STAR-CLUSTERS AND NEBULA. 



551 



the most celebrated for 



Fig. 623. Star-Map. 



its variable qualities, and its cycle of .change 

 occupies nearly one of our years. For a few 

 days it appears very bright, and then fades 

 away for about three months, to disappear 

 for five months, and then it reappears again, 

 increasing in brilliancy up to the second 

 magnitude for another three months or so. 

 Some people account for these phenomena by 

 stating that the sides of the star being less 

 luminous present the dark and light portions 

 in rotation ; but we can give no satisfactory 

 explanation of the reason, unless it be caused 

 by an aggregation of spots upon its surface, 

 like sun-spots on our sun, or perhaps by 

 eclipse. 



STAR-CLUSTERS AND NEBULA. 



NEBULAE and STAR-CLUSTERS are nume- 

 rous in the heavens. The most important are the 

 Great Nebulae in Orion and in Andromeda. But 

 there are other 

 very beauti- 

 ful "patches" 

 of luminous 

 matteror cloud 



appearances composed of minute stars 



invisible to the naked eye. We annex iteN-***fc 



specimens of the Nebulae, one or two having been already inserted. 



There must be thousands of these star-clouds, and they have been 



classified by Sir John Herschel from Sir William's discoveries as follows : 



(1) Clusters of stars, in 



which the stars are 

 clearly distinguish- 

 able, divided again 

 into regular and ir- 

 regular clusters. 



(2) Resolvable Nebulae, 



which may be sepa- 

 rated into distinct 

 stars under powerful 

 telescopes. 



Fig. 6 25 .-Dumb beii Nebui. (3) Nebulae, in which there 



is no appearance 

 whatever of stars, divided into classes according to brightness, etc. 



