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Astronomy. — ''The luminosity of stars of different types oj 

 spectrum." Bj Dr. A. Pannekoek. (Communicated by Prof. 

 H. G. VAN DE Sande Bakhuyzen). 



The investigation of the spectra of stars which showed that, with 

 a few exceptions, they can be arranged in a regular series, has led 

 to the general opinion that they represent different stages of develop- 

 ment gone through by each star successively. Vogel's classification 

 in three types is considered as a natural system because these types 

 represent the hottest and earliest, the further advanced, and the 

 coolest stage. This, however, does not hold for the subdivisions : 

 the difference in aspect of the lines, the standard in this case, does 

 not correspond to the different stages of development mentioned above. 

 Much more artificial is the classification with letters, which Pickering 

 has adopted in his Draper Catalogue; it arose from the practical 

 want to classify the thousands of stellar spectra photographed with 

 the objective prism. After we have allowed for the indistinctness 

 of the spectra which, arising from insufficient dispersion and brightness, 

 influenced this classification, the natural affinity between the spectra 

 will appear and then this classification has the advantage over that of 

 VoGEi, that die 2"^^ type is subdivided. The natural groups that can 

 be distinguished are: class A: the great majority of the white stars 

 (Sirius type), Vogel's la; class B: the smaller number of those stars 

 distinguished by the lines of helium, called Orion stars, Vogel's lb. 

 In the continuous series the latter ought to go before the first type 

 and therefore they are sometimes called type 0. Class F forms the 

 transition to the second type (Procyoii) ; class G is the type of the 

 sun and Capella (the E stars are the indistinct representatives of this 

 class); class K contains the redder stars of the 2^^ fjp^' which ap- 

 pi'oach to the 3^^ type, such as Arcturus (Pickering reckons among 

 them the H and I as indistinct representatives). The 3^ type is 

 called in the Draper Catalogue class M. 



The continuity of the stellar spectra is still more evident in the 

 classification given by Miss A. Maury. (Annals Harv. Coll. Obs. Bd. 28). 

 Miss Maury arranges the larger number of the stellar spectra in 20 

 consecutive classes, and accepts groups intermediate to these. The 

 classes I — IV are the Orion stars, VI —VIII constitute the first type, 

 IX— XI the transition to the 2^ type, XIII— XIV the 2^ type 

 itself such as the sun, XV corresponds to the redder Arcturus stars, 

 XVII — XX constitute the third type. If we consider that from class 

 I to III a group of Hues is gradually falling out, namely the hydrogen 

 lines of the other series, which are characteristic of the Wolf-Rayet 



