134 THE FUNCTION OF LARGE TELESCOPES. 



should be added that no bard and fast line can be drawn between 

 astronomy and astrophysics, as one of the principal problems of the 

 latter subject involves just such determinations of motion as are par- 

 ticularly to be desired for the purposes of the astronomy of position. 

 The subjects are thus intimately related and closely bound together, 

 and the bond between astrophysics and physics is hardly less strong. 

 They should thus be cultivated together, so that they may mutually 

 assist one another in bringing about the solution of the varied problems 

 with which they are concerned. 



It is particularly in astrophysical research that a great telescope is 

 advantageous. For the principal instrument of the astrophysicist, the 

 spectroscope, it is necessary to have as much light as can be gathered 

 into a single point. With sufficient light the chemical analysis of the 

 most distant star resolves itself into a comparatively simple problem. 

 But with small telescopes, and consequently faint star images, such 

 analysis, except of a roughly approximate character, is impossible with 

 the less brilliant stars 



One of the principal problems of the astrophysicist is to determine 

 the course of celestial evolution. It has been found that the spectra of 

 stars are susceptible of classification in a few well-defined types, which 

 seem to correspond with different periods in stellar development. 

 Starting from the great cloudlike masses of the nebuhe, it is supposed 

 that stars begin to form in regions of condensation, and that the great 

 masses of gas and vapor continue to contract under the action of gravi- 

 tation, meanwhile radiating heat into space. It is known from theo- 

 retical investigations that such cooling gaseous masses not only con- 

 tinue to grow smaller; they also rise in temperature with the advance 

 of time. Finally a certain point in their career is reached when the rise 

 in temperature ceases, though the contraction of the mass is not 

 arrested. The balls of condensing vapors continue to cool, losing more 

 and more heat, and becoming smaller and smaller in diameter. It is 

 perhaps at about this period in their history that they pass through 

 such a stage as is now exemplified by the sun, which has presumably 

 cooled from the condition of a white star like Sirius to that of a star of 

 the second or yellow class. The spectra of such hot stars as Sirius con- 

 tain little more than dark and exceedingly broad lines, grouped in rhyth- 

 mical order and due to the gas hydrogen. As these bodies continue to 

 cool, the strong lines of hydrogen become less prominent, and lines due 

 to metallic substances begin to appear. These become more and more 

 striking, until finally we reach such a type of spectrum as that of 

 Procyon, which is intermediate in character between the Sirian and the 

 solar stars. From this point on we find a continual approach to the 

 solar type, until at last stars are reached whose spectra agree line for 

 line with that of the sun. After passing through the condition of 

 the central body of the solar system, the yellow and orange color of 

 the stars becomes more pronounced, and subsequently a reddish tinge 



