THE NATURE OF THE NEBULAE — HUBBLE 143 



extreme north following end of the spiral is lightly veiled, and the 

 central half of the nebula, north preceding the major axis, is fainter 

 than the south following half. The latter feature is frequently used 

 in attempts to determine the true orientation — to identify the side of 

 the nebula which is nearest the observer. The question is still un- 

 answered, and with it another fundamental question concerning the 

 direction of rotation — whether the spiral is "winding up" or "un- 

 winding." 



In addition to the veiling of large areas, many obscuring clouds 

 are scattered over the nebula. The absorption has not been meas- 

 ured, but it seems likely that the "all or none" principle is approxi- 

 mated. On the one hand, certain clouds seem to exhibit complete 

 absorption and, moreover, the spiral obscures the faint, distant, back- 

 ground nebulae in a very conspicuous manner. On the other hand, 

 stars of various known types, widely scattered over the spiral, exhibit 

 only the normal dispersion about their mean luminosities, and the 

 behavior suggests that the systematic effects of absorption on these 

 particular stars are either negligible or curiously uniform. The ques- 

 tion of absorption is important and deserves further investigation, 

 for it is one of the factors involved in the precise determination of 

 the distance of the nebula. At present the effect is ignored in the 

 calculations but is recognized as an uncertainty which operates in a 

 specific direction. 



STAR CLOUDS AND OPEN CLUSTERS 



The upper limit of stellar luminosities in M31 is probably of the 

 order of M=— 6, or somewhat fainter than the corresponding limit 

 in the galactic system. Stars brighter than M= — 5 are not numerous, 

 but thereafter the numbers increase rapidly with diminishing lumi- 

 nosity as would be expected in a great stellar system. The distribu- 

 tion of the supergiants (the only stars which can be studied individu- 

 ally) follows the spiral arms much as, in our own system, they follow 

 the Milky Way. Star clouds are conspicuous ; one, for instance, was 

 listed in the early catalogs as a separate nebula, NGC206. Open 

 clusters are also found, and in at least one case a typical A-type 

 spectrum has been recorded. Emission nebulosities (of the type of 

 the Orion nebula) have not been recognized; but such objects are 

 known to be a characteristic feature of later type spirals, and their 

 absence in M31 is expected. 



CEPHEIDS 



Three types of objects have been found in large numbers and 

 studied in some detail, namely, Cepheid variables, novae, and globular 

 clusters. The first Cepheid was discovered in 1923; systematic 



