376 



NATURE 



[August 20, 1891 



In the case of the sun the force of gravity has already become 

 so great at the surface that the decrease of the density of the 

 gi^es must be extremely rapid, passing in the space of a few 

 miles from atmospheric pressure to a density infinitesimally 

 small ; consequently the temperature-gradient at the surface, if 

 determined solely by expansion, must be extremely rapid. The 

 gases here, however, are exposed to the fierce radiation of the 

 sun, and unless wholly transparent would take up heat, especially 

 if any solid or liquid particles were present from condensation or 

 convection currents. 



From these causes, within a very small extent of space at the 

 surface of the sun, all bodies with which we are acquainted 

 should fall to a condition in which the extremely tenuous gas 

 could no longer give a visible spectrum. The insignificance of 

 the angle subtended by this space as seen from the earth should 

 cause the boundary of the solar atmosphere to appear defined. 

 If the boundary which we see be that of the sun proper, the 

 matter above it will have to be regarded as in an essentially dy- 

 namical condition— an assemblage, so to speak, of gaseous pro- | 

 jectiles for the most part falling back upon the sun after a 

 greater or less range of flight. But in any case it is within , 

 a space of relatively small extent in the sun, and probably in the 

 other solar stars, that the reversion which is manifested by dark 

 lines is to be regarded as taking place. 



Passing backward in the star's life, we should find a gradual 

 weakening of gravity at the surface, a reduction of the tempera- 

 ture-gradient so far as it was determined by expansion, and con- 

 vection currents of less violence producing less interference with 

 ihe proportional quantities of gases due to their vapour densities, 

 while the effects of eruptions would be more extensive. 



At last we might come to a state of things in which, if the 

 star were hot enough, only hydrogen might be sufficiently cool 

 relatively to the radiation behind to produce a strong absorption. 

 The lower vapours would be protected, and might continue to 

 be relatively too hot for their lines to appear very dark upon the 

 continuous spectrum ; besides, their lines might be possibly to 

 some extent effaced by the co-ning in under such conditions in 

 the vapours themselves of a continuous spectrum. 



In such a star the light radiated towards the upper part of the 

 atmosphere may have come from portions lower down of the 

 atmosphere itself, or at least from parts not greatly hotter. 

 There may be no such great difference of temperature of the low 

 and less low portions of the star's atmosphere as to make the 

 darkening effect of absorption of the protected metallic vapours 

 to prevail over the illuminating effect of their emission. 



It is only by a vibratory motion corresponding to a very high 

 temperature that the bright lines of the first spectrum of hydro- 

 gen can be brought out, and by the equivalence of absorbing and 

 emitting power that the corresponding spectrum of absorption 

 should be produced ; yet for a strong absorption to show itself, 

 the hydrogen must be cool relatively to the source of radiatio.T 

 behind it, whether this be condensed particles or gas. Such 

 conditions, it seems to me, should occur in the earlier rather 

 than in the more advanced stages of condensation. 



The subject is obscure, and we may go wrong in our mode of 

 conceiving of the probable progress of events, but there can be 

 no doubt that in one remarkable instance the white-star spec- 

 trum is associated with an early stage of condensation. 



Sirius is one of the most conspicuous examples of one type of 

 this class of stars. Photometric ob-ervations combined with its 

 ascertained parallax show that this star emits from forty to sixty 

 times the light of our sun, even to the eye, which is insensible to 

 ultra-violet light, in which Sirius is very rich, while we learn 

 from the motion of its companion that its mass is not much 

 more than double that of our sun. It follows that, unless we 

 attribute to this star an improbably great emissive power, it 

 must be of immense size, and in a much more diffuse and there- 

 fore an earlier condition than our sun ; though probably at a 

 later stage than those white stars in which the hydrogen lines are 

 bright. 



A direct determination of the relative temperature of the 

 photospheres of the stars might possibly be obtained in some 

 cases from the relative position of maximum radiation of their 

 continuous spectra. Langley has shown that through the whole 

 range of temperature on which we can experiment, and presum- 

 ably at temperatures beyond, the maximum of radiation-power 

 in solid bodies gradually shifts upwards in the spectrum from 

 the infra-red through the red and orange, and that in the sun it 

 has'reached the blue. 



