Section III, 1922 [311] Trans. R.S.C. 



Sahas Ionization Hypothesis 



By H. H. Plaskett 



Presented by J. S. Plaskett, F. R.S.C. 



(Read May Meeting, 1922) 



The work of McLennan, Franck and Hertz and others has shown 

 that there are associated with the production of a single line spectrum, 

 an arc spectrum, an enhanced spectrum, certain definite amounts 

 of energy. Saha, using the Nernst Heat theorem and these known 

 ionization potentials of various elements, has obtained a physical 

 interpretation of stellar spectra. Inversely from the appearance of a 

 given spectrum in a star he has been able to compute the stellar 

 temperatures. The study of early type high temperature stars at 

 this observatory has shown, however, that this hypothesis is open 

 to two main criticisms. 



(1) The use of the Nernst Heat theorem implies that the cause 

 of ionization in stellar atmospheres is radiation from the photosphere 

 (J. A. Anderson, E. A. Milne). Now some B-type stars (effective 

 temperature lo,000°K) show the Balmer series as emission. From 

 Bohr's theory it is possible to compute the necessary energy to enable 

 one H atom to emit the Balmer series. From Planck's law it is like- 

 wise possible to compute the available radiation energy emitted at 

 any wave-length per sq. cm. per sec. Carrying out the necessary 

 computations it is found that at 15,000° K there is enough radiant 

 energy to place 10^^ H atoms per sq. cm. column in a disturbed con- 

 dition. Of the radiation proceeding radially outward from the photo- 

 sphere certain of these lO^" atoms will absorb at i^T 10^ ergs per sec. 

 The 10-" atoms will emit in all directions 10^ ergs per sec. of Hy radiation . 

 Of this 10^ emitted ergs only 10"^^ will proceed radially outward on 

 a star the size of the sun. The net result is that if radiation is the 

 only factor at work, at Hy 10^ ergs will be absorbed radially and only 

 10"^^ re-emitted radially. In other words, we should only obtain an 

 absorption spectrum. The fact that an emission spectrum is obtained 

 shows that there must be at work a far more powerful source of 

 ionization than radiant energy. The fact that at 15,000°K there are 

 probably some 10^^ free electrons per cu. cm. with velocities of thermal 

 agitation sufficient to ionize H atoms shows that electron collisions, 

 not absorption of radiation, is the probable cause of ionization. 



