June 1, 189-2.] 



KNOWLEDGE 



109 



have as to the intensely high temperature to which the 

 star was raised. Dr. Huggins has been able to photograph 

 as bright lines the riithr series of hydrogen lines which he 

 discovered in the ultra-violet spectra of the Sirian stars, 

 and he has also obtained the clearest tokens of the presence 

 of two groups of bright lines, apparently corresponding to, 

 but more pronounced and developed than, two groups of 

 dark lines which he lias but recently discovered in the 

 spectrum of Sirius, and which lie far beyond the last of the 

 hydrogen series. 



A third fact is perhaps the most striking of all, the most 

 " sensational," if we may be allowed the term. It is that 

 the lines of hydrogen are present both as bright and as 

 dark Imes — and the same is true of the lines of some other 

 elements, such as sodium and calcium for example — but 

 that the body or bodies giving the bright lines are moving 

 in the line of sight as compared with that giving the dark 

 lines at the incredible rate of 000 miles per second, or, in 

 round numbers, fifty millions of miles a day ! 



Clearly, we have not to do here with the breaking up 

 of a cooling crust and its sinking in a molten sea. Equally 

 clearly, no mere temporary brightening of a photospheric 

 surface is in question : and indeed, that idea is scarcely 

 admissible at all in view of the general opinion of the day 

 that photospheres are condensation surfaces. An increase 

 of temperature might raise the height at which that 

 condensation takes place, and so increase the radiation 

 from the particular star by making the radiating sm'face 

 greater, but could hardly increase the temperature at which 

 condensation sets in, or augment the brilliancy of the 

 condensed material. 



How is it with the meteoric theory '? The idea in this 

 case is that two meteor streams, the one dense and giving 

 the dark line spectrum approaching us, and the other 

 rarer and giving the bright line spectrum receding from 

 us, have rushed into each other end on, and their 

 collisions have given rise to the light which appears to us 

 as the star. But, if Prof. Vogel's observations are 

 accepted, the spectrum of the star gives us evidence of 

 not two bodies only but of three, moving in different 

 directions and with different speeds. Two meteor streams 

 might indeed run headlong into each other, and exactly in 

 the line of sight, but three could hardly be expected to do 

 so at the same point. Then, if we accept Prof. Vogel's 

 figures, two of these streams are travelling in opposite 

 directions with velocities the sum of which is sixty millions 

 of miles per day, a speed which would have sufficed to 

 carry a body, during the time that the star has been under 

 examination, right across the solar system, from beyond 

 the orbit of Neptune on the one side, to beyond the same 

 orbit on the other. And during the time the spectroscopic 

 observations have been carried on, this speed of travel has 

 been maintained practically unchanged. 



The collision of two meteor streams, therefore, is ruled 

 out of court on two grounds. First, we cannot imagine 

 that the Nova is composed of two streams, each moving 

 with a velocity of some thirty millions of miles a day, and 

 each having a length in a straight line equal to the 

 distance of Neptune from the sun, and encountering each 

 other in direct collision, " end on," and both streams lying 

 in the direction of the line of sight. But the second 

 objection is stronger still. The light and heat come 

 from some source, and arrested or retarded motion, or 

 collision — which need not, however, be held to involve the 

 direct impact of the two bodies upon each other- is the 

 source to which the meteoric theory would ascribe them. 

 But there must be the arrest or retardation of motion ; the 

 meteoric stream, or star, cannot spend its energy in light 

 and heat, and have its motion as well. And the develop- 



ment of that light or heat cannot precede the loss of speed. 

 The inconceivable rapidity of motion which the relative dis- 

 placement of the bright and dark lines indicates must be, on 

 this hypothesis, motion nftcr retardation ; whilst it does 

 not seem in the least likely that one meteor stream should 

 be purely gaseous, and the other give the continuous 

 spectrum, crossed by absorption lines, typical of the incan- 

 descent solid surrounded by absorbing vapours. 



A variation of this theory suggests one meteor stream 



plunging into a nebula. This is distinctly an advance, 



because we could imagine the meteors eonstnued as they 



reach and are arrested by the nebula. The bright lines, 



then, would express the local heating of the gases of the 



nebula by the friction of the meteorites as they plough their 



way into the nebula. But we should recognise tlie bright 



lines in this case as partly the lines of the elements 



characteristic of the meteorites, and partly as the typical 



[ nebular lines. This is not the case. The typical nebular 



' Unes — other than those of hydrogen, which are common to 



' most stars — are all wanting. There is no evidence of a 



collision between a meteoric swarm and a nebula. The 



hydrogen lines, the coronal line, many prominent chromo- 



; spheric lines are recognised, but thougli one bright line lies 



close to the chief nebular line the general verdict of the 



best observers is against any actual identity, and other 



lines, only less characteristic, are clearly not present. 



Yet another circumstance which tells against the theory 

 is, that though the continuous spectrum is over-borne 

 and disguised by the spectrum of bright lines, yet the 

 former is believed to show traces of that peculiar type of 

 spectrum so beatttitully shown by Mira t'eti, a Herculis, 

 and other variables. The same circumstance was reported 

 of the first Nova examined by the spectroscope, and the 

 appearance of the violet and ultra-violet hues of hydrogen 

 as bright lines in the spectra of variables like Mira seems 

 to establish an important link between variables of long 

 period, and " temporary " stars like T Coronaj, and our 

 present Nova. But the shaded bands of the third type 

 spectrum are certainly not what we expect from a nebula. 

 There can be no reasonable doubt, I thmk, that the 

 basis of our Nova is a real star or sun, and not a mere 

 gaseous nebtila, though in some way or other yet itn- 

 explained its luminosity was increased for a few weeks one 

 thousand fold. If we supjiose a sun, usually quiet and dull, 

 suddenly bursting out with prominences and metallic 

 eruptions on a scale utterly dwarfing anything ever 

 witnessed tipon our own siui, we should have a spectrum 

 very like that shown by the Nova, except in one most 

 important detail. We find, in the solar prominences, 

 evidence that the hurling forth of matter with tremendous 

 velocities so heats the cooler hydrogen atmosphere above 

 the chromosphere as to cause it to glow with those tiltra- 

 violet lines which are not, at least normally, found in the 

 general spectrum. A speed of 700 miles per second far 

 I exceeds anything recorded of the ejective spieed of our 

 solar storms, so that we have to conceive of far more 

 violent convulsions than the sun ever displays. But 

 supposing such convulsions brought about, it is easy to 

 imagine that for the time being a far greater part of 

 the light of the star might proceed from what we might 

 call its chromosphere, prominences and corona, than from 

 its photosphere. In such circumstances the bright line 

 spectrum, which would certainly in its principal Hnes, and 

 probably in its subordinate ones, closely resemble that of 

 our Nova, would be the most prominent feature. 



Unfortunately, in this case, the bright lines would be 

 displaced towards the blue instead of towards the red ; the 

 heated gases would be approaching us, not receding from 

 us, as we find to be actually the case. 



