198 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1933 



to these objects, since all show distinctly an extremely flattened disk- 

 like form with a marked central condensation from which spiral 

 arms, with knots or condensations distributed throughout their 

 length, unwind. It surely does not require a very vivid imagination 

 to picture our own galaxy as a great spiral nebula with a flattened 

 disklike form, the local cluster and the Milky Way clouds being the 

 condensations in the arms of this spiral. The main difficulty is the 

 question of size, as our galaxy is undoubtedly four or five times 

 greater in diameter than the largest known spiral, the Great Nebula 

 in Andromeda. 



And yet if we examine photographs of the two nearest spirals, 

 Messier 33 in Triangulum and Messier 31, the Great Nebula in Andro- 

 meda, we see so many similarities that the essential identity in struc- 

 ture of these spirals and our galaxy can scarcely be doubted. The 

 spiral in Triangulum with a distance of some 900,000 light-years and 

 a diameter of 15,000 is well resolved into stars and star clouds by 

 the 100-inch telescope, and I have always considered this spiral as 

 being a model about one-fifteenth in size of our own galaxy. But 

 the similarity in structural detail is even more marked in the An- 

 dromeda nebula, which at a distance of 900,000 light-years has a 

 diameter of 45,000, about one-fifth of the galaxy. A photograph of 

 part of the Andromeda nebula on a large scale with the 100-inch 

 telescope shows so striking a resemblance to the Milky Way clouds 

 that, especially if we remember the difference in scale and that the 

 fainter stars cannot show in the Andromeda on account of the great 

 distance, no one who sees them can doubt the essential similarity of 

 structure. 



We can then, I think, legitimately assume that the galaxy is a 

 great discoidal aggregation some 200,000 light-years in diameter and 

 10,000 in thickness, built up of stars and star clouds which gradually 

 merge into one another at the edges where the stars are thinner. It 

 also seems legitimate to assume, from analogy with the external 

 galaxies, the spiral nebulae, that it has a marked central condensa- 

 tion, indicated by the richness of the star clouds in the direction of 

 the center in Sagittarius, and a definite spiral form with the Milk^' 

 Way clouds as the condensations along the spiral arms. Our sun 

 is near the center of one of these condensations, the local cluster, 

 situated about halfway between the center and the edge of the whole 

 system. It seems obvious, from our position within the system, that 

 the structure of the adjacent condensations, particularly when com- 

 plicated by the presence of absorbing matter or clouds, so prominent 

 a feature of both the galaxy and the spirals, could easily reproduce 

 the structure we see in the Milky Way clouds. Although we cannot 

 prove positively that the galaxy is so constituted, any evidence we 



