416 



KNOWLEDGE. 



NOVEMUKH, 1911. 



probabilit\' with the iniiiact origin of variable and 

 double stars : in both of which cases the evidence is 

 so conclusive that it is certain that a very large 

 number of the wonder and double stars of the 

 heavens did actualh- originate from the grazing 

 impact of suns. 



Planetary Nebui.aic. 



There is \'erv strong e\idence, indeed, that most, 

 if not the whole of the so-called planetary nebulae, 

 originated from the impact of suns. It is almost 

 certain that Herschel's wonderful suggestion as to 

 the physical character of these bodies is true. He 

 examined these circular discs of faint even light uith 

 great care and he tells us that only one possible 

 suggestion of their character offers itself. The\- 

 cannot really be 



discs, or they would ; | 



sometimes be shown 

 to us more or less 

 edge on : they can- 

 not be continuous 

 spheres of gas, or 

 the edges would be 

 much less brilliant 

 than the central 

 portions. The onlv 

 explanation left, in- 

 conceivable as it ma\' 

 seem, is that they 

 are hollow shells of 

 gas. How such 

 bodies could be 

 formed appeared to 

 him a mystery, but 

 it is ob\iousl\' a 

 clear and simple deduction, 

 properties of the third bod\ 

 le of 



than the critical \-elocity will be carried to enormous 

 distances, and will not tend to concentrate back 

 again, because the rotation will give them an orbital 

 power. 



In every stage of atom sorting there will tend to 

 be ensphering shells, in which the lighter gases w ill 

 be the outer ones, and consequently many of the 

 planetarv nebulae wdl have a sphere in sphere 

 structure. 



The Centre oe the Planetary Nehclae. 



Thev should also often have centres consisting of 



Sometimes this centre 

 a rare meteoric swarm, such as seems 



material of greater density. 



would be 

 to be the 

 nebulae. 



case with 

 Both of 





- V n c zo^/. 



LKH _;. WVbl/s XfliuLu 



continumis spectrum tile mete<inc miclei within 

 tile ensphering; shells. 



:ao jot iM 



I I I. I M M 



4. Struve's Nebula. 



DiaL'ram sliuwin^ 



in spectrogram that concentric layers are of difterent 

 elements 



Struve's and Webb's planetar}- 

 these nebulae, as shown by 

 Keeler's drawings, 

 •» exhibit also the 



-J sphere in sphere 



«;. structure. (I'igures 



j and 4.) Sometimes 

 the meteoric swarm 

 would be dense 

 enough to appear 

 stellar, perhajis with 

 a velvetv structure. 

 Some of the planet- 

 ary nebulae actually 

 do exhibit such a 

 star. In some cases 

 the two suns would 

 be entrajiped into a 

 double star the dis- 

 tance of whose 

 constituents from 



t. 



w lien we 

 anil appK" 



app 



study 

 tci it 



the 

 the 



The Origin 



We have already 



ht gases, must under man\ 



OE Planetary Nehijlae. 



shown that the high kinetol of 

 the light gases, must under many and varied circum- 

 stances cause such elements as helium, hydrogen, 

 nebulium. and so on, to leave systems. We have 

 also seen that on the birth of the third body, the 

 atmospheres of the suns must be at the centre of the 

 mass, and as expansion occurs these light gases 



gra 



dualh 



ike their way out. By taking energy 

 from heavier elements these light atoms attain an 

 escaping velocit\'. It is clear that this inversion of 

 the position of the elements could not occur 

 suddenly. At the birth of the third body, the light 

 elements do not mo\e faster than the heavier ones. 

 This increase of speed is obtained as the 

 temperatures tend to become equal, consequently 

 much of the later escaping hydrogen will not possess 

 anything like the speed that some of the light 

 material, better situated, will attain, and hence 

 some of it niav only have a \elocity approximately 

 that of the critical velocity. The whole third body 

 is rotating, and hence particles with slightly less 



one another might 

 be \'er\' small conipared with the dimensions of 

 the planetar\- nebula, for some of these extraordinary 

 bodies have dimensions that are inconceivabl\' stu- 

 pendous. The orbit of the earth would be as a child's 

 hoop in a continent, compared with the vast dimen- 

 sions of some of these celestial shells of luminous gas. 



I think it extremeh' probable that every planetary 

 nebula is gas that has been produced by an impact 

 of suns, gas that has sorted itself, by selective 

 escape, into a condition of practical stability. The 

 planetar\- nebulae that are known to result as the 

 final stage of many temporary stars need not 

 necessarily be permanent shells ; the speed may 

 be so great that the material may be carried on 

 into distant space, or not great enough to form 

 permanent gas shells. The width of the lines in 

 the spectrograms would tell much as to this velocity. 



The same remark applies to planetary nebulae as 

 was made regarding star clusters. Their character 

 and motions should be studied in the light of the 

 new astron(jmy of impact, to ascertain how far they 

 tend to give solidity to this new theory, or to 

 modif\" some of its conclusions. 



White Nebulae. 

 Covering vast regions at the two poles of 



the 



