vi The Solar System and Universe 91 



collision on these occasions was a fine shower of shooting- 

 stars, through which the Earth sailed as safely as a locomotive 

 passes through a cloud of dust. Meteorites of all sizes, 

 from an invisible granule to masses of several tons 

 and moving in various directions, seem to be scattered 

 in infinite numbers through all space, and occasional denser 

 swarms moving together form comets. 



136. The Stars. The Sun, surrounded by its orderly 

 family of planets and an irregular host of attendant comets 

 and meteorites, is practically alone in the centre of the star- 

 sphere, forming one system isolated by inconceivable ex- 

 panses of space from the fixed stars. But the Sun and its 

 train are sweeping with tremendous velocity in the direc- 

 tion of the constellation Hercules. The number of stars 

 or fixed points of light on the star-dome which are visible 

 at any one time to the unaided eye of an observer on the 

 Earth is about 3000. More people in fact assemble to 

 hear a popular concert than there are stars in the heavens, 

 so far as our vision can tell. By the aid of an opera-glass 

 more than 120,000 stars, too feeble in their light to be seen 

 by the unaided eye, spring into sight. A million may be seen 

 through a small telescope ; in a large telescope the number 

 is enormously increased, and with every instrumental im- 

 provement smaller specks of light crowd in myriads on the 

 view. Some stars, invisible in the most powerful telescopes 

 to the eye, have been discovered by their effect on a 

 sensitive photographic plate. Altogether the existence of 

 something like 100,000,000 stars has been ascertained. 

 The telescope, no matter how powerful, fails to make even 

 the brightest star appear as a disc ; but it often shows that 

 what we see as a single star is actually double, triple, 

 quadruple, or multiple. In some cases this is an accidental 

 result of stars, perhaps very distant from one another, lying 

 nearly in the same line as seen from the Earth ; but there 

 are many " physical doubles " the associated stars of 

 which are seen to revolve round one another. This dis- 

 covery proves that these stars are subject to gravitation. 

 Several stars vary in their brightness at definite intervals, at 

 one time blazing out with extraordinary brilliance and then 



