STELLAR EVOLUTION. 155 



either on aecount of tlieir or(»ater distance or heecUise thc^ stars are 

 less widely spaced, the eentral reiij;ions are no lonoer dearly resolval)le 

 as separate objects. It is thus ([uite possil)le to imaoiue a cluster in 

 which the stars are so closely o-rou]X'd that no telescope, however 

 powerful, could distinguish them separately. 



Now, as a matter of fact, we tind in all parts of the heavens lumin- 

 ous objects which can i)ot l)e separated into stars. Some of these are 

 of definite outline and are perfectly synmietiical in form, in many 

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

 knowni as the planetary ne])ula'. Other nebuhe, like the yi^nit nebula 

 in Orion (PI. VI), are diffuse and irregular and extend over great 

 regions of the sky. It was long a question whether such ol)jects were 

 capable of resolution into stars with a sufficiently powerful telescope. 

 Herschel rightly concluded that an impoi'tant distinction can be drawn 

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

 distinction. 



It was only after Iluggins had applied the spectroscope to an analy- 

 sis of the light of a nelnda that it could be said without danger of con- 

 tradiction that the })henomenon is not one produced by the crowding 

 together of separate stars, l)ut is due to the presence of a mass of incan- 

 descent gas. Sir William lluggins's account of his first spectroscopic 

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

 tions of the Tulse Hill OV)servatory : 



""On the evening of August 29, 18(i4, 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 1 suspected some displacement of the prism and that I 

 was looking at a reflection of the illuminat(Kl 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 1 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 Avidth 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 rcd'rangibility. A little closer 

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

 all three lines being s(»p;irated by intervals relatively dark. The riddle 

 of the nebuhe was solved. The answer, which had come to us in the 

 light itself, read: Not an aggregation of sfars, but a luminous gas.'' 



With this achance a new era of progi'css began. The power of the 

 spectroscope^ to disfingnish between a glowing gas and a. mass of j)ar- 

 tially condensed vapors like a star established it at onci; in its ])lace as 

 the chief instrument of the student of st(dlar evolution. Jt became 

 apparent that the unformed nebula might furnish flu; stulV from which 

 stars are made. Obsei'vations tendinu" to this conclusion were not lonii" 



