210 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 3 3 



of distant stars. The effect of such an absorbing medium on the determina- 

 tions of the dimensions of the galaxy are fairly obvious when it is remembered 

 that these dimensions depend principally upon the known intrinsic brightness 

 of the Cepheid variables. The inverse square law enables the distance of 

 any star to be determined if its intrinsic and apparent brightnesses are known. 

 Any absorbing matter between us and the star will make it appear fainter and 

 its distance will hence be measured too great. 



Various determinations of the decrease in apparent brightness produced by 

 the galactic absorbing matter have been made, but the most recent and direct 

 is by Stebbins at Mount Wilson, who by means of a photoelectric photometer 

 determined how much the globular clusters, from whose distribution the esti- 

 mate of a diameter of the galaxy of 200,000 light-years was made, were red- 

 dened by this absorbing matter. The startling conclusion was reached that 

 the presence of this obscuring material had made the previous estimates of the 

 distances of the globular clusters about twice too great and that the actual 

 diameter of the galaxy is only about 100,000 light-years instead of the 200,000 

 obtained without consideration of the presence of absorbing material. 



This decreased diameter has been convincingly confirmed by a development 

 of the theory of the rotation of the galaxy discussed in the last section of 

 the lecture. If the galaxy is in rotation, and this is now almost universally 

 accepted by astronomers, the theory shows, and indeed it follows directly from 

 Kepler's third law, that the distance of the sun from the gravitational center 

 of the galaxy can be obtained from certain constants of the rotation. These 

 constants can be derived from an analysis of the radial velocities and proper 

 motions of distant stars, and this has been done by the author and Dr. J. A. 

 Pearce in collaboration at Victoria for the most distant stars that can be 

 readily observed. Earlier determinations by this method were considerably 

 smaller, but the Victoria value from increased and more accurate material 

 gives the distance of the sun to the center as some 33,000 light-years. The 

 sun, from reliable evidence, is about two-thirds of the distance outward from 

 the center of the edge of the galaxy, which makes the diameter 100,000 light- 

 years. The agreement by these two entirely different and independent methods 

 is strong evidence of the essential correctness of this revised diameter of 

 100,000 light-years for the galaxy. 



This reduces the disparity in size between the galaxy and the Andromeda 

 nebula to a little over 2 to 1 instead of the 4 or 5 to 1 given in the lecture. 

 But this disparity almost entirely disappears by reason of recent measures, 

 which double the apparent size of the Andromeda. Hubble has discovered 

 over 100 objects on photographs of this nebula, which he identifies as globular 

 clusters, and their distribution indicates that the nebula must extend to about 

 twice the size shown by long-exposure photographs. Similarly, within the last 

 few months Stebbins has measured by a photoelectric photometer the out- 

 lying regions of the Andromeda nebula and finds that the luminous star clouds 

 extend also to about twice its apparent diameter. 



This obviously makes the Andromeda nebula of practically the same dimen- 

 sions as the galaxy and enormously strengthens the conclusion reached in 

 the lecture of the identity in structure and general characteristics of the two 

 systems. It renders improbable and unnecessary the hypothesis of some 

 astronomers that the galaxy is not a single unit rotating in its own plane, 

 similar to the external nebulae we see all around us, but is a large complex 

 organization of loosely aggregated discrete nebulae. It seems to me that the 

 removal of the disparity in size between the Andromeda and the galaxy has 

 removed the necessity or probability of such hypotheses and that we may now 

 accept readily our inherent belief in the homogeneity of the cosmos. 



