X THE STAREY HEAVENS 425 



NEBULA 



From Stars we pass insensibly to Nebulse, 

 which are so far away that their distance 

 is at present quite immeasurable. All that 

 we can do is to fix a minimum, and this 

 is so great that it is useless to express it 

 in miles. Astronomers, therefore, take the 

 velocity of light as a unit. It travels at the 

 rate of 180,000 miles a second, and even at 

 this enormous velocity it must have taken 

 hundreds of years to reach us, so that we see 

 • them not as they now are but as they were 

 hundreds of years ago. 



It is no wonder, therefore, that in many of 

 these clusters it is impossible to distinguish 

 the separate stars of which they are composed. 

 As, however, our telescopes are improved, 

 more and more clusters are being resolved. 

 Photography also comes to our aid, and, as 

 already mentioned, by long exposure stars can 

 be made visible which are quite imperceptible 

 to the eye, even with aid of the most powerful 



telescope. 



Spectrum analysis also seems to show that 



