Comets 263 



FIG. 156. Great nebula in Andromeda. 



its motion around the sun, and becomes visible to us 

 as a shower of meteors when the earth gets in its way. 



Nebulae. Nebulae are like comets in appearing hazy, 

 but they are not parts of the solar system. More than 

 100,000 nebulae can be seen with the largest telescopes. 

 Probably none of them are as near as the brighter stars. 

 Some are green in color, but a great majority are white 

 and show a spiral form. These are believed to consist 

 of innumerable small particles of solid matter, and some 

 gas. The green nebulae are thought to be mainly gas- 

 eous. The great nebula in Andromeda is the only one 

 that is large and bright enough to be easily seen with 

 the naked eye. The distance across it is probably hun- 

 dreds of thousands of times as great as the distance 

 from the earth to the sun. It is approaching the solar 

 system at the rate of 185 miles a second. 



