142 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



complementary distribution — supergiants in the outer region, dwarfs 

 and (possibly) giants in the nuclear region. Among nebulae in 

 general, the relative distribution varies systematically through the 

 sequence of structural forms, the supergiants working inward as the 

 spirals unwind. The characteristic behavior offers a possible observa- 

 tional approach to the fundamental question of nebular evolution, 

 the first problem being a study of the systematic variation in stellar 

 contents of nebulae along the sequence of forms. This problem 

 requires the greatest possible light-gathering power and will be espe- 

 cially suitable for the 200-inch reflector when that telescope is com- 

 pleted. Meanwhile, the preliminary investigations are carried on 

 with the 100-inch, and some of the results for M31 will presently 

 be discussed. 



NGC205 



First, however, some mention should be made of the thud member 

 of the triple system, the fainter satellite, NGC205. It lies about 37' 

 north preceding the nucleus of the main spiral, almost exactly along 

 the minor axis. As in the case of M32, the actual position in the line 

 of sight is wholly speculative. However, the radial velocity is 

 appreciably the same as that of the spiral, and, consequently, the 

 orbital motion must be almost entirely across the line of sight. This 

 situation is consistent with the assumption that the satellite may lie 

 close to the fundamental plane of the spiral. The distance between 

 the two nuclei would then be of the order of 30,000 light-years, and 

 the satellite would be located within the extremely faint, tenuous, 

 outer extensions of the spiral. 



NGC205 is a nondescript system. In form and general texture, it 

 resembles an elliptical type (E5) with a ratio of axes about 2 to 1, 

 but the concentration toward the center is very low, small obscuring 

 clouds are involved, and several individual stars and small nebulous 

 objects (presumably globular clusters) seem to be associated with the 

 satellite. The dimensions, as usual, vary with exposures, but the 

 diameters are generally quoted as 8'X4' or 1,600X800 light-years, and 

 the total luminosity as about 7 million suns. 



STELLAR CONTENTS OF MESSIER 31 



OBSCURATION 



Since the two satellites are unresolved, the main spiral alone offers 

 an opportunity to examine the stellar contents of a nebula. The 

 investigation reveals many analogies with the galactic system, con- 

 sistent with the current picture of our own nebula as a spiral at a 

 stage in the sequence somewhat later than M31. One of the most 

 conspicuous features is the great amount of obscuration. The 



