THE NATURE OF THE NEBULAE HUBBLE 147 



The outbursts occur with a frequency of the general order of one per 

 stellar system per several (possibly four or five) centuries. The fre- 

 quency in giant nebulae, such as M31 and our own galactic system, is 

 presumably greater than the average, and, consequently, we might 

 expect to find records of several examples scattered through the 

 history of the last 2,000 years. Plausible instances have been assem- 

 bled (for example, a nova outburst in 1054, whose remnants are 

 probably now observed in the Crab nebula, and Tycho's nova of 1572), 

 but in general it is not possible to establish specific, individual cases. 



GLOBULAR CLUSTERS 



A large number of objects have been found in M31 which, from 

 their forms, structures, colors, and spectra, have been provisionally 

 identified as globular clusters, although no individual stars can be 

 seen. The 140 known objects form a homogeneous group in which 

 the intrinsic luminosities range from M=— 4.7 to —7 and thus 

 exhibit a relatively small dispersion around the mean value, which is 

 about —5.3. 



That globular clusters in the galactic system also form a homo- 

 geneous group is indicated by Shapley's classical investigation of their 

 distances. When his peculiar scale of total luminosities (which he 

 humorously calls convenient rather than conventional) is corrected, 

 the absolute magnitudes range from about —5 to —9, with an ex- 

 ceptional cluster, Omega Centauri at —9.8. The mean value is 

 near —7.3. 



Thus the two groups overlap to a considerable extent, but the 

 clusters in the galactic system are systematically brighter than those 

 in the spiral by about two magnitudes. The discrepancy is fully 

 established, and represents an intrinsic difference in the group char- 

 acteristics. The brightest of the objects in M31 is fainter than the 

 mean of the galactic globular clusters. The discrepancy, however, is 

 not so disturbing as that formerly presumed in the case of the novae, 

 because globular clusters are known in the Magellanic Clouds, and 

 they also appear to be systematically fainter than the galactic objects — 

 comparable, in fact, to the objects in M31. Moreover, similar objects 

 are recognized in several of the nearer nebulae, and the average lumi- 

 nosities vary from system to system. These considerations detract 

 from the value of globular clusters as criteria of distances, but they 

 suggest a new field of investigation, namely, the comparative study 

 of the group characteristics of globular clusters in different environ- 

 ments. 



Unlike the clusters in the galactic system, which are believed to 

 exhibit a spherical distribution, those in M31 follow the flattened, 

 lens-shaped distribution of the general luminosity. The novae 



114728—39 11 



