HOW FAR AWAY ARE THE STARS? 



193 



the observatory to Chicago. Here it operated machinery 

 which turned on the lights and opened the Fair. Each 

 night the great batteries of electric lights at the Century 

 of Progress Exposition were turned on by means of the 

 light from this same star sent from one or more of the 

 observatories which co-operated in this interesting service. 

 If the distance to Arcturus were expressed in miles, it 

 would be about forty times six million million. Can 

 you express this in figures ? 



Star Magnitudes. Any one who has seen the heavens on 

 a clear night knows that the brightness of the stars varies 

 greatly. The faintest star visible to the unaided eye is 

 called a sixth magnitude star. This furnishes the basis 

 of classifying them. The table which follows gives a 

 rough comparison of magnitudes or brightness. 



The apparent brightness of a star depends upon its 

 temperature, size, and distance. Other things being 

 equal, the nearer the star to us, the brighter it seems. 

 The North Star is about as bright as Betelgeuse, but it 

 appears much dimmer because it is more than twice as 

 far away from us, and yet it appears brighter than some 

 nearer stars which are smaller and cooler. 



What the Color of Stars Tells Us. The unaided eye can 

 easily notice a difference in color of some of the stars. 

 When an iron rod is heated in the furnace, the first 



H. & W. SCI. I 14 



