332 NEWCOMB 



distance of the sun. This is a distance through which light 

 would travel in about 3300 years. 



It is not impossible that the number of stars is much 

 greater than that we have supposed. Let us grant that there 

 are eight times as many, or 1,000,000,000. Then we should 

 have to extend the boundary of our universe twice as far, 

 carrying it to a distance which light would require 6600 

 years to travel. 



There is another method of estimating the thickness with 

 which stars are sown through space, and hence the extent of 

 the universe, the result of which will be of interest. It is 

 based on the proper motion of the stars. One of the 

 greatest triumphs of astronomy of our time has been the 

 measurement of 1 the actual speed at which many of the stars 

 are moving to or from us in space. These measures are 

 made with the spectroscope. Unfortunately, they can be 

 best made only on the brighter stars becoming very diffi- 

 cult in the case of stars not plainly visible to the naked eye. 

 Still the motions of several hundreds have been measured and 

 the number is constantly increasing. 



A general result of all these measures and of other esti- 

 mates may be summed up by saying that there is a certain 

 average speed with which the individual stars move in space ; 

 and that this average is about twenty miles per second. 

 We are also able to form an estimate as to what proportion 

 of the stars move with each rate of speed from the lowest 

 up to a limit which is probably as high as 150 miles per 

 second. Knowing these proportions we have, by obser- 

 vation of the proper motions of the stars, another method of 

 estimating how thickly they are scattered in space; in other 

 words, what is the volume of space which, on the average, 

 contains a single star. This method gives a thickness of 

 the stars greater by about twenty-five per cent, than that 

 derived from the measures of parallax. That is to say, a 

 sphere like the second we have proposed, having a radius 

 800,000 times the distance of the sun, and therefore a di- 

 ameter 1,600,000 times this distance, would, judging by the 

 proper motions, have ten or twelve stars contained within it, 

 while the measures of parallax only show eight stars within 

 the sphere of this diameter having the sun as its centre. 



