THE MEASURE OF STELLAR SPACES 341 



conceive it. Here is a body, possibly three or four thousand 

 times the diameter of the earth. If its density be somewhat 

 similar to that of our sun, its mass then is perhaps a hundred 

 or two hundred million times that of the earth. The energy 

 of a moving body is proportionate to its mass first, and secondly, 

 to the square of its speed. A rifle-bullet moving at less than 

 half a mile per second, and striking a steel target, becomes 

 very hot. At a dozen times this speed it would develop enough 

 heat to melt itself. 



Arcturus is moving six hundred times as fast. Were it to 

 collide with a stationary object, it would develop then thirty- 

 six thousand times as much heat per unit of mass as the rifle- 

 bullet. We have then an idea of what stellar collisions may 

 mean. Arcturus has, let us say, forty to fifty thousand times 

 the attractive force of our sun. It would draw another sun 

 or planet towards it with proportional speed. 



There is some evidence that such collisions take place. It 

 is doubtful if tire nearness of our nearest star is exceptional. 

 Were Arcturus at the distance of alpha Centauri, and coming 

 towards us, it would need but two or three thousand years to 

 cross the intervening space a fraction of a second in the life- 

 history of the stars. Assuming that the average spacing of the 

 suns is not much greater it may be much less it is evident, 

 with many hundreds of millions of them flying each whither 

 at tremendous speeds, such collisions might be frequent. As 

 we shall see upon a later page, it is probable that they are. 



Estimates of such unthinkable motions of colossal masses 

 might well raise a doubt as to their reality. At three hundred 

 miles per second, a body several miles in diameter flashing 

 across the line of sight a few feet before our eyes, would move 

 so swiftly that it would not be perceived. In our endeavour 

 to obtain a mental presentation of the motion of such a mass 

 as Arcturus, we might well believe that the observations were 

 an error, or the whole idea an illusion. Happily there came 

 a discovery which was not only to confirm the fact of stellar 

 motion ; but to disclose that motion, when it lay directly in 

 and not across the line of sight. It was to do much besides. 

 It came within a few years of the disclosures of Bessel and 

 Struve, though it was not put into effect for many years there- 

 after. It was from such a strange quarter of the sky that men 

 could scarcely then believe, and the wonder has not yet ceased. 



