110 



KNOWLEDGE 



[June 1, 1892. 



This circumstance is one which leads me to make a 

 suggestion, which I put forward with much diffidence and 

 a clear apprehension of many defects in it, for indeed we 

 are yet far from possessing enough material to theorize 

 upon . 



We have seen it happen again and again, in our own 

 solar system, that a comet has swejit round the sun at so 

 short a perihelion distance that the two bodies must 

 almost have grazed each other. Suppose we put instead 

 of a comet a long and dense stream of meteors ; suppose 

 too that the perihelion distance of the meteor stream lies 

 iilthin the radius of the star, and that the orbit of the 

 meteors is so presented to us that their motion at x^eri- 

 helion is almost entirely in the direction of our line of 

 sight. The maximum speed in the orbit which such a 

 stream could attain in our sj'stem is 380 miles per second. 

 It is readily imaginable that other suns than ours might 

 be able to raise double such a velocity, that depending 

 upon the mass of the star and the radius of its photo- 

 sphere. It is quite outside the limits of probability to 

 suppose that two meteor streams should both be moving 

 with a speed of some 350 miles to the second and almost 

 precisely in the line of sight. But there is nothing diffi- 

 cult at all in the supposition that there are many stars 

 which are capable of inducing a velocity of 700 miles per 

 second at perihelion. If then we had a long and dense 

 stream of meteors travelling towards a star, and rushing 

 into it, or just grazing it, as they passed periastron 

 we should have the ordinary spectrum of the star, and 

 superposed upon it the spectrum of the glowing meteorites, 

 and of the components of the stellar atmosphere through 

 which they were rushing. The meteorites would there- 

 fore give us a spectrum just as their orbital motion was 

 the highest. This suggestion overcomes the diSiculty of 

 supposing that the two colliding bodies are moving both 

 before and after collision with the enormous relative speed 

 which the displacement of the bright lines, as referred to 

 their dark companions, would indicate. 



There is another point. A large proportion of the meteors 

 would probably escape capture by the star, and would be 

 seen after periastron on the other side of the stellar disc. 

 But they would be now travelling far more slowly in their 

 orbit, and that orbit would be inclined at a very consider- 

 able angle of the line of sight, so that their motion from us 

 would then be at a comparatively slow rate. The principal 

 bright lines, the hydrogen lines certainly, but probably not 

 the lines correspondmg to the lower strata of the stellar 

 atmosphere, would then be doubled ; the line near the red cor- 

 responding to the meteors at i3eriastron,and seen on the one 

 side of the star, and the line nearer the blue corresponding 

 to the meteors after periastron, and seen on the other side 

 of the star. And this is the very appearance which Father 

 Sidgreaves, I'rof. Vogel, and other observers have actually 

 recorded. 



I should not at all wish to press this suggestion, for the 

 matter is not really ripe for solution. But I should like 

 to point out how vastly more powerful our means of stellar 

 spectrum analysis have become. The present Nova only 

 reached the fifth magnitude, and was entirely overlooked 

 for six weeks after it had reached its maximum. Never- 

 theless we have acquired stores of spectroscopic informa- 

 tion with regard to it which will take long to properly 

 discuss. Its history — especially when the mode of its 

 discovery by an amateur astronomer, armed only with a 

 half-guinea spy-glass, is borne in mind — points therefore to 

 the extreme importance of a diligent watch being kept upon 

 the sky, and especially upon the Milky Way and its 

 offshoots, for who can tell how many Novm have flashed 

 out during the past few years which, though not bright 



enough to attract universal attention, like Tycho's Pilgrim 

 Star of 1572, were yet quite bright enough to have lent a 

 rich harvest of information to the prism and the sensitive 

 plate. 



TEMPORARY STARS. 



By A. V. Ranyakd. 



WE have t(i thank Prof. E. C. Pickering for 

 the photographs which illustrate this uinuber 

 of Kxowled(;e. There have been great diffi- 

 culties in reproducing them, and I must 

 apologize for the unsatisfactory appearance 

 of many of the plates. It lias been found very difficult to 

 show the numerous lines and great variations of brightness 

 in the spectrum of the Nova, and at the same time to keep 

 the background of sky in the star plate black. 1 have 

 preferred that the printing should be done so as to bring 



I out the best effect in the spectrum of the Nova. 



I On examining the reproduction of the Nova spectrum 

 given in our plate, it will be noticed that there are at 

 least eight bright lines, bounded in every case by dark lines 

 on their more refrangible sides. This seems to be more 

 than a chance coincidence, and may probably* be taken 



! as indicating that we have before us the superposed 

 spectra of two bodies, one giving bright lines and moving 

 away from us so that all its lines are pushed from their 

 natural places towards the blue end of the spectrum by 

 the apparent shortening of the wave-lengths — very much 

 in the same manner as the note of a steam-whistle is 

 raised as a whistling locomotive approaches us, and de- 



: pressed as it rushes away from us— and the other body 

 giving dark absorption lines and moving towards us. 



j The lines F, G, /(, Hj, and a are characteristic of the 

 element hydrogen, and, according to Dr. ^'ogel of 

 Potsdam, who has compared the place of some of these 

 lines with the places of the corresponding lines in the 

 spectrum of stationai-y hydrogen in a vacuum tube, the 

 hydrogen of the bright-lme-giving star is rushing away 

 from us at the extraordinary rate of 230 miles a second, 

 while the hydrogen giving rise to the absorption lines is 

 approaching us with a velocity of 320 miles a second. 



I The phenomena observed may, it seems to me, be 

 accounted for by the disturbance produced by the motion of 

 a comparatively small star moving away from us through a 

 nebula which was moving towards us. When a large meteor 

 plunges into our atmosphere, the meteor and a considerable 

 mass of gas di'iven up before it become, for a second or more, 

 intensely luminous. Large meteors frequently leave a trail 

 of luminous gas around their paths, which remains glowing 

 in the cold regions of the upper air for sometimes twenty 

 minutes or half-an-hour. Such trails generally become 

 gradually broader and fainter, and before their final 

 disappearance they not infrequently subtend an angle of 

 half a degree in diameter, as seen from a distance of 70 or 

 80 miles — that is, they have an actual diameter of about 

 three-quarters of a mile, although the meteoric body 

 whose motion through the air gave rise to the trail may only 



* We ought not to speak with eertainty until the intervals between 

 the neighbouring bright and dark lines have been more accurately 

 compared. If the interval between the components of different lines 

 is foiuid to be proportional to the wave-lengths of those lines, the 

 phenomenon may be ascribed with much confidence to relative 

 motion; if not, or if the character of the bright and dark lines does 

 not in every case correspond, some other exiilanation might be 

 preferred. Father Sidgreaves' negatives, which he has kindly lent 

 me for examination, show that there are at least twentyfive pairs of 

 adjacent bright and dark lines, the dark line being uniformly on 

 the move refrangible side ; this evidently points to some physical 

 connection. 



