128 ANNUAL REPORT SMITHSONIAN INSTITUTION, 194 5 



(If there is obscuring matter between us and the star its dimming effect 

 must be found and allowed for before the comparison of real and ap- 

 parent brightness is made.) 



Table 1. — Brightness of Cepheid variables of different periods 

 _. . . . , Number of times brighter 



Period in days: than sun 



2#___." 350 



5 550 



10 830 



20 1,500 



40 2,500 



The two nearest of the spiral nebulae were in this way found to be 

 so distant as to be well separated from the outermost stars of our 

 galaxy. The estimated distance from side to side of the Milky Way 

 system is about 100,000 light years. Seven or eight such systems 

 could be laid side by side in empty space between our stars and those 

 of the great nebula in Andromeda, one of our two nearest neighbors, 

 before the gap would be bridged. 



A second point of similarity between our galaxy and the spiral 

 nebulae is size. The sizes of extragalactic nebulae, as found from 

 their angular diameters and distances, approach nearly enough the 

 size of the stellar system to warrant their being classed with it rather 

 than with even the largest of our own star clusters or star clouds, such 

 as are seen along the Milky Way. 



Third, the similarity of our star system and the spiral nebulae is 

 shown in the flattened shape of each (plate 1). Herschel, with un- 

 limited patience, counted stars in all directions and found that the 

 stellar system is "shaped like a grindstone," to use his own compari- 

 son. More recent counts amplify Herschel's discovery by showing 

 that the sun and the solar system are not centrally situated in our 

 galaxy. To the naked eye or with a small telescope there seem to be 

 about as many stars in one direction as in another in the plane of the 

 Milky Way, but with a telescope that will penetrate to greater depths 

 many more stars can be seen in the direction of Sagittarius than in 

 that of Auriga. The reason is that we are far out from the center in 

 Sagittarius toward the "anticenter" (the point opposite the center) 

 near Auriga. Measurements of a different kind place us about two- 

 thirds of the way from the center of the circumference. But still we 

 are nearly in the plane of the Milky Way. 



Finally, the motion of our galaxy is similar to the motion of the 

 spiral nebulae. It is now definitely known that the spirals are rotat- 

 ing. For this belief is there not strong presumptive evidence in the 

 appearance of the spiral nebulae ? There seems little doubt that they 

 have acquired their flattened pin wheel appearance (plate 2) from 

 rotation about a center, for a group of stars would in time collapse to 

 their common center if it were not for the centrifugal effect of motion 



