THE EARTH AS A PART OF THE SOLAR SYSTEM 531 



Only about two tnousand stars can be seen by the eye at 

 any one time. There are many other stars not visible to 

 the eye that can be seen by the telescope ; it has been esti- 

 mated that there are one hundred million stars that may be 

 seen by using this instrument. The camera takes pictures 

 of stars that not even the telescope can detect. 



Difference in brightness. Stars differ in their brightness. 

 Some are very bright and can be easily seen, while others 

 can just barely be seen and most cannot be seen at all with- 

 out the telescope. This difference may be due to two things, 

 to difference in distance and to a difference in the light- 

 giving power of the star. Stars are self-luminous like our 

 sun. Indeed our sun is a small star, but it looks bright 

 because it is so near. We may think of stars, then, as large 

 suns scattered through space. The brightest of all the stars 

 is Sirius, the Dog Star, seen during the winter months. It 

 has been estimated that this gives out as much light as 

 thirty of our suns. 



Constellations. Every one has observed that stars are 

 grouped in clusters, which are called constellations, each of 

 which has been named. There is usually little similarity 

 between the arrangement of the stars and the object for 

 which the constellation is named. One of the best known 

 of these is the Great Bear or Dipper. All the stars seem to 

 revolve around a point in the heavens near the North Star. 

 This star may be easily found by means of the Great Dipper. 

 The two outer stars on the bowl of the Great Dipper are 

 called pointers, because they point to the North Star. All 

 the constellations found between that star and the horizon 

 never set, and so they can be seen during all seasons of the 

 year. On the other hand, there are stars near the south pole 

 of the heavens, seen from the southern hemisphere, which we 

 never see in this latitude. 



There are many conspicuous constellations, and the ones 

 that can be seen change from month to month. The stars 



