140 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



determined, the real dimensions of the system were readily calculated 

 from the apparent dimensions. 



The main body of the spiral, the portion that is conspicuous on well- 

 exposed photographs, has a diameter of about 35,000 to 40,000 light- 

 years, and a total luminosity of the order of 1,000 million suns. 

 Extremely faint outer regions, whose existence can be detected by 

 refined methods of investigation, more than double the over-all diam- 

 eter, and add perhaps 60 percent to the luminosity. Thus, the spiral is 

 revealed as a giant nebula, the greatest of all those whose individual 

 distances are known, and comparable with the galactic system itself. 

 It is surprising and even disturbing to find the two greatest known 

 systems both within the local group but, on the basis of our present 

 information, the conclusion seems unavoidable. To this extent we 

 seem to inhabit a favored position in the universe. 



The mass of so immense a system as M31 must also be abnormally 

 large, although the precise quantity has not been determined. The 

 spiral is rotating, and the speed of rotation can be measured with the 

 spectrograph, but, as Zwicky has shown, such data alone do not furnish 

 an unambiguous value of the mass — they may suggest, at best, only a 

 lower limit. The relative, presumably orbital motions of the satellites 

 bear directly on the problem, but we can measure only one component 

 (namely, that in the line of sight) and, moreover, the planes of the 

 orbits together with the protection factors are unknown. Finally, we 

 can suppose that, among stellar systems in general, there exists a 

 characteristic relation between total mass and total luminosity which 

 may be evaluated with the aid of data from the galactic system. This 

 method also is uncertain because it involves several unverified assump- 

 tions such as the detailed similarity of stellar contents and the ratio of 

 dark to luminous material. Nevertheless, the different methods, 

 none of them individually reliable, agree in suggesting a mass of the 

 general order of 100,000 million suns, and we are inclined to believe 

 that the number of ciphers is about correct. 



MESSIER 32 



The two satellites of the giant spiral are both fainter than the 

 average nebula, and one of them, NGC205, is one of the faintest of 

 the known dwarfs. In other words, the triple system covers practi- 

 cally the whole observed range in total luminosities, namely, six 

 magnitudes, or about 1 to 250. The brighter satellite, M32, is 

 superposed on the outer arm of the main spiral, about 24 ' south of 

 the nucleus. The actual location in the line of sight is a matter of 

 speculation. M32 is not a spiral but is a typical example of an 

 elliptical nebula (E2) with a ratio of axes about 8 to 10. It exhibits 

 no structure save for the smooth fading of the luminosity from the 



