THE HISTORY OF A STAR. 77 



pear as stars in our telescopes, their spectrum closely resembles 

 that of the nebula. Going still further still increasing the con- 

 densation, still increasing the temperature the region of stars 

 properly so called is reached, until at last we find those which are 

 represented at the top of the curve. These results have been ar- 

 rived at by spectroscopic work, and the facts recorded have been 

 the chemical changes which take place in these swarms as their 

 temperature increases, from the most sparse condition at the bot- 

 tom of the curve to the most condensed one at the top. 



In the sparsest swarms, in the so-called nebulse, and those 

 which are so dim as to be with difficulty visible, indications are 

 found of the so far unknown substance or substances to which I 

 have referred at the beginning of this article, together with car- 

 bon and hydrogen, and, in all probability, magnesium, one of the 

 most common metals in meteorites, which has a bright spectrum 

 visible at a low temperature ; though I should add that the visible 

 presence of magnesium has recently been contested. Its visible 

 presence or absence, however, is not of fundamental importance. 

 As the temperature increases, we find carbon more abundant, 

 and traces of manganese and lead, metals which volatilize at a 

 low temperature. 



The next greatest change that supervenes is the addition of 

 more familiar indications of the metals magnesium, manganese, 

 and sodium, while the spaces between the meteorites glow more 

 intensely with the light of hydrogen and carbon, probably brought 

 about by some electrical action. Here the sparseness is still so 

 great that we have little to do with the absorption of light ; we 

 simply deal with incandescent vapors due to the high temperature 

 brought about by collisions among the meteorites and to the glow 

 of the gases between the meteorites. But although the particles 

 of meteoritic dust are so far apart that there is no possibility of 

 any obvious absorption of their light occurring at this stage, to 

 any large extent, the story is soon changed, for, when real conden- 

 sation begins, the light of the meteoritic dust itself is absorbed by 

 the vapors produced at low temperatures which lie between each 

 particle of dust and our eyes. The whole theory of absorption is 

 dependent upon the fact that light must come from the light- 

 source through a vapor which is cooler than the light-source 

 itself. 



Thus we get a clear indication that, when this stage is reached, 

 the meteoritic dust is very much closer together, and is on this 

 account capable of forming a background enabling us to see these 

 light-absorption phenomena. Absorption of light by the vapors 

 of substances known to exist in meteorites, such as manganese 

 and lead, is the first to occur, and these absorption phenomena 

 gradually preponderate, and indicate change from low to high 



