THE NATURE OF THE NEBULAE — HUBBLE 141 



nucleus outward to undefined borders. The size of the image in- 

 creases steadily with exposure times (the isophotal contours remaining 

 similar ellipses), as far as the exposures have been pushed. Although 

 the dimensions have little meaning apart from the observing condi- 

 tions, the values usually quoted are a major diameter of 4' or 800 

 light-years, and a total luminosity of the order of 20 million suns. 

 The values refer to the conspicuous portion of the image on well- 

 exposed photographs. 



Very short exposures with the 100-inch telescope show a round, 

 semistellar nucleus, about 2" in diameter and about 13.4 magnitude. 

 This diameter merely reflects the resolving power of the photographic 

 plate, for visual examination shows a still smaller nucleus. Sinclair 

 Smith, using an interferometer on the 100-inch, found no trace of a 

 strictly stellar center, and concluded that the true nucleus is probably 

 a globular mass with a diameter of perhaps 0''8, or about 2 light-years. 

 The concentration of luminosity within such a nucleus would be 

 somewhat greater than that in the central region of the most compact 

 globular cluster — in other words, not inconsistent with the assump- 

 tion that the nebula is a system of stars. 



The nucleus of M32 has been described at some length because, as 

 far as observations go, it appears to be typical of nebulae in general, 

 and, being relatively near, can be studied in greater detail. The 

 nucleus of our own system is presumably hidden behind dark obscur- 

 ing clouds in the Milky Way. Baade, using red-sensitive plates and 

 heavy filters, has partially penetrated the clouds, and, in the direction 

 of the galactic center and at latitude 4°, has recorded a star-density 

 which is of the order of 800,000 per square degree. Such a density 

 at perhaps 2,000 light-years from the actual center requires no un- 

 reasonable extrapolation to suggest a nuclear density comparable 

 with those in M32 and M31. 



Since the satellite is superposed on the outer, partially resolved 

 region of the main spiral, the search for individual stars is somewhat 

 confused. It can be stated that there is no definite trace of resolu- 

 tion in the satellite, and, consequently, there are probably no super- 

 giants in the system. The available photographs probably establish 

 this conclusion down to M=— 2, and possibly to M= — 1. Never- 

 theless, Sinclair Smith's investigations indicate that the assumption 

 of a stellar constitution offers the most plausible and consistent 

 working hypothesis that has been formulated. Thus we are pre- 

 sented with the problem of a stellar system in which supergiants 

 are not present. 



The problem assumes general significance because the satellite 

 closely resembles the nuclear region of the main spiral in color and 

 spectrum as well as texture, and both nebulae, as previously men- 

 tioned, are typical examples of their classes. The spiral exhibits a 



