Decembeb 1, 1892.] 



KNOWLEDGE 



237 



nitude which give other kinds of light, this class probably 

 consists of cooled-down stars. 



It will be long before our observations on parallax are 

 sufficiently uumerous and accurate to enable us to decide 

 this question directly ; but there are two other indications 

 of nearness to which we can appeal during the interval. 

 One of these is large proper motion. The other is a com- 

 putable orbit in the case of a binary star ; for I think a 

 pair of stars, separable in the telescope, but revolving so 

 rapidly as to enable an orbit to be computed from the 

 comparatively short range over which our observations 

 extend, must be among our nearer neighbours. 



I have tried both these tests and am still making some 

 computations on the subject. The result of both tests is 

 the same. The stars which are nearer to us than other 

 stars of the same magnitude are those whose spectrum is 

 of the type of Capella or Procyou, designated by the letter 

 F in the Draper Cittaloijue. 



Comparing the spectrum of a star of this class with that 

 of Aldebaran for instance, we can easily see how the latter 

 might be transformed into the former by the process of 

 cooling. The great number of dark lines in the spectrum 

 of .\ldebaran are due to absorption by gases in the 

 atmosphere of the star. As the star cools, many of these 

 gases will condense and the lines due to their absorption 

 will disappear from the spectrum. Partial condensation, 

 again, will render the lines thinner and less dark. These 

 changes will make the spectrum more similar to that of 

 Capella. I do not, indeed, see how a Sirian star could be 

 changed into one of the type of Aldebaran. Mr. Maunder 

 is very probably right m thinking that Sirian and solar 

 stars are physically distinct and do not represent different 

 stages of star-life ; but it may, notwithstanding, be true 

 that the Capellan type represents the cooled-down condition 

 of a solar star, and perhaps the higher temperature of the 

 surface of emission in the case of stars of the type of 

 Arcturus or Aldebaran may be indicated by a greater 

 extension of the ultra-violet spectrum. Such an extension 

 undoubtedly exists in the case of Arctm-us. 



The sun may be regarded as a Capellan star. Mr. Gore 

 not long since maintained in your columns that the sun 

 was a very dull star compared with all others in respect 

 of which we possessed the means of comparison. Should 

 furtlier observations confirm this result, it will strengthen 

 the evidence for the cooled-down condition of the Capellans. 

 With regard to the binary stars to which I have referred 

 the extraordinary brilliancy of y Leonis has been regarded 

 as an exception to the rule as to the greater brilliancy of 

 the Sirians. This star, however, is one of the few binaries 

 with computed orbits whose spectrum is of the Arcturian 

 type, and therefore does not affect my conclusion as to the 

 dulness of the Capellans. 



I may perhaps remark that Sirian stars of the class B, 

 which exhibit a number of fine lines in addition to the 

 hydrogen-lines in their spectra, have less proper motion 

 than ordinary Sirian stars. The difference here also may 

 result from condensation. 



Should this hypothesis be borne out by further obser- 

 vation the history of the sun may assist in explaining the 

 past geological changes on the earth. There would not be 

 a continuous loss of heat and light in consequence of the 

 cooling. The condensation of vapours in the sun's 

 atmosphere, and consequent diminution in the absorption, 

 might even have the contrary effect ; while if they formed 

 into clouds above the photosphere before sinking beneath 

 it we might have a season of great cold. 



Truly yours, W. H. S. Monck. 



[I do not agree with my friend Mr. Monck that we 

 may assume that the oldest stars are the coolest. The 



late Mr. Homer Lane, of Washington, showed that a 

 gaseous sphere, losing temperature by radiation and con- 

 tracting under its own gravity, would actually grow hotter 

 until it ceased to be a " perfect gas " obeying Boyle's law 

 of pressure and density. Mr. Lane's theory is a necessary 

 consequence of the kinetic theory of gases, and those who 

 accept the nebular hypothesis and beheve that stars have- 

 developed from faintly glowing gaseous masses are forced 

 to assume that during a long period they have grown 

 hotter, and that such stars as are still gaseous are still 

 growing hotter. 



We must not be misled by any supposed analogy between 

 the change in tint of a cooling solid and a cooling mass of 

 gas. While a cooling solid passes from white down to a 

 dull cherry-red colour before it ceases to be luminous, 

 some vapours actually give in the electric arc longer lines 

 at the blue end of their spectrum than at the red — that 

 is, the vapour gives out blue light in an outer and cooler 

 region of the electric arc than that in which it gives out 

 red and blue light. But if the stars are like our sun, it is 

 probable that we do not receive the greater part of the 

 light given out by their gaseous masses, which is nearly 

 all intercepted by their photospheres. It is the con- 

 tinuous spectrum of the solar photospheric cloud layer 

 which constitutes the chief light of the sun, and even 

 this reaches us through the solar corona and chromo- 

 sphere, which subtract many of the wave-lengths. 



Therefore, in looking at the light of the stars, if we are 

 to judge by solar analogies, we are observing the continuous 

 spectrum of a comparatively thin layer of incandescent 

 liquid or solid particles, through various depths and 

 various types of stellar coronas and stellar chromospheres. 

 The stellar spectra, when we succeed in fully deciphering 

 and interpreting them, will enable us to determine the 

 qualities and condition of the matter surrounding the 

 stellar photospheres rather than the constituent elements 

 and condition of the matter contained within the stellar 

 photospheres. — A. C. Raxyard.] 



STAR - CLU3TER NEAR SIRIUS OBSERTED BY 

 ARISTOTLE. 



To the Editor of Knowledge. 



Sir, — I cannot find any reference in modern astronomical 

 works to a very ancient observation of the star-cluster near 

 Sirius, which may possibly be of interest to some of your 

 readers. The description is by Aristotle, who has been too 

 much depreciated as a natural historian and philosopher in 

 modern times in consequence of the absurd claim to almost 

 infallibility made on his behalf in the Middle Ages. 



The passage to which I refer is in Book I., chapter 6, of 

 his " Meteorologies," where arguing against those who 

 thought that comets were produced by a conjunction of 

 planets, he says that some of the fixed stars have a tail, 

 and that not only the Egyptians assert this, but he can 

 confirm it by his own observation. "For a certain star of 

 those in the thigh of the Dog has a tail, though a dim one ; 

 for its light appears feeble to those who fix their gaze upon 

 it, but greater to those who regard it more indirectly." 



It is evident that the object thus spoken of is the cluster 

 numbered 41 in the catalogue of Messier, who says that it 

 resembles a nebula under a small power, but appears as a 

 cluster of stars when a higher one is used. In Herschel's 

 "General Catalogue" it is No. 1454; in that of Dr. 

 Dreyer (where there is a misprint in the numerical refer- 

 ence to Messier's list) it is Xo. 2287. The cluster is readily 

 visible to the naked eye, and is situated about 4° to the 

 south of Sirius. Yours faithfully, 



Blackheath, November 7th, 1892. W. T. Ly.vn. 



