THE EARTH'S PLACE IN THE UNIVERSE 19 



to recognize the groups, it is easy to find the "star in the end 

 of the dipper handle," "the brightest star in Taifrus," or 

 "the belt of Orion," even if the group is in a different 

 part of the sky from that in which we saw it last. 



Another reason why we should know something about the 

 constellations is the fact that references to the stars are 

 found in the poetry and the history of all ages since men 

 began to express their thoughts and record their observations 

 in writing. We can better understand these references if 

 we know the stars which, long before the pyramids of Egypt 

 were built, were shining upon the earth just as they are to- 

 day. Many of the names by which the stars were known 

 to the Persians, the Egyptians, and the Greeks have been 

 handed down to us. 



7. The Brightness of the Stars. Early astronomers 

 thought the brightness of stars was due to their size, so they 

 called the brightest ones first-magnitude stars, those a little 

 less bright second-magnitude stars, and so on down to the 

 sixth magnitude. In the city, where there is so much arti- 

 ficial light, we rarely see stars fainter than those of the third 

 magnitude. Sirius, Vega, Regulus, and Aldebaran are first- 

 magnitude stars; the North Star and the stars of the Big 

 Dipper and of Cassiopeia are some of the second-magnitude 

 stars. 



It is now known that the brightness of a star depends not 

 only on its size, but on its distance from the earth and on its 

 temperature. Many of the stars are larger than the sun 

 but are much more distant; they seem to us like mere points 

 of light. Enough has been learned about the stars to make 

 it clear that they are very unlike the earth in size, tempera- 

 ture, and condition. 



8. The Distance of the Stars. The stars are so far 

 away that their distances cannot be expressed in any number 

 that we can comprehend. When we say that the nearest 

 star is millions of millions of miles away, it is impossible to 



