lO 



NA TURE 



[November 2, 1899 



•nebula there. This has always been so ; and therefore 

 an the case of new stars we must always expect to get 

 indications of the existence of two bodies, the intruder 

 and the body intruded upon. 



We must also expect, if we are dealing with small 

 particles of meteoritic dust, that the action will be very 

 quick, and that the war will be soon over. All this really 

 agrees with the facts. I will, just in order to point my 

 ■remarks, show what happened in the case of the new 

 star we were fortunate enough to have the opportunity 

 of observing in the northern hemisphere not very long 

 ago, the new star in the constellation Auriga. We have 

 in the diagram the stars in the region in question ; a 

 black arrow indicates a dark space in the heavens where 

 there is no star. The next drawing shows the same stars 

 and the same region of the heavens ; but we observe that 

 in the centre is a star, which is the new star. In the 

 spectrum of it we obtained undoubted indications of the 

 fact that we were dealing with two different masses of 

 matter ; for the reason that if you take the chief spectral 

 ■Knes marked G, //, H and K, that is to say, the lines of 

 hydrogen and of calcium, we find both bright lines and 

 •dark lines, which being interpreted means that hydrogen 

 and calcium were both giving out light and stopping 

 light. We cannot imagine that the same particles of 

 calcium and of hydrogen were both giving out light and 

 stopping light ; there must have been different particles 

 of hydrogen and calcium giving light and different particles 

 of hydrogen and calcium stopping light ; and if we look at 

 the photograph carefully we find that the bright lines and 

 the dark lines are side by side, and we know that that 

 means a change of wave-length in consequence of move- 

 ment, and we also know from the change of wave-length 

 iindicated that the differential velocity of the particles 

 which gave us the bright hydrogen and calcium, and the 

 dark hydrogen and calcium, must have been something 

 like 500 miles a second. In that way we obtained indis- 

 putable proof that we were really dealing with two perfectly 

 •different series of particles moving in opposite directions, 

 and that that was the reason we got that sudden illumin- 

 ation in the heavens which as suddenly died out until 

 finally a nebula previously undiscovered was found to 

 occupy the place. The nebula is really not the result, 

 the nebula was the cause, but we did not know of its 

 •existence until our special attention had been drawn to 

 that part of the heavens. 



So much then for the first statement of facts relating 

 ■to the distribution of the various star groups and nebular 

 groups in the most general form. The next question is. 

 Can we say anything about the distances of these gaseous 

 stars, bright line stars, and other types? The way 

 in which an astronomer attempts to determine the 

 •different distances which the stars occupy in relation to 

 the earth may really be very well grasped, I think, by 

 ■considering what happens to one when travelling in a 

 railway train. If the train is going fairly quickly, and we 

 Jook at the near objects, we find that they appear to rush 

 by so rapidly that they tire the eye, and one naturally 

 looks at the objects which are more distant ; the more 

 -distant the object we look at is the more slowly it appears 

 to move, and the less the eye is fatigued. Now, suppose 

 that instead of the train rushing through the country and 

 passing the objects which we regard under these different 

 conditions, the different objects are rushing past us at 

 rest. Then, obviously, those things which appear to 

 be moving most quickly will be those nearest, and the 

 more distant objects, just because they are distant, will 

 appear to move more slowly ; that is to say, we shall get 

 what is called a large "proper motion" — in the case of 

 ■the objects nearest to us— and a small " proper motion" 

 — in the case of the bodies which are further away. 



This question has been attacked with regard to the 

 ■stars in magnificent fashion by a great number of 

 astronomers. A photograph will show in a diagrammatic 



NO. 1566, VOL. 61] 



form the very various rates of proper motion which have 

 been assigned by careful observation to a very great 

 number of the stars. In the chart the amount of proper 

 motion of the various stars is indicated by the lengths of 

 the lines which proceed from them, and the direction in 

 which the various stars appear to be moving is also indi- 

 cated by the direction which these lines take. Some of 

 the lines are extremely long ; they seem to stretch over a 

 large part of the sky. Of course the scale is an 

 exaggerated one, but it is the relative motion that we 

 have chiefly to deal with, and we find that on the same 

 scale in some cases the lines are extremely short ; so 

 that the diagram tells us that the amount of proper 

 motion is apt to vary very considerably. We have large 

 proper motions and small proper motions among the 

 stars. 



It was Mr. Monck who was the first to show in 1892^ 

 that the gaseous stars had the smallest proper motion ; 

 that is to say, that the hottest stars were further away 

 from us than the cooler ones. That is a good, definite 

 statement, and one which everybody can understand. He 

 next found that the proto-metallic stars— that is to say, 

 the stars not so hot as the gaseous ones, but hotter than 

 the metallic ones — had the next smaller proper motion. 

 This, of course, indicates that the metallic stars are the 

 nearest to us unless proper motion does not depend upon 

 distance, but rather upon a greater average velocity in 

 space. It has been shown, however, by considering the 

 sun's movement in space, that this view probably may be 

 neglected. The first discussion of proper motion then 

 went to show, roughly, that the hotter a star is the 

 further away from us it is ; and it made out a fair case 

 for the conclusion that the sun forms one of a group or 

 cluster of stars in which the predominating type of 

 spectrum is similar to its own. 



Kapteyn carried the inquiry a stage further.^ Working 

 upon the idea that stars with the greatest proper motion 

 are on the average the nearest, the part of the proper 

 motion due to the sun's translation in space he considered 

 must depend strictly upon the distance, and he determined 

 this by resolving the observed proper motion along a great 

 circle passing through the point of space towards which 

 the sun is moving, which is called the apex of the sun's 

 way, and reducing to a. point 90° from the apex. His 

 results were practically the same as those obtained by 

 taking the individual proper motions. He also found 

 that stars with the greatest proper motion are mainly 

 metallic, and have no regard at all to the Milky Way ; 

 that stars with the smallest and no observable proper 

 motion are gaseous and proto-metallic, including a few 

 metallic ones which have collected in the galactic plane. 

 In this he agrees with the prior observations to which I 

 have drawn attention. In the table which I now give 

 the mean proper motion is shown. 



The table deals with something over a second, which 

 may be looked upon as a great proper motion, down to the 

 tenth of a second, which may be regarded as a small one ; 



1 "Astronomy and Astro- Physics," xviii., 2, p. 876. 



2 Amsterdam Academy of Science, 1893. - . ' 



