SIZE AND AGE OF THE UNIVERSE— JEANS 



127 



way we find that the distance of the nearest of these nebulae is about 

 770,000 light-years. 



This is the nebula M 33 in the constellation Triangulum. The 

 second nearest nebula is the well-known "Great" nebula in Androm- 

 eda; this is at a distance about 3 percent greater. In this last 

 nebula, no fewer than 40 Cepheid variables can be detected, but as we 

 pass to more distant nebulae, the number of identifiable Cepheids nat- 

 urally decreases, and this particular method becomes less reliable. 

 Finally it fails altogether through the impossibility of discovering 

 Cepheid variables at all. 



Yet many stars are even brighter than Cepheid variables, and 

 these enable us to carry on with the same method to even greater 

 distances. 



In table 1, the second column shows the distances of eight near 

 objects as determined from the Cepheid variables observed in them. 

 The last column shows the candlepower of the brightest stars ob- 

 served in these objects, that of the sun being taken as unity. 



Table 1 



Object 



Distance in light- 

 years 



Candlepower of 



brightest star 



(Sun=l) 



Large Magellanic cloud 



Small Magellanic cloud 



Globular cluster N. G. C. 6822 



Nebula M 33 



Nebula M 31 (Andromeda) 



Nebula M 101 



Nebula N. G. C. 2403 



Nebula N 81 



85, 000 



95, 000 



620, 000 



770, 000 



800, 000 



1, 300, 000 



2, 000, 000 

 2, 400, 000 



100, 000 

 18, 000 

 15, 000 

 29, 000 

 18, 000 

 22, 000 

 22, 000 

 18, 000 



With one exception, the brightest star in each of these objects 

 has about 20,000 times the candlepower of the sun. Now stars can 

 be identified in 40 nebulae in all, and if we assume that in each 

 of these the brightest star has about 20,000 times the candlepower of 

 the sun, we can immediately estimate the distance of these nebulae 

 also. 



We have been gradually moving farther out into space, and if we 

 still continue our journey, we come in time to nebulae in which even 

 the brightest of stars are invisible. How then can we discover the 

 distances of these nebulae ? 



The answer is that the nebulae — like the stars — appear to be built 

 to pattern. Wlien two stars show the same spectrum and the same 

 period of variability, they belong to the same category, and are of 

 approximately the same candlepower. In the same way, wiien tw-o 

 nebulae show the same build — the same shape and distribution of 



