92 The Realm of Nature CHAP. 



fading down to invisibility. This happens so regularly in 

 some as to suggest that a dark body revolving round the 

 star comes between it and us. In other stars the increase 

 in brightness is accompanied, according to the spectroscope, 

 by a change in chemical constitution and a great increase 

 of temperature, as if perhaps swarms of meteorites flying in 

 opposite directions had come into collision. 



137. Distance of the Stars. The stars are so remote 

 that when corrected for aberration ( 108) there is, as a rule, 

 no apparent parallax. This means that the displacement 

 of our eye by 186,000,000 miles from one side of the 

 Earth's orbit to the opposite does not alter their apparent 

 position on the star-dome. In several cases a minute 

 parallax has been measured. The largest, barely i", was 

 found in the case of a Centauri, one of the brightest stars 

 visible in the southern hemisphere. The parallax of Sirius, 

 the brightest star in the sky, is \ of a second, that of the 

 Pole Star only -^ of a second. Light which travels at 

 186,000 miles per second requires 8 minutes to flash from 

 the Sun to the Earth, and would require 9 hours to traverse 

 the diameter of Neptune's orbit. Yet the light from a Cen- 

 tauri, the nearest star, has been more than 3 years on its 

 way to us. We see Sirius by the rays sent out more than 

 1 7 years ago, and for nearly half a century the light-waves 

 which are now arriving from the Pole Star have been shooting 

 with lightning speed across the awful voids of space. Other 

 stars are perhaps a hundred or a thousand times more 

 remote than these. Although the star-dome may be spoken 

 of as a vastly remote whole with reference to the solar 

 system, it is really made up of remotely isolated objects 

 placed at different distances and seen by us at different 

 dates. For all our sight can tell us to the contrary, every 

 star that shines placidly in the sky may have grown cold 

 years or centuries ago, and snapped the thread of light 

 the end of which may now be fast approaching our Earth. 



138. Stars as Suns. For classifying the stars the 

 spectroscope has entirely superseded the telescope. By its 

 means great differences have been detected in the chemical 

 composition and physical states of various stars, and the 



