MEASURING HEAT FROM STARS 439 



down to the 5.3 magnitude. It is to be remembered, in considering 

 this data, that a star of say the second magnitude is only %.5 as bright 

 as a star of the first magnitude, i. e., one magnitude differs from 

 another by the factor 2.5 in brightness. However the total radiation 

 may be entirely different as may be noticed in Table I. In this table 

 the last column gives the spectral classification used by astronomers. 

 The blue stars have the classification B, A. These stars pass into the 

 yellow gradation F, G, K, the latter being yellowish-red. The red stars 

 are Class -1/ and the deep red ones are Class N. Keeping in mind 

 this classification, it may be noticed that the red star a Orionis emits 

 about eight times as much total radiation as the blue star, jS Orionis, 

 which is much brighter to the eye. A similar example is the yellow 

 star a Auriga (Capella) and the beautiful red star, a Tauri (Alde- 

 baran), both of which stars are familiar objects. The latter is only 

 about one-half as bright to the eye; and yet it emits the same amount 

 of total radiation as does the brighter star. Comparing stars of the 

 same photometric brightness the 5.3-magnitude stars (f> Pegasi and 

 19 Piscium are interesting examples because of their smallness, and 

 because the latter is of a deep red color. The latter follows the general 

 rule that the redder the star the greater the amount of total radiation 

 received. The number of these very red stars is very small and they 

 were not conveniently situated for observation. However from the 

 observations on numerous stars of Class M as compared with blue and 

 yellow stars of the same photometric magnitude, it is to be expected 

 that these very red stars, Class N, will be found to have, as a general 

 rule, the highest emissivity of all. Among all the data collected, on 

 105 stars, there are no exceptions to the general classification, viz., the 

 redder the star the greater the amount of total radiation emitted. To 

 some, of course, this information is not unexpected. However, if the 

 reader will pause for a moment and consider that some of these measure- 

 ments were made on starlight which left its source more than 160 years 

 ago, and that stellar distances are so inconceivably great that another 

 160 years must elapse before the arrival of starlight which is being 

 emitted at the present moment, it will be evident that every measure- 

 ment has a value of far greater importance than merely confirming our 

 expectations which are based upon our preconceived notions of what is 

 occurring on a star and in passing through interstellar space. 



The second method of studying the quality of the radiations of red 

 and of blue stars, by means of the absorption cell of water, is more 

 limited in range, because of the weakness of the radiations received. 

 Only a few stars could therefore be investigated by this method. From 

 the first method of observation it is to be expected that the total radia- 

 tion from a red star contains more infra-red rays than does the total 

 radiation from a blue star, and hence the amount transmitted bv the 



