142 AJSFNUAL REPORT SMITHSONIAN mSTITUTTON,, 1942 



they can be used as distance indicators because at maximum brightness 

 the ordinary nova comes to about the same candlepower every time. 

 Comparing the apparent brightness with the real brightness, the dis- 

 tance in light-years can readily be computed. Next to the Cepheid 

 variables, novae are the best criteria for measuring distances of gal- 

 axies, providing there are enough found in any given system to stabil- 

 ize the statistics. 



4. Sortie other supergiants. — Two or three magnitudes fainter than 

 the notable S Doradus in the Large Magellanic Cloud are a number 

 of others of the same peculiar spectrum, which are termed the P-Cygni 

 type. There is evidence, not quite conclusive, that these stars are re- 

 lated to the novae, differing most conspicuously, of course, in the 

 property of remaining at high luminosities and not fading away after 

 (he impulsive outburst. But in addition to these bluish hot stars we 

 find in the Magellanic Clouds supergiant red stars, almost as bright. 

 Some of them are variable, and some are, indeed, long-period Cepheid 

 variables, 10,000 times as bright as the sun. Many resemble the 

 famous red giants of our neighborhood, Antares and Betelgeuse. A 

 few greatly exceed the local red supergiants in volume as well as in 

 radiation output. Diameters of the order of the radius of Jupiter's 

 orbit, volumes 10 million times that of the sun, are indicated. These 

 preposterous dimensions are derived by knowing the distance of the 

 Magellanic Cloud, the total candlepowers of these supergiant stars, 

 and the spectra which indicate low efficiency as radiators. In order 

 to give out so much radiation, the emitting surfaces must be exceed- 

 ingly large. The star S Doradus, on the other hand, which is a more 

 efficient radiator, is much smaller and denser than the Antares-like 

 supergiants. 



Adjacent to the Small Magellanic Cloud is the great globular clus- 

 ter, 47 Tucanae, already mentioned as an intermediate between normal 

 galaxies and normal star clusters. Its three brightest stars appear to 

 be typical long-period variable stars, but they are not typical in one 

 respect. Their luminosities at maximum are three or four magnitudes 

 brighter than most long-period variable stars, which are typified by 

 Mira (The Wonderful) Ceti, the first of known variables. It is pecu- 

 liar, too, that the three supergiant variables in the cluster rise to 

 almost exactly the same magnitude at maximum, and all have periods 

 of about 200 days. This is one of the coincidences that the laws of 

 chance do not easily condone ; there must be some deep significance for 

 stars or star clusters in the unusual performance of these supergiants. 



Both blue and red supergiants appear sporadically in our own ga- 

 lactic system; probably the occasionally detectable highly luminous 

 stars in external galaxies also belong to various spectral classes. Dr. 

 Hubble has effectively used the luminosities of these invariable stars. 



