STELLAR LABORATORIES — DUNHAM 261 



It is asking a great deal to expect to find out the detailed structure 

 of an object which never appears otherwise than as a point of light, 

 even in the largest telescope, but I hope to show that some of our 

 questions about such an object can nevertheless be answered. 



The distance of a star was first measured by the amount of change 

 in its apparent direction when the observer was carried from one side 

 of the earth's orbit to the other side six months later. Even before 

 the distance of any star was known, astronomers had noticed that 

 some stars had changed their positions over a period of years. Arc- 

 turus had moved rather more than 1 degree since the beginning 

 of the Christian era. This might mean either that Arcturus was 

 close to us and moved slowly, or that it was far away and traveled 

 very fast. As soon as the distance became known, however, the 

 apparent motion could at once be translated into true motion, and 

 this turns out to be 84 miles per second. Most stars are found to be 

 moving at a more deliberate pace, but 10 to 20 miles per second is 

 very common among the stars^ 



It is easier to picture how far away a star like Arcturus must be, 

 if we realize that even when moving at 8i miles a second it takes 

 about 800 years for its direction to change by as much as the moon's 

 apparent diameter. 



Knowledge of the distance of a star at once unlocks another most 

 important fact, namely, its true luminosity or candlepower. A star 

 will look bright either if it is very close and of moderate brightness 

 or if it is far away and extremely brilliant. If the distance can 

 be measured, we can decide immediately between these two possibili- 

 ties, and from the apparent brightness we can find the actual candle- 

 power of the star. 



It turns out that the candlepower of the stars varies to a remark- 

 able degree. There is, in fact, as much difference between the faint- 

 est and the brightest as between a firefly and a powerful searchlight. 

 Our sun holds a place half-way between these limits and would 

 correspond to an ordinary household electric light. 



In marked contrast to this great range in intrinsic brightness 

 among the stars is their unexpected similarity in mass. It would 

 be extremely difficult, and perhaps impossible, to determine the mass 

 of a star were it not for the fortunate circumstance that many pairs 

 of stars are known in which the two are held together by their 

 mutual attraction while they revolve around a common center of 

 gravity in elliptical orbits. If we know how far away such a pair 

 of stars is from us, we can find out their masses from the speed with 

 which they swing each other around in their orbits. The interesting 

 result is that when we compare the masses of the stars with that 

 of the sun we find them all very much alike. For there are a few 



