150 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1935 



stars are found. These two classes of stars, to which the names 

 " giants " and "dwarfs " have been given by astronomers, differ 

 enormously in luminosity, the average low-temperature giant being 

 at least 10,000 times as bright as the corresponding dwarf. This 

 conclusion, originally drawn from trigonometric observations, is 

 fully confirmed by the luminosities of nearly 4,200 stars recently 

 determined by the spectroscopic method. 



In general, stars may be divided according to luminosity into 

 three classes, dwarfs, giants, and extremely bright stars that may be 

 called supergiants. The separation between dwarfs and giants is 

 widest among the stars of lowest temperature and becomes pro- 

 gressively less for stars of higher temperature, the dwarfs becoming 

 brighter and the giants slightly fainter. Some stars of intermediate 

 luminosity appear among these stars of increasing temperature, and 

 the two chains of dwarfs and giants come together and may even 

 cross among stars somewhat hotter than the sun with temperatures 

 of 7,000° or 8,000° C. The supergiants, which include the most 

 luminous stars in the sky, are found among stars of all tempera- 

 tures and show a much greater range in luminosity than either the 

 giants or the dwarfs of the same spectral type or temperature. 

 They are especially numerous among the stars with temperatures 

 slightly higher than that of the sun; in fact, stars of these types 

 seem to contain few normal giants, the great majority being either 

 dwarfs or stars of the supergiant class. Another interesting fact 

 about the supergiants is that a large proportion of them vary in 

 light, and it seems probable that such variation indicates a kind of 

 instability that is associated with great size and high luminosity. 



Perhaps the single most interesting result which comes from a 

 study of the luminosities of stars is the remarkable tendency to 

 group around definite values of brightness. This is especially 

 marked in the case of giant stars, although very evident among the 

 dwarfs. About 90 percent of giant stars with temperatures near 

 4,000° C. have the same luminosity within a range of twofold or 

 threefold. In other words, most stars of a given temperature, like 

 the bulbs of our electric lamps, are built to give out a definite num- 

 ber of candlepower and do not show the almost infinite range in 

 luminosity we might so readily expect. The luminosity especially 

 favored among the giant stars with temperatures less than our sun 

 is about 100 to 150 times that of the sun. It does not change rapidly 

 with decrease of temperature but on the whole increases slightly for 

 the cooler giants. This is doubtless due to the larger size of these 

 stars, the increase in the area which emits light more than counter- 

 balancing the smaller amount given out by each unit of surface. 



