VARIABLE STARS 



By Dr. L. V. Robinson 

 Astronomer, Harvard College Observatory 



The layman who has casually observed that the Big Dipper is 

 always a dipper regardless of its position in the sky may also have 

 noticed that all the stars appear to keep the same positions relative 

 to one another. In general, the only exceptions are the planets. 

 The layman, from somewhat hurried glances, may also be inclined to 

 think that all the stars remain unchanged in brightness. If he could 

 observe the stars a few thousand 3'ears hence, however, he would be 

 convinced that " the stars do move," and a few days or weeks of con- 

 tinuous observation would also show changes in brightness of many 

 of the brighter stars. The sun shares the motions of the other stars ; 

 in fact, it is dragging our little earth through space with a velocity 

 of 12 miles per second relative to the other stars; but we move at 

 a very slow rate compared with some of the suns, which are so far 

 away in space that we call them stars. 



The reason why the stars do not appear to move is that they are so 

 exceedingly far away. Even the nearest is 275,000 times as far away 

 as the sun is from us (that is 275,000X92,900,000 miles), and light 

 from this star does not reach us until it has been on its journey four 

 and one-third years. Light from the sun reaches us after a short 

 journey of only eight minutes. By the way of contrast, it is said that 

 the sensation of a burn travels so slowly relative to the speed of 

 light, that an infant with an arm long enough to reach the sun 

 would live his allotted time of three score and ten years and die 

 in ripe old age before learning that his fingers had been burned. 



But what of the brightness of all these far-away suns ? By actu- 

 ally measuring their distances, astronomers can compare the bright- 

 ness of each one with that of the sun. The intrinsically brightest 

 star now known is one that is apparently associated with the great 

 Magellanic Clouds of the southern sky. At maximum brightness 

 this star, S Doradus, is half a million times as bright as our sun 

 and, according to the theory of relativity in which mass and light 



• Reprinted by permission, witli author's alterations, from The Scientific Monthly, vol. 

 34, pp. 343-350, April, 1032. 



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