotted over it as on a shielil. 

 Il is cpiite obvious that we caimol, in such a structure as thai, 

 exi>ecl to gel the s;inie conditions that we met with in tlic 

 nebula of .\ndronieda, anil in the planetary nebula-. And, jii 

 fact, we do not. In this nebula, which speaks of disturbance 

 in every inch of il, we have spectroscopic indications of verj 

 high temperature indeed. Carlxm is replaced by hyilrogen. In 

 such a nebula as this, il is impossible for us to pick out the 

 place of condensiition : the condensation may be held to bi 

 anywhere, for ilislurbances are obviously everywhere. And yoii 

 ' remember, I hope, that I pointed out to you llial Ihe pari .it 



