Chapter VIII 



MAN'S PLACE IN THE UNIVERSE 



The Sidereal Universe. The largest telescopes reveal 

 about a thousand million stars. Each increase in tele- 

 scopic power adds to the number and we can scarcely 

 set a limit to the multitude that must exist. Nevertheless 

 there are signs of exhaustion, and it is clear that the 

 distribution which surrounds us does not extend uni- 

 formly through infinite space. At first an increase in 

 light-grasp by one magnitude brings into view three 

 times as many stars; but the factor diminishes so that 

 at the limit of faintness reached by the giant telescopes 

 a gain of one magnitude multiplies the number of stars 

 seen by only 1.8, and the ratio at that stage is rapidly 

 decreasing. It is as though we are approaching a limit 

 at which increase of power will not bring into view very 

 many additional stars. 



Attempts have been made to find the whole number 

 of stars by a risky extrapolation of these counts, and 

 totals ranging from 3000 to 30,000 millions are some- 

 times quoted. But the difficulty is that the part of the 

 stellar universe which we mainly survey is a local con- 

 densation or star-cloud forming part of a much greater 

 system. In certain directions in the sky our telescopes 

 penetrate to the limits of the system, but in other direc- 

 tions the extent is too great for us to fathom. The 

 Milky Way, which on a dark night forms a gleaming 

 belt round the sky, shows the direction in which there 

 lie stars behind stars until vision fails. This great 

 flattened distribution is called the Galactic System. It 

 forms a disc of thickness small compared to its areal 



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