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ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 2 



strated that a relation exists between period and acfiud or absolute 

 brightness. In other words, if we Imow the actual brightness of one 

 Cepheid variable, then we can compute the distance of any other 

 star of this type, once its period and apparent brightness are known. 

 This is equivalent to saying that we must know the " zero point " of 

 the period-luminosity relation in order to use it effectively. The 

 zero point is determined from the apparent motions of the stars, 

 since those nearest to us appear to move the fastest; hence the dis- 

 tance and therefore the brightness of an average star may be known. 



This important relation be- 

 tween brightness and period 

 is illustrated in Figure 2. 

 The period-luminosity re- 

 lation proved to be the key 

 that unlocked one of the 

 secrets of the universe. The 

 key was immediately seized 

 by a young astronomer at 

 Mount Wilson Observatory, 

 and soon w^e knew the dis- 

 tances of all the far-away 

 clouds in which the telescope 

 revealed the presence of 

 Cepheid variables whose pe- 

 riods could be determined. 

 This young astronomer was 

 Dr. Harlow Shapley, the 

 present director of Harvard 

 College Observatory. 

 The enormous brightness of Cepheids favors their use as yard- 

 sticks in measuring the size of the universe. The faintest of these 

 stars, the so-called cluster-type Cepheids, are intrinsically about a 

 hundred times as bright as our sun; consequently one of these stars 

 can be seen wdien it is ten times as far away as a star of the bright- 

 ness of our sun. The brightest representatives of the Cepheid class, 

 which are about 10,000 times the brightness of the sun, can be seen 

 at vastly greater distances. The huge 100-inch Mount Wilson tele- 

 scope shows stars which give less than one-millionth the amount of 

 light of the faintest stars visible to the unaided eye, and conse- 

 quently with it one sees stars over a thousand times more distant. 

 In fact, Cepheids on the outskirts of our universe and among the 

 great star clouds of the Milky Way system are visible in the Mount 

 Wilson telescope. The farther cluster of stars measured by Shapley 



^ 1 2 1 8 16 32 DAYS 

 Figure 2. — The period-luminosity relation 

 Horizontal axis is period in days, and verti- 

 cal axis is intrinsic luminosity in terms of sun's 

 brightness. The zero point is fixed when it is 

 kjiown, for example, that the average Cepheid 

 with a period of two days is 200 times as bright 

 as the sun. 