The defined character, as a rule, of the stellar lines of absorp- 



NO. I 138, VOL. 44] 



tion suggests that the vapours producing them do not at the 

 ',ome time exerl any strong power of general absorption. Con- 

 sequently, we should probably not go far wrong, when the pho- 

 tosphere consists of liquid or solid particles, if we could compare 

 select parts of the continuous spectrum between the stronger 

 lines, or where they are fewest. It is obvious that, if extended 

 portions of different stellar spectra were compared, their true 

 relation would be obscured by the line-absorption. 



The increase of temperature, as shown by the rise in the 

 ;pectrum of the maximum of radiation, may not always be ac- 

 companied by a corresponding greater brightness of a star as 

 estimated by the eye, which is an extremely imperfect photo- 

 metric instrument. Not only is the eye blind to large regions 

 of radiation, but even for the small range of light that we can 

 see the visual effect varies enormously with its colour. Accord- 

 ing to Prof Langley, the same amount of energy which just 

 enables us to perceive light in the crimson at A would in the 

 green produce a visual effect loo.ooo times greater. In the 

 violet the proportional effect would be i6oo, in the blue 62,000, • 

 in the yellow 28,000, in the orange 14,000, and in the rel 1200. 

 Captain Abney's recent experiments make the sensitiveness of the 

 eye for the green near F to be 750 times greater than for the red 

 about C. It is for this reason, at least in part, that I suggested 

 in 1864, and have since shown by direct observation, that the 

 spectrum of the nebula in Andromeda, and presumably of 

 similar nebulee, is, in appearance, only wanting in the red. 



The stage at which the maximum radiation is in the green, 

 corresponding to the eye's greatest sensitiveness, would be that 

 in which it could be most favourably maasured by eye-photome- 

 try. As the maximum rose into the violet and beyond, the star 

 would increase in visual brightness, but not in proportion to the 

 increase of energy radiated by it. 



The brightness of a star would be affected by the nature of 

 the substance by which the light was chiefly emitted. In thi 

 laboratory, solid carbon exhibits the highest emissive power. A 

 stellar stage in which radiation comes, to a large extent, from a 

 photosphere of the solid particles of this substance, would be 

 favourable for great brilliancy. Though the stars are built up 

 of matter essentially similar to that of the sun, it does not follow 

 that the proportion of the different elements is everywhere the 

 same. It may be that the substances condensed in the photo- 

 spheres of different stars may differ in their emissive powers, but 

 probably not to a great extent. 



All the heavenly bodies are seen by us through the tinted 

 medium of our atmosphere. According to Langley, the solar 

 stage of stars is not really yellow, but, even as gauged by our 

 imperfect eyes, would appear bluish-white if we could free our- 

 selves from the deceptive influences of our surroundings. 



From these considerations it follows that we can scarcely 

 infer the evolutional stages of the stars from a simple comparison 

 of their eye-magnitudes. We should expect the white stars to 

 be, as a class, less dense than the stars in the solar stage. As 

 great mass might bring in the solar type of spectrum at a rela- 

 tively earlier time, some of the brightest of these stars may bj 

 very massive, and brighter than the sun — for example, the bril- 

 liant star Arcturus. For these reasons the solar stars should 

 not only be dense than the white stars, but perhaps, as a class, 

 surpass them in mass and eye-brightness. 



It has been shown by Lane that, so long as a condensing 

 gaseous mass remains subject to the laws of a purely ga'^eous 

 body, its temperature will continue to rise. 



The greater or less breadth of the lines of absorption of 

 hydrogen in the white stars may be due to variations of the 

 depth of the hydrogen in the line of sight, arising from the 

 causes which have been discussed. At the sides of the lines 

 the absorption and emission are feebler than in the middle, and 

 would come out more strongly with a greater thickness of gas. 



The diversities among the white stars are nearly as numerous 

 as the individuals of the class. Time does not permit me to 

 do more than to record that, in addition to the three sub-classes 

 into which' they have been divided by Vogel, Scheiner has re- 

 cently investigated minor differences as suggested by the charac- 

 ter of the third line of hydrogen near G. He has pointed out, 

 too, that so far as his observations go the white stars in the 

 constellation of Orion stand alone, with the exception of Algol, 

 in possessing a dark line in the blue which has apparently the 

 same position as a bright line in the great nebula of the same 

 constellation ; and Pickering finds in his photographs of the 

 spectra of these stars dark lines corresponding to the principal 

 lines of the bright-line stars, and the planetary nebulae with the 



