STELLAR EVOLUTION. 301 



Fig. 4. His telescope was a large one, but it can safely be said that 

 he never saw a cluster so well as this object can be perceived through 

 the aid of photograpliy. He found in studying object after object in 

 all parts of the heavens that many clusters could be resolved into their 

 constituent stars. ■ In some of these clusters the stars are widely sepa- 

 rated by a powerful instrument, as they appear in this photograph. In 

 others, either on account of their greater distance or because the stars 

 are less widely spaced, the central regions are no longer clearly re- 

 solvable as separate objects. It is thus quite possible to imagine a 

 cluster in which the stars are so closely grouped that no telescope, how- 

 ever powerful, could separately distinguish them. 



Now as a matter of fact we find in all parts of the heavens luminous 

 objects which can not be separated into stars. Some of these are of 

 definite outline and are perfectly symmetrical in form, in many cases 

 with a brilliant star-like nucleus at their center. These are known as 

 the planetary nebula. Other nebulae, like the great nebula in Orion 

 (Fig. 6), are diffuse and irregular and extend over great regions of 

 the sky. It was long a question whether such objects were capable of 

 resolution into stars with a sufficiently powerful telescope. Herschel 

 rightly concluded that an important distinction can be drawn between 

 a nebula and a star cluster, though his son did not admit this dis- 

 tinction. 



It was only after Huggins had applied the spectroscope to an 

 analysis of the light of a nebula that it could be said without danger of 

 contradiction that the phenomenon is not one produced by the crowding 

 together of separate stars, but is due to the presence of a mass of incan- 

 descent gas. Sir William Huggins' account of his first spectroscopic 

 examination of a nebula is recorded in the first volume of the 'Publica- 

 tions of the Tulse Hill Observatory' : 



''On the evening of August 29, 1864, I directed the spectroscope for 

 the first time to a planetary nebula in Draco. I looked into the spec- 

 troscope, No spectrum such as I had expected ! A single bright line 

 only ! At first I suspected some displacement of the prism, and that I 

 was looking at a reflection of the illuminated slit from one of its faces. 

 This thought was scarcely more than momentary; then the true inter- 

 pretation flashed upon me. The light of the nebula was monochro- 

 matic and so, unlike any other light I had as yet subjected to prismatic 

 examination, could not be extended out to form a complete spectrum. 

 After passing through the two prisms it remained concentrated into a 

 single bright line, having a width corresponding to the width of the 

 slit, and occupying in the instrument a position at that part of the 

 spectrum to which its light belongs in refrangibility. A little closer 

 looking showed two other bright lines on the side towards the blue, 

 all three lines being separated by intervals relatively dark. The riddle 



