LEARNING ABOUT THE STARS 



177 



out the year. Infer a cause for the change you note in 

 its position. 



Exercise. A person familiar with the stars can tell 

 the hour of the night by observing the position of Ursa 

 Major. Can you tell why this is possible? 



What is the nature of a star? Stars are intensely 

 hot, self-luminous bodies that give off enormous quan- 

 tities of energy. By means of an instrument called the 

 spectroscope, astronomers analyze light waves that 

 stars send out and in this way determine their chemi- 

 cal constitution. Stars are composed of iron, carbon, 

 hydrogen, sodium, and other chemical elements of 

 which the earth is made. In fact, all heavenly bodies 

 are built up from the same substances. This is one of 

 the great scientific discoveries of the past century, and 

 it shows a wonderful unity in the universe. 



Stars vary in their color. Stars that give off a bluish 

 white light are young stars made up of intensely 

 heated gases. As they become older they lose heat 

 and change in color from white to yellow and even- 

 tually to a deep red. When they become very, very 

 old they change to a solid state and give out no light. 

 Not all stars, however, exhibit such a regular life his- 

 tory. Stars whose brightness changes irregularly are 

 called variable stars. Occasionally a new star blazes 

 out where none had been seen before. About forty 

 stars of this class have been observed since 1572, most 

 of them during the past century. Most of these tempo- 

 rary stars soon lose their brightness. 



The stars are often called "fixed stars" because 

 they seem to maintain the same relative positions to 



Convex lens 



Plane mirror 



Eyepiece 



^Eyepiece Concave 



mirror" 



FIG 292. PRINCIPLE OF REFLECTING AND 

 REFRACTING TELESCOPES 



one another. They are, however, moving rapidly 

 among themselves. The relative motion of the stars 

 with respect to one another is about 600,000,000 miles 

 a year, but because of their enormous distances from 

 us the distances they move during a lifetime are not 

 perceptible to us. Even the Egyptians during the time 

 they were building the pyramids saw the constella- 

 tions almost exactly as they are seen today. If, how- 

 ever, we could live on the earth 100,000 years from 

 now, we should find the stars of the constellations in 

 different positions with relation to each other. 



How are telescopes constructed? The ancients were 

 greatly limited in their observations of the heavenly 

 bodies because they had only their eyes to collect star- 

 light. About 1610 Galileo made a telescope which en- 

 abled him to see thousands of stars never before seen 

 by man. Galileo's telescope revolutionized thought. 

 He not only saw many new stars but he also saw the 

 planet Jupiter with four satellites revolving around 

 it. This gave strong support to the Copernican view 

 that the sun is the center of our solar system and that 

 the planets and satellites revolve around it. This dis- 

 covery marked the downfall of the ancient notion 

 that the' earth is the center of our universe and estab- 

 lished the study of the heavens on a firm foundation. 



Courtesy Lick Observatory 



FIG. 293. THE LICK 36-INCH REFRACTING TELESCOPE 



