ASTRO-PHYSICS 289 



brightness, for on the average the fainter a star 

 is the greater is its distance. 



Certain stars show variations in the intensity 

 of their light, with short periods ranging from 

 a day to one or two months. A definite relation 

 has been discovered between this period and the 

 absolute brightness of the star in cases where 

 the distances are known. Extending the same 

 relation to a variable star at an unknown distance, 

 an observation of the period gives its absolute 

 brightness, which, compared with the apparent 

 brightness, indicates the distance. 



Globular clusters of stars of one type are 

 found to possess approximately equal dimensions, 

 so that the apparent diameter of the cluster 

 enables an observer to make a guess at its 

 distance. 



The average velocity of the arms of spiral 

 nebulae in the line of sight as measured spectro- 

 scopicallyis several hundred kilometres per second. 

 This gives a rough value for the distance of 

 those nebulae which show an angular movement 

 of the arms across the line of sight, and a lower 

 limit for the distance of those too far away for 

 those movements to be measurable. 



We can now appreciate the methods by 

 means of which the dimensions of our stellar 

 system have been estimated, and may pass to 

 consider some of the results. 



The mean diameter of the earth's orbit is 

 185.6 million miles. That of the orbit of 

 Neptune, the outermost known planet, is about 

 5600 million miles, and this may be taken as 

 the size of our solar system. Light would take 

 S.^6 hours to travel across it. 



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