70 NebulcB and Nebular Hypotheses. [Sess. 



bodies which are not to be found here, and which make their 

 presence known by spectral lines for which we have no 

 counterparts on earth. 



The principal difference between substances as found on 

 our earth and as found in the stars is one of temperature. 

 We consider a temperature of 80° Eahr. to be very hot 

 indeed ; but the temperature of the sun's surface — i.e., its 

 coldest region — is 11,000° Fahr., and that of many of the 

 stars is higher still. In such a furnace our well - known 

 substances change their condition altogether ; they cannot 

 exist as solids, or even as liquids, but are boiled into vapours 

 at once, just as ice may be boiled into water- vapour. Unlike 

 the latter, however, the vapour of a metal is not invisible, 

 but is vividly incandescent ; and it is to the presence in 

 them of such luminous vapours that the sun and the stars 

 owe their glorious brilliancy. 



The result of spectroscopic investigation is to prove that 

 stars pass through a process of evolution. When they are 

 young their spectra contain few lines, and these are due 

 mostly to the presence of helium and hydrogen. Such stars 

 are bluish-white in colour. As they grow older the lines 

 due to metallic vapours make their appearance, and the star 

 becomes yellower in colour. At a still later stage lines as 

 such give place to certain fluted bands, which are now recog- 

 nised as the indication of stellar senility and the herald of 

 approaching death. At this stage the star becomes redder, 

 as its internal fires begin slowly to die down. The last stage 

 of all — the inevitable terminus ad quern — is the dark stage, 

 when the surface of the one-time star has become cool enough 

 to form a crust which covers the still hot interior and shuts 

 out its light. That such dark stars exist we know for a fact, 

 since they make their presence evident to us in several ways, 

 although we can never actually see them. 



Since, then, the stars have a life-history, they must as- 

 suredly have had a birth. We cannot imagine that they are 

 launched into existence as complete and finished entities. 

 Everything in nature grows, — development is gradual. How, 

 then, do the stars come to be stars ? 



It is in the attempt to answer this question that we find 

 the necessity for " nebulae." We have seen that a star gets 



