548 SCIENTIFIC RECREATIONS. 



mentioned. As a consequence of their proper motion the binary stars 

 appear to vary in their distances from each other, as in the topmost of the 

 three cuts on the (opposite) page representing y Virginis. The stars have 



Fig. 615. it Coronae. Fig. 616. Boijtes. Fig. 617. Castor. Fig. 618. Rigel. 



gradually approached each other, and so are the stars in Castor approximating, 

 and when they have closed, and have appeared almost as a single star, as 

 they will do, they will take open order again. 



The shortest time occupied by a double star in its revolution is thirty- 

 five years, and we have already given some of those which have been 

 ascertained. We will close this section with a few other examples. For 

 instance, y Virginis revolves in one hundred and fifty years, Castor in 

 two hundred and forty years, 4 Aquarii in three hundred years, 37 Pegasi 

 in five hundred years. There are numerous other instances up to a period 

 of three thousand years, and about eight hundred of these binary systems 

 are known. We have mentioned that there are two or more suns in the 

 multiple systems. These suns are the cause of the different colours of the 

 stars. 



COLOURS OF THE STARS. 



The question of star-colour follows naturally the consideration of the 

 multiple stars ; for although single stars have been observed of a ruddy 

 colour, there are no instances of a blue or green one unattended by a companion. 

 This colouring has been attributed to the contrast between multiple stars, for 

 the colours are frequently complementary ; but investigation has shown that 

 this cannot be the case. For instances have been known in which, when 

 two are thus associated, and one is concealed from us, the other is just as 

 bright, and retains its former colour. 



Of course in cases in which colour is apparent to the unaided vision, only 

 the brightest stars betray colour. Antares, Betelgeuse and Aldebran are red 

 (orange) colour. Sirius and Canopus are white. Arcturus and Capella are 

 yellowish, so is Pollux. Vega is bluish-white. These appearances are, of 

 course, much more marked when the stars are examined through the 

 telescope, and telescopic stars which are stars unobservable without a 

 glass are very much coloured, and the multiple stars give us blue, green, 

 violet, and other tints, besides those already mentioned. 



Again, these coloured stars do not always remain the same colour. Sirius 

 was once red ; Mars was at times white. Spectrum analysis shows that 



