Of course in cases in which colour is apparent to the unaided vision, only the brightest stars betray colour. Antares, Betelgeuse and Aldebaran 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 the colours of many are due to absorption by the vapours of some of the rays; and the existence of certain vapours may cut off some, and at other times other vapours may exist and cut off other rays, and so the colours may be changed. Struve gives the following list of binary complements of “multiple” stars:—
| Pairs consisting of a blue and white principal star | 53 |
| Pairs consisting of a blue and a light yellow principal star | 52 |
| Pairs consisting of a blue and yellow or red principal star | 52 |
| Pairs consisting of a blue and green principal star | 16 |
Fig. 619.—Position of the two stars of γ Virginis.*
Fig. 620.—Position of the two stars of Castor.*
Fig. 621.—Position of the two stars of ξ Ursæ Majoris.
We need scarcely pursue this question farther, though many ideas concerning the coloured stars will arise in every thoughtful reader’s mind. Supposing that every system has its sun or suns, can we fancy the effects of a green or blue or violet sunlight—a light unmixed? To employ the words of Sir John Herschel—“It may be more easily suggested in words than conceived in imagination what variety of illumination two suns, a red and a green, or a yellow and blue one, must afford to a planet circulating round either—or what charming contrasts and graceful vicissitudes a red and a green day, for instance, alternating with a white one and with darkness, might arise from the presence or absence of one or other or both above the horizon.”