ADVANCE OF ASTRONOMY. 195 



the Copernican system and as a suggestion of the 

 sunlike nature of the stars. 



Life of Stars. — If the view be accepted that the 

 sun was once a diffused body of gas extending be- 

 yond the present limits of the solar system, and that 

 it has slowly shrunk, giving rise to the present phase 

 of things, and if the stars be regarded as sunlike, we 

 should expect to find in the immensity of the heavens 

 some confirmatory evidence. In other words, we 

 should expect to see stars a-making and others a-dy- 

 ing. The former are now familiar to astronomers, 

 and the existence of dead stars is generally admitted. 



Nebulce. — It is generally agreed that the faint 

 clouds of light called nebula}, which occur scattered 

 in the sky, are in many cases at least early stages of 

 star-making, — embryo stars in an undifferentiated 

 state. Two of these nebula? are visible to the un- 

 aided eye on clear dark nights, namely, in the con- 

 stellations of Orion and of Andromeda. 



In the seventeenth century, after Galilei had intro- 

 duced the use of the telescope, many nebulae were de- 

 tected, but they were generally passed over quickly 

 as " diffusions of self-luminous matter," or " shining 

 fluid,'' or " fire-mist,'' and so forth. Towards the 

 end of the eighteenth century (1780) William Her- 

 schel began his study of nebulae, and not only in- 

 creased the list from 150 to 2,500 in about a score 

 of years, but showed that many of them had a de- 

 tailed structure. At first he regarded nebulae as 

 clusters of stars, and stated the evolutionary idea 

 that stars and clusters of stars arose from nebular 

 condensations. Subsequently, however, he reverted 

 to the older view in regard to many nebulae, includ- 

 ing that of Orion. In the first half of the nineteenth 

 century it was Herschel's earlier view that prevailed ; 



