May 4. 1893] 



NA TURE 



December 8, 1892). He shows that the statical theory of 

 tides that has been applied is entirely inappropriate to 

 the case, and also that the hypothesis involves assump- 

 tions amounting almost to impossibilities. In the first 

 place, the pairing of the bright and dark lines makes it 

 necessary to assume that the two bodies engaged were of 

 similar chemical constitution, one having an absorption 

 spectrum and the other an equivalent radiation spectrum. 

 But even if we make this unthinkable supposition, a fatal 

 objection has been pointed out by Mr. Maunder {^Know- 

 ledge, June 1892). It is that the bright lines ought to 

 have their refrangibility increased, not decreased as the 

 spectroscopic observations show them to be. In other 

 words, the erupted matter would approach the earth, not 

 recede from it. This single undisputable fact effectually 

 disposes of the chromospheric hypothesis to which refer- 

 ence has been made. 



Another chromospheric theory in which only a single 

 star is involved has been put forward by Father Sid- 

 greaves {Tlie Observatory, October, 1S92). After de- 

 scribing the spectrum he says, " It is only necessary, 

 therefore, to consider the conditions under which the 

 blue-side shift of the Nova's lines should produce the 

 absorption effect while the red-side parts show unclouded 

 radiation. A great cyclonic storm of heated gases would 

 produce this double if the heated gases were rushing 

 towards us in the lower depths of the atmosphere trend- 

 ing upwards and returning over the stellar limb. In the 

 lower positions the advancing outrush would be screened 

 by a great depth of absorbing atmosphere, while as a high 

 retreating current its radiation would be along a clear 

 line to our spectroscopes." This explanation is plausible 

 enough, but it does not go to the root of the matter. 

 How, for instance, does Father Sidgreaves account for 

 such a tremendous eruption as that required by his 

 hypothesis ? It is difficult to believe that internal forces 

 could sustain, for two months, a stream of gas rushing 

 earthwards with a velocity of about 400 miles per second, 

 and then curving round and receding at the rate of 300 

 miles per second. And the idea becomes still more in- 

 comprehensible when we remember that the body 

 possessing this marvellous store of energy was quite in- 

 visible before December, 1891. Until Father Sidgreaves 

 explains the machinery by which the terrific whirl of 

 chromospheric matter was started and kept up, his theory 

 can hardly be seriously discussed. 



As has already been remarked, Mr. Lockyer was the 

 originator of the theory that Novas represent the result 

 of the collisions of small masses. On this theory the 

 broadened character of the lines in the spectrum of Nova 

 Aurigaj is explained by supposing that different parts of 

 the colliding swarms of meteorites were moving with 

 different velocities, or with the same velocity in different 

 •directions. Several modifications of the meteoritic theory 

 have been published. Mr. W. H. Monck has suggested 

 that a star, or a swarm of meteors, rushing through a 

 gaseous nebula afford the best explanation of the phe- 

 nomena. The only difference between this idea and that 

 of Mr. Lockyer's is that the nebula is supposed to con- 

 sist of gaseous instead of meteoritic particles. But, 

 from a dynamical point of view, there is no distinction 

 between the two, for it is well known that Prof G. H. 

 Darwin has proved that the individual meteorites of a 

 swarm would behave like the individual particles of a 

 gas. Referring to the collision with a gaseous nebula, 

 Mr. Monck says {.Journal of the British Astronomical 

 Association, ]sx\\xA\y, 1893): "The previous absence of 

 nebular lines, even if clearly proved, would not be con- 

 clusive as to the non-existence of such a nebula, for its 

 temperature may not be high enough to produce these 

 lines until raised by the advent of the star. A consider- 

 able proportion of Nova;, however, appear to be con- 

 nected with known nebulae. Irregularities in the nebula; 

 would produce the observed fluctuations of light, and if 

 iNQ 12^7, VOT.. 48] 



the relative velocity was considerable the bright gas-lines 

 of the nebula would be distinguishable from the dark 

 absorption lines of the star. The bright lines would be 

 broader than usual, because the velocity of the portion 

 of the nebula adjoining the star would be partially 

 destroyed and the luminous gas would thus be moving 

 with different velocities. The heating being confined to 

 the surface of the star, the cooling would take place more 

 rapidly than after an ordinary collision. But if the star 

 travelled far through the nebula in a state of intense in- 

 candescence, portions of the surface would from time to 

 time be vaporised and captured by the nebula, the mass 

 of the moving star thus diminishing at every step. It 

 might even end in complete vaporisation, as meteors are 

 sometimes vaporised in our atmosphere. Herr Seelin- 

 ger has worked out mathematically a theory {Astr. Nach. 

 No. 31 18, and Nature, vol. xlvii. p. 137) very similar to 

 that of Mr. Monck. He supposes that a body enters a 

 cosmic cloud, such as Dr. Max Wolf's photographs show 

 to be widely scattered through space. Whatever the 

 constitution of such a nebulous mass, collision with it 

 causes an increase of temperature, and a vaporisation 

 of some of the constituents of the colliding body. The 

 process is precisely similar to the entrance of a meteor 

 into the earth's atmosphere. According to HerrSeelinger, 

 Nova Aurigte was produced in this wise. A dark body 

 was rushing earthwards through space ; it came to a mass 

 of nebulosity, the light of which was so feeble that the 

 eye could not appreciate it ; the collision caused an in- 

 crease of temperacure and of luminosity ; the heaping up 

 of the glowing vapours in front of the colliding body pro- 

 duced the spectrum of dark lines, and the bright-line 

 spectrum was given by the vapours left behind as the 

 body moved onwards. These vapours would quickly 

 assume the velocity of adjacent parts of the nebula, 

 hence the dark lines would appear on the more refrangible 

 sides of the bright ones in the manner observed. 



Mr. Maunder also favours a collision theory {Knowledge, 

 June 1892), his idea being that a long and dense swarm 

 of meteors rushed through the atmosphere of a star, and 

 produced the phenomena exhibited by Nova Aurigte 

 As the stream passed periastron, the spectrum of the 

 glowing meteorites, and that of the constituents of the 

 stellar atmosphere with which they were colliding, would 

 appear together with the absorption spectrum of the star. 



From what has been said it will be seen that none of 

 the collision theories are substantially different from that 

 laid down by Mr. Lockyer in 1877. It has been asserted 

 that the meteoritic theory is not competent to explain the 

 observed facts, but the opponents have generally omitted 

 to specify its imperfections. One of the commonest 

 objectionsisthat the collision of two meteor swarms would 

 be accompanied by a very considerable slackening of the 

 rate of movement. Against this can be urged Seelinger's 

 proof that the great relative velocity indicated by the 

 spectrum could remain practically unchanged, and, in 

 spite of this, enough kinetic energy could be transformed 

 into heat to cause a superficial incandescence. Another 

 objection is that it is impossible to conceive of meteor 

 swarms of such magnitude that though rushing through 

 one another with a relative velocity of more than seven 

 hundred miles per second, disentanglement did not take 

 place until two or three months had elapsed. In the 

 light of latter-day revelations of astronomical photography, 

 this objection becomes a mere cavil. The long-exposure 

 photographs taken in recent years show that space is full 

 of nebulous matter, and the " stream of tendency " is 

 towards the idea that such masses are not gaseous but of 

 meteoritic constitution. Now a simple calculation proves 

 that even if Nova Auriga: had a parallax of one second of 

 arc, the whole of the luminosity received up to the end of 

 April, 1892, could have been produced by the collision of 

 two bits of nebulous matter, each of which would subtend 

 an angle at the earth of less than half a minute of arc. 



