438 



THE POPULAR SCIENCE MONTHLY 



class emit a far greater amount of total radiation than do the blue 

 stars; that they have a higher emissivity, or, in other words, that they 

 are cooling faster than the blue stars. (In parenthesis it may be added 

 that it is doubtful whether the interior of a red star is cooler than a 

 blue star. The whole mass is no doubt shrinking and the temperature 

 may be actually rising. But the idea to be conveyed is that in the 

 various stages of stellar evolution, the " red star stage " seems to be the 

 one in which a star is "burning out" the fastest.) The first method, 

 viz., measuring the total radiation from red and blue stars having the 

 same photometric brightness, as already mentioned, is somewhat uncer- 

 tain, because the radiation received from a star is a function of its size, 

 distance, temperature and especially its emissive power. Some of the 

 largest stars are no doubt the farthest from us. The second method of 

 observation consisted in roughly separating the star's rays into a 

 spectrum, by means of an absorption cell of water which absorbs most 

 of the infra-red rays and transmits the visible rays. Hence, by measur- 

 ing the total radiation from a star, and also the part which is trans- 

 mitted by the absorption cell of water we obtain an estimate of the rela- 

 tive amounts of energy in these two parts of the spectrum. This 

 measurement is a ratio of two quantities of energy; and hence is inde- 

 pendent of the size, the distance and the temperature of the star. It 

 gives us direct information of the emissivity of the different parts of 

 the star's spectrum. It is true that it gives us information of only two 

 parts of the spectrum; but, from our knowledge of the solar spectrum, 

 and of the spectra of terrestrial substances, this information enables us 

 to make important deductions as regards the distribution of energy in 



the spectra of stars. 



Table I 



Object 



Magni- 

 tude 



/3 Ononis 0.34 



aOrionis 0.92 



e^Tauri 3.62 



eTauri 3.63 



,/Tauri 3.94 



^Taiiri 3.86 



jTauri I 3.93 



a Auriga 0.21 



aTauri 1.06 



Deflec- 

 tion 



SCeti 

 V Ceti 

 <P 



Pegasi , 5 



4.04 

 4.18 

 23 



2.501 



22.4 



0.18 



0.35 



0.12 



0.36 



0.52 



6.14 



6.78 



6.84 



0.08 



0.31 



0.22 



Type 



£8 p. 



Ma 



Ab 



K 



A 



G 



K 



G 



Kb 



B2 

 Ma 

 Ma 



Object 



19 Piscium 



-y Aquarii 



X Aquarii 



5 Capricorni 



/3 Aquarii 



/3 Ophiuchi 



6 Ophiuchi 



a Goronse Borealis... 



y Draconis 



/3 Ur^aj Majoris 



y Draconis 



Magni- 

 tude 



5.30 



3.97 

 3.84 

 2.98 

 3.07 

 2.94 

 3.03 

 2.32 

 2.42 

 2.44 

 2.42 



Deflec- 

 tion 



0.46 

 0.24 

 1.02 

 0.28 

 0.55 

 0.37 

 1.37 

 0.48 

 1.59 

 0.37 

 1.58 

 1.67 

 1.64 



Type 



N 



A 



Ma 



Ab 



G 



K 



Ma 



A 



Kb 



A 



Kb 



Some of the data obtained by measuring the total radiation from 

 blue and from red stars, having the same brightness, will now be dis- 

 cussed. It was possible to make quantitative measurements on stars 



1 Galv. Sensitivity i = 1 X lO"'" Amp. 



