vi The Solar System and Universe 93 



classification now viewed with most favour is of a bio- 

 graphical character, referring the star to its position in the 

 long evolution or series of changes through which our Sun 

 is passing ( 118). In arranging the stars in the order of 

 their evolution their state at the period their light left them 

 is of course referred to. Stars of youth, or the earlier stages, 

 are comparatively cool and diffused agglomerations of 

 matter gradually condensing and rising in temperature. 

 Stars of middle life, or the central stages, are intensely hot, 

 invested with a glowing atmosphere of gas which gives 

 bright lines in the spectra of their light. Stars of old age, 

 or the later stages of evolution, have survived the period of 

 maximum temperature and are steadily consolidating and 

 cooling down. There is reason to believe that many stars 

 are invisible to us because they have ceased to glow. We 

 may infer, from their general similarity to our Sun, that stars 

 of the central and later stages at least are accompanied by 

 systems of planets. Some double stars present much the 

 same appearance as the Sun would have done at a similar 

 distance when Jupiter was still brilliantly incandescent. 

 Many of the stars have a rapid motion through space as 

 shown by the displacement of their spectral lines. This is 

 termed their proper motion, to distinguish it from the various 

 apparent movements, but though it is inconceivably swift it 

 has produced very little change in the appearance of the 

 constellations in 2000 years. 



139. Charting the Heavens. Although the constella- 

 tions remain of the same form as when first described by 

 astronomers, some change must be taking place. Common 

 star-maps fail to let the changes appear, but a series of 

 large photographic charts of the sky would probably show a 

 definite alteration of position amongst the stars on account 

 of their proper motion in a few years. An International 

 Astronomical Congress held in 1891 decided that in several 

 observatories such photographs should be taken with the 

 ultimate object of completing a photographic survey of the 

 entire star-dome. In order to prevent confusion from chance 

 specks and to detect asteroids, a device has been suggested 

 by which the photographic plate is exposed in the telescope 



